Move GoogleUtilities out of Firebase directory (#1516)

10 files changed, 1561 insertions, 0 deletions

diff --git a/GoogleUtilities/Network/GULMutableDictionary.m b/GoogleUtilities/Network/GULMutableDictionary.m
new file mode 100644
index 0000000..d281eb4
--- /dev/null
+++ b/GoogleUtilities/Network/GULMutableDictionary.m
@@ -0,0 +1,97 @@
+// Copyright 2017 Google
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#import "Private/GULMutableDictionary.h"
+
+@implementation GULMutableDictionary {
+  /// The mutable dictionary.
+  NSMutableDictionary *_objects;
+
+  /// Serial synchronization queue. All reads should use dispatch_sync, while writes use
+  /// dispatch_async.
+  dispatch_queue_t _queue;
+}
+
+- (instancetype)init {
+  self = [super init];
+
+  if (self) {
+    _objects = [[NSMutableDictionary alloc] init];
+    _queue = dispatch_queue_create("GULMutableDictionary", DISPATCH_QUEUE_SERIAL);
+  }
+
+  return self;
+}
+
+- (NSString *)description {
+  __block NSString *description;
+  dispatch_sync(_queue, ^{
+    description = self->_objects.description;
+  });
+  return description;
+}
+
+- (id)objectForKey:(id)key {
+  __block id object;
+  dispatch_sync(_queue, ^{
+    object = self->_objects[key];
+  });
+  return object;
+}
+
+- (void)setObject:(id)object forKey:(id<NSCopying>)key {
+  dispatch_async(_queue, ^{
+    self->_objects[key] = object;
+  });
+}
+
+- (void)removeObjectForKey:(id)key {
+  dispatch_async(_queue, ^{
+    [self->_objects removeObjectForKey:key];
+  });
+}
+
+- (void)removeAllObjects {
+  dispatch_async(_queue, ^{
+    [self->_objects removeAllObjects];
+  });
+}
+
+- (NSUInteger)count {
+  __block NSUInteger count;
+  dispatch_sync(_queue, ^{
+    count = self->_objects.count;
+  });
+  return count;
+}
+
+- (id)objectForKeyedSubscript:(id<NSCopying>)key {
+  // The method this calls is already synchronized.
+  return [self objectForKey:key];
+}
+
+- (void)setObject:(id)obj forKeyedSubscript:(id<NSCopying>)key {
+  // The method this calls is already synchronized.
+  [self setObject:obj forKey:key];
+}
+
+- (NSDictionary *)dictionary {
+  __block NSDictionary *dictionary;
+  dispatch_sync(_queue, ^{
+    dictionary = [self->_objects copy];
+  });
+  return dictionary;
+}
+
+@end
diff --git a/GoogleUtilities/Network/GULNetwork.m b/GoogleUtilities/Network/GULNetwork.m
new file mode 100644
index 0000000..233500b
--- /dev/null
+++ b/GoogleUtilities/Network/GULNetwork.m
@@ -0,0 +1,388 @@
+// Copyright 2017 Google
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#import "Private/GULNetwork.h"
+#import "Private/GULNetworkMessageCode.h"
+
+#import <GoogleUtilities/GULLogger.h>
+#import <GoogleUtilities/GULNSData+zlib.h>
+#import <GoogleUtilities/GULReachabilityChecker.h>
+#import "Private/GULMutableDictionary.h"
+#import "Private/GULNetworkConstants.h"
+
+/// Constant string for request header Content-Encoding.
+static NSString *const kGULNetworkContentCompressionKey = @"Content-Encoding";
+
+/// Constant string for request header Content-Encoding value.
+static NSString *const kGULNetworkContentCompressionValue = @"gzip";
+
+/// Constant string for request header Content-Length.
+static NSString *const kGULNetworkContentLengthKey = @"Content-Length";
+
+/// Constant string for request header Content-Type.
+static NSString *const kGULNetworkContentTypeKey = @"Content-Type";
+
+/// Constant string for request header Content-Type value.
+static NSString *const kGULNetworkContentTypeValue = @"application/x-www-form-urlencoded";
+
+/// Constant string for GET request method.
+static NSString *const kGULNetworkGETRequestMethod = @"GET";
+
+/// Constant string for POST request method.
+static NSString *const kGULNetworkPOSTRequestMethod = @"POST";
+
+/// Default constant string as a prefix for network logger.
+static NSString *const kGULNetworkLogTag = @"Google/Utilities/Network";
+
+@interface GULNetwork () <GULReachabilityDelegate, GULNetworkLoggerDelegate>
+@end
+
+@implementation GULNetwork {
+  /// Network reachability.
+  GULReachabilityChecker *_reachability;
+
+  /// The dictionary of requests by session IDs { NSString : id }.
+  GULMutableDictionary *_requests;
+}
+
+- (instancetype)init {
+  return [self initWithReachabilityHost:kGULNetworkReachabilityHost];
+}
+
+- (instancetype)initWithReachabilityHost:(NSString *)reachabilityHost {
+  self = [super init];
+  if (self) {
+    // Setup reachability.
+    _reachability = [[GULReachabilityChecker alloc] initWithReachabilityDelegate:self
+                                                                        withHost:reachabilityHost];
+    if (![_reachability start]) {
+      return nil;
+    }
+
+    _requests = [[GULMutableDictionary alloc] init];
+    _timeoutInterval = kGULNetworkTimeOutInterval;
+  }
+  return self;
+}
+
+- (void)dealloc {
+  _reachability.reachabilityDelegate = nil;
+  [_reachability stop];
+}
+
+#pragma mark - External Methods
+
++ (void)handleEventsForBackgroundURLSessionID:(NSString *)sessionID
+                            completionHandler:(GULNetworkSystemCompletionHandler)completionHandler {
+  [GULNetworkURLSession handleEventsForBackgroundURLSessionID:sessionID
+                                            completionHandler:completionHandler];
+}
+
+- (NSString *)postURL:(NSURL *)url
+                   payload:(NSData *)payload
+                     queue:(dispatch_queue_t)queue
+    usingBackgroundSession:(BOOL)usingBackgroundSession
+         completionHandler:(GULNetworkCompletionHandler)handler {
+  if (!url.absoluteString.length) {
+    [self handleErrorWithCode:GULErrorCodeNetworkInvalidURL queue:queue withHandler:handler];
+    return nil;
+  }
+
+  NSTimeInterval timeOutInterval = _timeoutInterval ?: kGULNetworkTimeOutInterval;
+
+  NSMutableURLRequest *request =
+      [[NSMutableURLRequest alloc] initWithURL:url
+                                   cachePolicy:NSURLRequestReloadIgnoringLocalCacheData
+                               timeoutInterval:timeOutInterval];
+
+  if (!request) {
+    [self handleErrorWithCode:GULErrorCodeNetworkSessionTaskCreation
+                        queue:queue
+                  withHandler:handler];
+    return nil;
+  }
+
+  NSError *compressError = nil;
+  NSData *compressedData = [NSData gul_dataByGzippingData:payload error:&compressError];
+  if (!compressedData || compressError) {
+    if (compressError || payload.length > 0) {
+      // If the payload is not empty but it fails to compress the payload, something has been wrong.
+      [self handleErrorWithCode:GULErrorCodeNetworkPayloadCompression
+                          queue:queue
+                    withHandler:handler];
+      return nil;
+    }
+    compressedData = [[NSData alloc] init];
+  }
+
+  NSString *postLength = @(compressedData.length).stringValue;
+
+  // Set up the request with the compressed data.
+  [request setValue:postLength forHTTPHeaderField:kGULNetworkContentLengthKey];
+  request.HTTPBody = compressedData;
+  request.HTTPMethod = kGULNetworkPOSTRequestMethod;
+  [request setValue:kGULNetworkContentTypeValue forHTTPHeaderField:kGULNetworkContentTypeKey];
+  [request setValue:kGULNetworkContentCompressionValue
+      forHTTPHeaderField:kGULNetworkContentCompressionKey];
+
+  GULNetworkURLSession *fetcher = [[GULNetworkURLSession alloc] initWithNetworkLoggerDelegate:self];
+  fetcher.backgroundNetworkEnabled = usingBackgroundSession;
+
+  __weak GULNetwork *weakSelf = self;
+  NSString *requestID = [fetcher
+      sessionIDFromAsyncPOSTRequest:request
+                  completionHandler:^(NSHTTPURLResponse *response, NSData *data,
+                                      NSString *sessionID, NSError *error) {
+                    GULNetwork *strongSelf = weakSelf;
+                    if (!strongSelf) {
+                      return;
+                    }
+                    dispatch_queue_t queueToDispatch = queue ? queue : dispatch_get_main_queue();
+                    dispatch_async(queueToDispatch, ^{
+                      if (sessionID.length) {
+                        [strongSelf->_requests removeObjectForKey:sessionID];
+                      }
+                      if (handler) {
+                        handler(response, data, error);
+                      }
+                    });
+                  }];
+  if (!requestID) {
+    [self handleErrorWithCode:GULErrorCodeNetworkSessionTaskCreation
+                        queue:queue
+                  withHandler:handler];
+    return nil;
+  }
+
+  [self GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                    messageCode:kGULNetworkMessageCodeNetwork000
+                        message:@"Uploading data. Host"
+                        context:url];
+  _requests[requestID] = fetcher;
+  return requestID;
+}
+
+- (NSString *)getURL:(NSURL *)url
+                   headers:(NSDictionary *)headers
+                     queue:(dispatch_queue_t)queue
+    usingBackgroundSession:(BOOL)usingBackgroundSession
+         completionHandler:(GULNetworkCompletionHandler)handler {
+  if (!url.absoluteString.length) {
+    [self handleErrorWithCode:GULErrorCodeNetworkInvalidURL queue:queue withHandler:handler];
+    return nil;
+  }
+
+  NSTimeInterval timeOutInterval = _timeoutInterval ?: kGULNetworkTimeOutInterval;
+  NSMutableURLRequest *request =
+      [[NSMutableURLRequest alloc] initWithURL:url
+                                   cachePolicy:NSURLRequestReloadIgnoringLocalCacheData
+                               timeoutInterval:timeOutInterval];
+
+  if (!request) {
+    [self handleErrorWithCode:GULErrorCodeNetworkSessionTaskCreation
+                        queue:queue
+                  withHandler:handler];
+    return nil;
+  }
+
+  request.HTTPMethod = kGULNetworkGETRequestMethod;
+  request.allHTTPHeaderFields = headers;
+
+  GULNetworkURLSession *fetcher = [[GULNetworkURLSession alloc] initWithNetworkLoggerDelegate:self];
+  fetcher.backgroundNetworkEnabled = usingBackgroundSession;
+
+  __weak GULNetwork *weakSelf = self;
+  NSString *requestID = [fetcher
+      sessionIDFromAsyncGETRequest:request
+                 completionHandler:^(NSHTTPURLResponse *response, NSData *data, NSString *sessionID,
+                                     NSError *error) {
+                   GULNetwork *strongSelf = weakSelf;
+                   if (!strongSelf) {
+                     return;
+                   }
+                   dispatch_queue_t queueToDispatch = queue ? queue : dispatch_get_main_queue();
+                   dispatch_async(queueToDispatch, ^{
+                     if (sessionID.length) {
+                       [strongSelf->_requests removeObjectForKey:sessionID];
+                     }
+                     if (handler) {
+                       handler(response, data, error);
+                     }
+                   });
+                 }];
+
+  if (!requestID) {
+    [self handleErrorWithCode:GULErrorCodeNetworkSessionTaskCreation
+                        queue:queue
+                  withHandler:handler];
+    return nil;
+  }
+
+  [self GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                    messageCode:kGULNetworkMessageCodeNetwork001
+                        message:@"Downloading data. Host"
+                        context:url];
+  _requests[requestID] = fetcher;
+  return requestID;
+}
+
+- (BOOL)hasUploadInProgress {
+  return _requests.count > 0;
+}
+
+#pragma mark - Network Reachability
+
+/// Tells reachability delegate to call reachabilityDidChangeToStatus: to notify the network
+/// reachability has changed.
+- (void)reachability:(GULReachabilityChecker *)reachability
+       statusChanged:(GULReachabilityStatus)status {
+  _networkConnected = (status == kGULReachabilityViaCellular || status == kGULReachabilityViaWifi);
+  [_reachabilityDelegate reachabilityDidChange];
+}
+
+#pragma mark - Network logger delegate
+
+- (void)setLoggerDelegate:(id<GULNetworkLoggerDelegate>)loggerDelegate {
+  // Explicitly check whether the delegate responds to the methods because conformsToProtocol does
+  // not work correctly even though the delegate does respond to the methods.
+  if (!loggerDelegate ||
+      ![loggerDelegate
+          respondsToSelector:@selector(GULNetwork_logWithLevel:messageCode:message:contexts:)] ||
+      ![loggerDelegate
+          respondsToSelector:@selector(GULNetwork_logWithLevel:messageCode:message:context:)] ||
+      !
+      [loggerDelegate respondsToSelector:@selector(GULNetwork_logWithLevel:messageCode:message:)]) {
+    GULLogError(kGULLoggerNetwork, NO,
+                [NSString stringWithFormat:@"I-NET%06ld", (long)kGULNetworkMessageCodeNetwork002],
+                @"Cannot set the network logger delegate: delegate does not conform to the network "
+                 "logger protocol.");
+    return;
+  }
+  _loggerDelegate = loggerDelegate;
+}
+
+#pragma mark - Private methods
+
+/// Handles network error and calls completion handler with the error.
+- (void)handleErrorWithCode:(NSInteger)code
+                      queue:(dispatch_queue_t)queue
+                withHandler:(GULNetworkCompletionHandler)handler {
+  NSDictionary *userInfo = @{kGULNetworkErrorContext : @"Failed to create network request"};
+  NSError *error =
+      [[NSError alloc] initWithDomain:kGULNetworkErrorDomain code:code userInfo:userInfo];
+  [self GULNetwork_logWithLevel:kGULNetworkLogLevelWarning
+                    messageCode:kGULNetworkMessageCodeNetwork002
+                        message:@"Failed to create network request. Code, error"
+                       contexts:@[ @(code), error ]];
+  if (handler) {
+    dispatch_queue_t queueToDispatch = queue ? queue : dispatch_get_main_queue();
+    dispatch_async(queueToDispatch, ^{
+      handler(nil, nil, error);
+    });
+  }
+}
+
+#pragma mark - Network logger
+
+- (void)GULNetwork_logWithLevel:(GULNetworkLogLevel)logLevel
+                    messageCode:(GULNetworkMessageCode)messageCode
+                        message:(NSString *)message
+                       contexts:(NSArray *)contexts {
+  // Let the delegate log the message if there is a valid logger delegate. Otherwise, just log
+  // errors/warnings/info messages to the console log.
+  if (_loggerDelegate) {
+    [_loggerDelegate GULNetwork_logWithLevel:logLevel
+                                 messageCode:messageCode
+                                     message:message
+                                    contexts:contexts];
+    return;
+  }
+  if (_isDebugModeEnabled || logLevel == kGULNetworkLogLevelError ||
+      logLevel == kGULNetworkLogLevelWarning || logLevel == kGULNetworkLogLevelInfo) {
+    NSString *formattedMessage = GULStringWithLogMessage(message, logLevel, contexts);
+    NSLog(@"%@", formattedMessage);
+    GULLogBasic((GULLoggerLevel)logLevel, kGULLoggerNetwork, NO,
+                [NSString stringWithFormat:@"I-NET%06ld", (long)messageCode], formattedMessage,
+                NULL);
+  }
+}
+
+- (void)GULNetwork_logWithLevel:(GULNetworkLogLevel)logLevel
+                    messageCode:(GULNetworkMessageCode)messageCode
+                        message:(NSString *)message
+                        context:(id)context {
+  if (_loggerDelegate) {
+    [_loggerDelegate GULNetwork_logWithLevel:logLevel
+                                 messageCode:messageCode
+                                     message:message
+                                     context:context];
+    return;
+  }
+  NSArray *contexts = context ? @[ context ] : @[];
+  [self GULNetwork_logWithLevel:logLevel messageCode:messageCode message:message contexts:contexts];
+}
+
+- (void)GULNetwork_logWithLevel:(GULNetworkLogLevel)logLevel
+                    messageCode:(GULNetworkMessageCode)messageCode
+                        message:(NSString *)message {
+  if (_loggerDelegate) {
+    [_loggerDelegate GULNetwork_logWithLevel:logLevel messageCode:messageCode message:message];
+    return;
+  }
+  [self GULNetwork_logWithLevel:logLevel messageCode:messageCode message:message contexts:@[]];
+}
+
+/// Returns a string for the given log level (e.g. kGULNetworkLogLevelError -> @"ERROR").
+static NSString *GULLogLevelDescriptionFromLogLevel(GULNetworkLogLevel logLevel) {
+  static NSDictionary *levelNames = nil;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    levelNames = @{
+      @(kGULNetworkLogLevelError) : @"ERROR",
+      @(kGULNetworkLogLevelWarning) : @"WARNING",
+      @(kGULNetworkLogLevelInfo) : @"INFO",
+      @(kGULNetworkLogLevelDebug) : @"DEBUG"
+    };
+  });
+  return levelNames[@(logLevel)];
+}
+
+/// Returns a formatted string to be used for console logging.
+static NSString *GULStringWithLogMessage(NSString *message,
+                                         GULNetworkLogLevel logLevel,
+                                         NSArray *contexts) {
+  if (!message) {
+    message = @"(Message was nil)";
+  } else if (!message.length) {
+    message = @"(Message was empty)";
+  }
+  NSMutableString *result = [[NSMutableString alloc]
+      initWithFormat:@"<%@/%@> %@", kGULNetworkLogTag, GULLogLevelDescriptionFromLogLevel(logLevel),
+                     message];
+
+  if (!contexts.count) {
+    return result;
+  }
+
+  NSMutableArray *formattedContexts = [[NSMutableArray alloc] init];
+  for (id item in contexts) {
+    [formattedContexts addObject:(item != [NSNull null] ? item : @"(nil)")];
+  }
+
+  [result appendString:@": "];
+  [result appendString:[formattedContexts componentsJoinedByString:@", "]];
+  return result;
+}
+
+@end
diff --git a/GoogleUtilities/Network/GULNetworkConstants.m b/GoogleUtilities/Network/GULNetworkConstants.m
new file mode 100644
index 0000000..90bd03d
--- /dev/null
+++ b/GoogleUtilities/Network/GULNetworkConstants.m
@@ -0,0 +1,40 @@
+// Copyright 2017 Google
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#import "Private/GULNetworkConstants.h"
+
+#import <Foundation/Foundation.h>
+
+NSString *const kGULNetworkBackgroundSessionConfigIDPrefix = @"com.gul.network.background-upload";
+NSString *const kGULNetworkApplicationSupportSubdirectory = @"GUL/Network";
+NSString *const kGULNetworkTempDirectoryName = @"GULNetworkTemporaryDirectory";
+const NSTimeInterval kGULNetworkTempFolderExpireTime = 60 * 60;  // 1 hour
+const NSTimeInterval kGULNetworkTimeOutInterval = 60;            // 1 minute.
+NSString *const kGULNetworkReachabilityHost = @"app-measurement.com";
+NSString *const kGULNetworkErrorContext = @"Context";
+
+const int kGULNetworkHTTPStatusOK = 200;
+const int kGULNetworkHTTPStatusNoContent = 204;
+const int kGULNetworkHTTPStatusCodeMultipleChoices = 300;
+const int kGULNetworkHTTPStatusCodeMovedPermanently = 301;
+const int kGULNetworkHTTPStatusCodeFound = 302;
+const int kGULNetworkHTTPStatusCodeNotModified = 304;
+const int kGULNetworkHTTPStatusCodeMovedTemporarily = 307;
+const int kGULNetworkHTTPStatusCodeNotFound = 404;
+const int kGULNetworkHTTPStatusCodeCannotAcceptTraffic = 429;
+const int kGULNetworkHTTPStatusCodeUnavailable = 503;
+
+NSString *const kGULNetworkErrorDomain = @"com.gul.network.ErrorDomain";
+
+GULLoggerService kGULLoggerNetwork = @"[GULNetwork]";
diff --git a/GoogleUtilities/Network/GULNetworkURLSession.m b/GoogleUtilities/Network/GULNetworkURLSession.m
new file mode 100644
index 0000000..cb8a204
--- /dev/null
+++ b/GoogleUtilities/Network/GULNetworkURLSession.m
@@ -0,0 +1,669 @@
+// Copyright 2017 Google
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#import <Foundation/Foundation.h>
+
+#import "Private/GULNetworkURLSession.h"
+
+#import <GoogleUtilities/GULLogger.h>
+#import "Private/GULMutableDictionary.h"
+#import "Private/GULNetworkConstants.h"
+#import "Private/GULNetworkMessageCode.h"
+
+@implementation GULNetworkURLSession {
+  /// The handler to be called when the request completes or error has occurs.
+  GULNetworkURLSessionCompletionHandler _completionHandler;
+
+  /// Session ID generated randomly with a fixed prefix.
+  NSString *_sessionID;
+
+  /// The session configuration.
+  NSURLSessionConfiguration *_sessionConfig;
+
+  /// The path to the directory where all temporary files are stored before uploading.
+  NSURL *_networkDirectoryURL;
+
+  /// The downloaded data from fetching.
+  NSData *_downloadedData;
+
+  /// The path to the temporary file which stores the uploading data.
+  NSURL *_uploadingFileURL;
+
+  /// The current request.
+  NSURLRequest *_request;
+}
+
+#pragma mark - Init
+
+- (instancetype)initWithNetworkLoggerDelegate:(id<GULNetworkLoggerDelegate>)networkLoggerDelegate {
+  self = [super init];
+  if (self) {
+    // Create URL to the directory where all temporary files to upload have to be stored.
+    NSArray *paths =
+        NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, YES);
+    NSString *applicationSupportDirectory = paths.firstObject;
+    NSArray *tempPathComponents = @[
+      applicationSupportDirectory, kGULNetworkApplicationSupportSubdirectory,
+      kGULNetworkTempDirectoryName
+    ];
+    _networkDirectoryURL = [NSURL fileURLWithPathComponents:tempPathComponents];
+    _sessionID = [NSString stringWithFormat:@"%@-%@", kGULNetworkBackgroundSessionConfigIDPrefix,
+                                            [[NSUUID UUID] UUIDString]];
+    _loggerDelegate = networkLoggerDelegate;
+  }
+  return self;
+}
+
+#pragma mark - External Methods
+
+#pragma mark - To be called from AppDelegate
+
++ (void)handleEventsForBackgroundURLSessionID:(NSString *)sessionID
+                            completionHandler:
+                                (GULNetworkSystemCompletionHandler)systemCompletionHandler {
+  // The session may not be Analytics background. Ignore those that do not have the prefix.
+  if (![sessionID hasPrefix:kGULNetworkBackgroundSessionConfigIDPrefix]) {
+    return;
+  }
+  GULNetworkURLSession *fetcher = [self fetcherWithSessionIdentifier:sessionID];
+  if (fetcher != nil) {
+    [fetcher addSystemCompletionHandler:systemCompletionHandler forSession:sessionID];
+  } else {
+    GULLogError(kGULLoggerNetwork, NO,
+                [NSString stringWithFormat:@"I-NET%06ld", (long)kGULNetworkMessageCodeNetwork003],
+                @"Failed to retrieve background session with ID %@ after app is relaunched.",
+                sessionID);
+  }
+}
+
+#pragma mark - External Methods
+
+/// Sends an async POST request using NSURLSession for iOS >= 7.0, and returns an ID of the
+/// connection.
+- (NSString *)sessionIDFromAsyncPOSTRequest:(NSURLRequest *)request
+                          completionHandler:(GULNetworkURLSessionCompletionHandler)handler {
+  // NSURLSessionUploadTask does not work with NSData in the background.
+  // To avoid this issue, write the data to a temporary file to upload it.
+  // Make a temporary file with the data subset.
+  _uploadingFileURL = [self temporaryFilePathWithSessionID:_sessionID];
+  NSError *writeError;
+  NSURLSessionUploadTask *postRequestTask;
+  NSURLSession *session;
+  BOOL didWriteFile = NO;
+
+  // Clean up the entire temp folder to avoid temp files that remain in case the previous session
+  // crashed and did not clean up.
+  [self maybeRemoveTempFilesAtURL:_networkDirectoryURL
+                     expiringTime:kGULNetworkTempFolderExpireTime];
+
+  // If there is no background network enabled, no need to write to file. This will allow default
+  // network session which runs on the foreground.
+  if (_backgroundNetworkEnabled && [self ensureTemporaryDirectoryExists]) {
+    didWriteFile = [request.HTTPBody writeToFile:_uploadingFileURL.path
+                                         options:NSDataWritingAtomic
+                                           error:&writeError];
+
+    if (writeError) {
+      [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                                   messageCode:kGULNetworkMessageCodeURLSession000
+                                       message:@"Failed to write request data to file"
+                                       context:writeError];
+    }
+  }
+
+  if (didWriteFile) {
+    // Exclude this file from backing up to iTunes. There are conflicting reports that excluding
+    // directory from backing up does not excluding files of that directory from backing up.
+    [self excludeFromBackupForURL:_uploadingFileURL];
+
+    _sessionConfig = [self backgroundSessionConfigWithSessionID:_sessionID];
+    [self populateSessionConfig:_sessionConfig withRequest:request];
+    session = [NSURLSession sessionWithConfiguration:_sessionConfig
+                                            delegate:self
+                                       delegateQueue:[NSOperationQueue mainQueue]];
+    postRequestTask = [session uploadTaskWithRequest:request fromFile:_uploadingFileURL];
+  } else {
+    // If we cannot write to file, just send it in the foreground.
+    _sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
+    [self populateSessionConfig:_sessionConfig withRequest:request];
+    _sessionConfig.URLCache = nil;
+    session = [NSURLSession sessionWithConfiguration:_sessionConfig
+                                            delegate:self
+                                       delegateQueue:[NSOperationQueue mainQueue]];
+    postRequestTask = [session uploadTaskWithRequest:request fromData:request.HTTPBody];
+  }
+
+  if (!session || !postRequestTask) {
+    NSError *error = [[NSError alloc]
+        initWithDomain:kGULNetworkErrorDomain
+                  code:GULErrorCodeNetworkRequestCreation
+              userInfo:@{kGULNetworkErrorContext : @"Cannot create network session"}];
+    [self callCompletionHandler:handler withResponse:nil data:nil error:error];
+    return nil;
+  }
+
+  // Save the session into memory.
+  NSMapTable *sessionIdentifierToFetcherMap = [[self class] sessionIDToFetcherMap];
+  [sessionIdentifierToFetcherMap setObject:self forKey:_sessionID];
+
+  _request = [request copy];
+
+  // Store completion handler because background session does not accept handler block but custom
+  // delegate.
+  _completionHandler = [handler copy];
+  [postRequestTask resume];
+
+  return _sessionID;
+}
+
+/// Sends an async GET request using NSURLSession for iOS >= 7.0, and returns an ID of the session.
+- (NSString *)sessionIDFromAsyncGETRequest:(NSURLRequest *)request
+                         completionHandler:(GULNetworkURLSessionCompletionHandler)handler {
+  if (_backgroundNetworkEnabled) {
+    _sessionConfig = [self backgroundSessionConfigWithSessionID:_sessionID];
+  } else {
+    _sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
+  }
+
+  [self populateSessionConfig:_sessionConfig withRequest:request];
+
+  // Do not cache the GET request.
+  _sessionConfig.URLCache = nil;
+
+  NSURLSession *session = [NSURLSession sessionWithConfiguration:_sessionConfig
+                                                        delegate:self
+                                                   delegateQueue:[NSOperationQueue mainQueue]];
+  NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithRequest:request];
+
+  if (!session || !downloadTask) {
+    NSError *error = [[NSError alloc]
+        initWithDomain:kGULNetworkErrorDomain
+                  code:GULErrorCodeNetworkRequestCreation
+              userInfo:@{kGULNetworkErrorContext : @"Cannot create network session"}];
+    [self callCompletionHandler:handler withResponse:nil data:nil error:error];
+    return nil;
+  }
+
+  // Save the session into memory.
+  NSMapTable *sessionIdentifierToFetcherMap = [[self class] sessionIDToFetcherMap];
+  [sessionIdentifierToFetcherMap setObject:self forKey:_sessionID];
+
+  _request = [request copy];
+
+  _completionHandler = [handler copy];
+  [downloadTask resume];
+
+  return _sessionID;
+}
+
+#pragma mark - NSURLSessionTaskDelegate
+
+/// Called by the NSURLSession once the download task is completed. The file is saved in the
+/// provided URL so we need to read the data and store into _downloadedData. Once the session is
+/// completed, URLSession:task:didCompleteWithError will be called and the completion handler will
+/// be called with the downloaded data.
+- (void)URLSession:(NSURLSession *)session
+                 downloadTask:(NSURLSessionDownloadTask *)task
+    didFinishDownloadingToURL:(NSURL *)url {
+  if (!url.path) {
+    [_loggerDelegate
+        GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                    messageCode:kGULNetworkMessageCodeURLSession001
+                        message:@"Unable to read downloaded data from empty temp path"];
+    _downloadedData = nil;
+    return;
+  }
+
+  NSError *error;
+  _downloadedData = [NSData dataWithContentsOfFile:url.path options:0 error:&error];
+
+  if (error) {
+    [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                                 messageCode:kGULNetworkMessageCodeURLSession002
+                                     message:@"Cannot read the content of downloaded data"
+                                     context:error];
+    _downloadedData = nil;
+  }
+}
+
+#if TARGET_OS_IOS || TARGET_OS_TV
+- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
+  [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                               messageCode:kGULNetworkMessageCodeURLSession003
+                                   message:@"Background session finished"
+                                   context:session.configuration.identifier];
+  [self callSystemCompletionHandler:session.configuration.identifier];
+}
+#endif
+
+- (void)URLSession:(NSURLSession *)session
+                    task:(NSURLSessionTask *)task
+    didCompleteWithError:(NSError *)error {
+  // Avoid any chance of recursive behavior leading to it being used repeatedly.
+  GULNetworkURLSessionCompletionHandler handler = _completionHandler;
+  _completionHandler = nil;
+
+  if (task.response) {
+    // The following assertion should always be true for HTTP requests, see https://goo.gl/gVLxT7.
+    NSAssert([task.response isKindOfClass:[NSHTTPURLResponse class]], @"URL response must be HTTP");
+
+    // The server responded so ignore the error created by the system.
+    error = nil;
+  } else if (!error) {
+    error = [[NSError alloc]
+        initWithDomain:kGULNetworkErrorDomain
+                  code:GULErrorCodeNetworkInvalidResponse
+              userInfo:@{kGULNetworkErrorContext : @"Network Error: Empty network response"}];
+  }
+
+  [self callCompletionHandler:handler
+                 withResponse:(NSHTTPURLResponse *)task.response
+                         data:_downloadedData
+                        error:error];
+
+  // Remove the temp file to avoid trashing devices with lots of temp files.
+  [self removeTempItemAtURL:_uploadingFileURL];
+
+  // Try to clean up stale files again.
+  [self maybeRemoveTempFilesAtURL:_networkDirectoryURL
+                     expiringTime:kGULNetworkTempFolderExpireTime];
+}
+
+- (void)URLSession:(NSURLSession *)session
+                   task:(NSURLSessionTask *)task
+    didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
+      completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition,
+                                  NSURLCredential *credential))completionHandler {
+  // The handling is modeled after GTMSessionFetcher.
+  if ([challenge.protectionSpace.authenticationMethod
+          isEqualToString:NSURLAuthenticationMethodServerTrust]) {
+    SecTrustRef serverTrust = challenge.protectionSpace.serverTrust;
+    if (serverTrust == NULL) {
+      [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                                   messageCode:kGULNetworkMessageCodeURLSession004
+                                       message:@"Received empty server trust for host. Host"
+                                       context:_request.URL];
+      completionHandler(NSURLSessionAuthChallengePerformDefaultHandling, nil);
+      return;
+    }
+    NSURLCredential *credential = [NSURLCredential credentialForTrust:serverTrust];
+    if (!credential) {
+      [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelWarning
+                                   messageCode:kGULNetworkMessageCodeURLSession005
+                                       message:@"Unable to verify server identity. Host"
+                                       context:_request.URL];
+      completionHandler(NSURLSessionAuthChallengeCancelAuthenticationChallenge, nil);
+      return;
+    }
+
+    [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                                 messageCode:kGULNetworkMessageCodeURLSession006
+                                     message:@"Received SSL challenge for host. Host"
+                                     context:_request.URL];
+
+    void (^callback)(BOOL) = ^(BOOL allow) {
+      if (allow) {
+        completionHandler(NSURLSessionAuthChallengeUseCredential, credential);
+      } else {
+        [self->_loggerDelegate
+            GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                        messageCode:kGULNetworkMessageCodeURLSession007
+                            message:@"Cancelling authentication challenge for host. Host"
+                            context:self->_request.URL];
+        completionHandler(NSURLSessionAuthChallengeCancelAuthenticationChallenge, nil);
+      }
+    };
+
+    // Retain the trust object to avoid a SecTrustEvaluate() crash on iOS 7.
+    CFRetain(serverTrust);
+
+    // Evaluate the certificate chain.
+    //
+    // The delegate queue may be the main thread. Trust evaluation could cause some
+    // blocking network activity, so we must evaluate async, as documented at
+    // https://developer.apple.com/library/ios/technotes/tn2232/
+    dispatch_queue_t evaluateBackgroundQueue =
+        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
+
+    dispatch_async(evaluateBackgroundQueue, ^{
+      SecTrustResultType trustEval = kSecTrustResultInvalid;
+      BOOL shouldAllow;
+      OSStatus trustError;
+
+      @synchronized([GULNetworkURLSession class]) {
+        trustError = SecTrustEvaluate(serverTrust, &trustEval);
+      }
+
+      if (trustError != errSecSuccess) {
+        [self->_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                                           messageCode:kGULNetworkMessageCodeURLSession008
+                                               message:@"Cannot evaluate server trust. Error, host"
+                                              contexts:@[ @(trustError), self->_request.URL ]];
+        shouldAllow = NO;
+      } else {
+        // Having a trust level "unspecified" by the user is the usual result, described at
+        // https://developer.apple.com/library/mac/qa/qa1360
+        shouldAllow =
+            (trustEval == kSecTrustResultUnspecified || trustEval == kSecTrustResultProceed);
+      }
+
+      // Call the call back with the permission.
+      callback(shouldAllow);
+
+      CFRelease(serverTrust);
+    });
+    return;
+  }
+
+  // Default handling for other Auth Challenges.
+  completionHandler(NSURLSessionAuthChallengePerformDefaultHandling, nil);
+}
+
+#pragma mark - Internal Methods
+
+/// Stores system completion handler with session ID as key.
+- (void)addSystemCompletionHandler:(GULNetworkSystemCompletionHandler)handler
+                        forSession:(NSString *)identifier {
+  if (!handler) {
+    [_loggerDelegate
+        GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                    messageCode:kGULNetworkMessageCodeURLSession009
+                        message:@"Cannot store nil system completion handler in network"];
+    return;
+  }
+
+  if (!identifier.length) {
+    [_loggerDelegate
+        GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                    messageCode:kGULNetworkMessageCodeURLSession010
+                        message:
+                            @"Cannot store system completion handler with empty network "
+                             "session identifier"];
+    return;
+  }
+
+  GULMutableDictionary *systemCompletionHandlers =
+      [[self class] sessionIDToSystemCompletionHandlerDictionary];
+  if (systemCompletionHandlers[identifier]) {
+    [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelWarning
+                                 messageCode:kGULNetworkMessageCodeURLSession011
+                                     message:@"Got multiple system handlers for a single session ID"
+                                     context:identifier];
+  }
+
+  systemCompletionHandlers[identifier] = handler;
+}
+
+/// Calls the system provided completion handler with the session ID stored in the dictionary.
+/// The handler will be removed from the dictionary after being called.
+- (void)callSystemCompletionHandler:(NSString *)identifier {
+  GULMutableDictionary *systemCompletionHandlers =
+      [[self class] sessionIDToSystemCompletionHandlerDictionary];
+  GULNetworkSystemCompletionHandler handler = [systemCompletionHandlers objectForKey:identifier];
+
+  if (handler) {
+    [systemCompletionHandlers removeObjectForKey:identifier];
+
+    dispatch_async(dispatch_get_main_queue(), ^{
+      handler();
+    });
+  }
+}
+
+/// Sets or updates the session ID of this session.
+- (void)setSessionID:(NSString *)sessionID {
+  _sessionID = [sessionID copy];
+}
+
+/// Creates a background session configuration with the session ID using the supported method.
+- (NSURLSessionConfiguration *)backgroundSessionConfigWithSessionID:(NSString *)sessionID {
+#if (TARGET_OS_OSX && defined(MAC_OS_X_VERSION_10_10) &&         \
+     MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_10) || \
+    TARGET_OS_TV ||                                              \
+    (TARGET_OS_IOS && defined(__IPHONE_8_0) && __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_8_0)
+
+  // iOS 8/10.10 builds require the new backgroundSessionConfiguration method name.
+  return [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:sessionID];
+
+#elif (TARGET_OS_OSX && defined(MAC_OS_X_VERSION_10_10) &&        \
+       MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_10) || \
+    (TARGET_OS_IOS && defined(__IPHONE_8_0) && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_8_0)
+
+  // Do a runtime check to avoid a deprecation warning about using
+  // +backgroundSessionConfiguration: on iOS 8.
+  if ([NSURLSessionConfiguration
+          respondsToSelector:@selector(backgroundSessionConfigurationWithIdentifier:)]) {
+    // Running on iOS 8+/OS X 10.10+.
+    return [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:sessionID];
+  } else {
+    // Running on iOS 7/OS X 10.9.
+    return [NSURLSessionConfiguration backgroundSessionConfiguration:sessionID];
+  }
+
+#else
+  // Building with an SDK earlier than iOS 8/OS X 10.10.
+  return [NSURLSessionConfiguration backgroundSessionConfiguration:sessionID];
+#endif
+}
+
+- (void)maybeRemoveTempFilesAtURL:(NSURL *)folderURL expiringTime:(NSTimeInterval)staleTime {
+  if (!folderURL.absoluteString.length) {
+    return;
+  }
+
+  NSFileManager *fileManager = [NSFileManager defaultManager];
+  NSError *error = nil;
+
+  NSArray *properties = @[ NSURLCreationDateKey ];
+  NSArray *directoryContent =
+      [fileManager contentsOfDirectoryAtURL:folderURL
+                 includingPropertiesForKeys:properties
+                                    options:NSDirectoryEnumerationSkipsSubdirectoryDescendants
+                                      error:&error];
+  if (error && error.code != NSFileReadNoSuchFileError) {
+    [_loggerDelegate
+        GULNetwork_logWithLevel:kGULNetworkLogLevelDebug
+                    messageCode:kGULNetworkMessageCodeURLSession012
+                        message:@"Cannot get files from the temporary network folder. Error"
+                        context:error];
+    return;
+  }
+
+  if (!directoryContent.count) {
+    return;
+  }
+
+  NSTimeInterval now = [NSDate date].timeIntervalSince1970;
+  for (NSURL *tempFile in directoryContent) {
+    NSDate *creationDate;
+    BOOL getCreationDate =
+        [tempFile getResourceValue:&creationDate forKey:NSURLCreationDateKey error:NULL];
+    if (!getCreationDate) {
+      continue;
+    }
+    NSTimeInterval creationTimeInterval = creationDate.timeIntervalSince1970;
+    if (fabs(now - creationTimeInterval) > staleTime) {
+      [self removeTempItemAtURL:tempFile];
+    }
+  }
+}
+
+/// Removes the temporary file written to disk for sending the request. It has to be cleaned up
+/// after the session is done.
+- (void)removeTempItemAtURL:(NSURL *)fileURL {
+  if (!fileURL.absoluteString.length) {
+    return;
+  }
+
+  NSFileManager *fileManager = [NSFileManager defaultManager];
+  NSError *error = nil;
+
+  if (![fileManager removeItemAtURL:fileURL error:&error] && error.code != NSFileNoSuchFileError) {
+    [_loggerDelegate
+        GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                    messageCode:kGULNetworkMessageCodeURLSession013
+                        message:@"Failed to remove temporary uploading data file. Error"
+                        context:error.localizedDescription];
+  }
+}
+
+/// Gets the fetcher with the session ID.
++ (instancetype)fetcherWithSessionIdentifier:(NSString *)sessionIdentifier {
+  NSMapTable *sessionIdentifierToFetcherMap = [self sessionIDToFetcherMap];
+  GULNetworkURLSession *session = [sessionIdentifierToFetcherMap objectForKey:sessionIdentifier];
+  if (!session && [sessionIdentifier hasPrefix:kGULNetworkBackgroundSessionConfigIDPrefix]) {
+    session = [[GULNetworkURLSession alloc] initWithNetworkLoggerDelegate:nil];
+    [session setSessionID:sessionIdentifier];
+    [sessionIdentifierToFetcherMap setObject:session forKey:sessionIdentifier];
+  }
+  return session;
+}
+
+/// Returns a map of the fetcher by session ID. Creates a map if it is not created.
++ (NSMapTable *)sessionIDToFetcherMap {
+  static NSMapTable *sessionIDToFetcherMap;
+
+  static dispatch_once_t sessionMapOnceToken;
+  dispatch_once(&sessionMapOnceToken, ^{
+    sessionIDToFetcherMap = [NSMapTable strongToWeakObjectsMapTable];
+  });
+  return sessionIDToFetcherMap;
+}
+
+/// Returns a map of system provided completion handler by session ID. Creates a map if it is not
+/// created.
++ (GULMutableDictionary *)sessionIDToSystemCompletionHandlerDictionary {
+  static GULMutableDictionary *systemCompletionHandlers;
+
+  static dispatch_once_t systemCompletionHandlerOnceToken;
+  dispatch_once(&systemCompletionHandlerOnceToken, ^{
+    systemCompletionHandlers = [[GULMutableDictionary alloc] init];
+  });
+  return systemCompletionHandlers;
+}
+
+- (NSURL *)temporaryFilePathWithSessionID:(NSString *)sessionID {
+  NSString *tempName = [NSString stringWithFormat:@"GULUpload_temp_%@", sessionID];
+  return [_networkDirectoryURL URLByAppendingPathComponent:tempName];
+}
+
+/// Makes sure that the directory to store temp files exists. If not, tries to create it and returns
+/// YES. If there is anything wrong, returns NO.
+- (BOOL)ensureTemporaryDirectoryExists {
+  NSFileManager *fileManager = [NSFileManager defaultManager];
+  NSError *error = nil;
+
+  // Create a temporary directory if it does not exist or was deleted.
+  if ([_networkDirectoryURL checkResourceIsReachableAndReturnError:&error]) {
+    return YES;
+  }
+
+  if (error && error.code != NSFileReadNoSuchFileError) {
+    [_loggerDelegate
+        GULNetwork_logWithLevel:kGULNetworkLogLevelWarning
+                    messageCode:kGULNetworkMessageCodeURLSession014
+                        message:@"Error while trying to access Network temp folder. Error"
+                        context:error];
+  }
+
+  NSError *writeError = nil;
+
+  [fileManager createDirectoryAtURL:_networkDirectoryURL
+        withIntermediateDirectories:YES
+                         attributes:nil
+                              error:&writeError];
+  if (writeError) {
+    [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                                 messageCode:kGULNetworkMessageCodeURLSession015
+                                     message:@"Cannot create temporary directory. Error"
+                                     context:writeError];
+    return NO;
+  }
+
+  // Set the iCloud exclusion attribute on the Documents URL.
+  [self excludeFromBackupForURL:_networkDirectoryURL];
+
+  return YES;
+}
+
+- (void)excludeFromBackupForURL:(NSURL *)url {
+  if (!url.path) {
+    return;
+  }
+
+  // Set the iCloud exclusion attribute on the Documents URL.
+  NSError *preventBackupError = nil;
+  [url setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:&preventBackupError];
+  if (preventBackupError) {
+    [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                                 messageCode:kGULNetworkMessageCodeURLSession016
+                                     message:@"Cannot exclude temporary folder from iTunes backup"];
+  }
+}
+
+- (void)URLSession:(NSURLSession *)session
+                          task:(NSURLSessionTask *)task
+    willPerformHTTPRedirection:(NSHTTPURLResponse *)response
+                    newRequest:(NSURLRequest *)request
+             completionHandler:(void (^)(NSURLRequest *))completionHandler {
+  NSArray *nonAllowedRedirectionCodes = @[
+    @(kGULNetworkHTTPStatusCodeFound), @(kGULNetworkHTTPStatusCodeMovedPermanently),
+    @(kGULNetworkHTTPStatusCodeMovedTemporarily), @(kGULNetworkHTTPStatusCodeMultipleChoices)
+  ];
+
+  // Allow those not in the non allowed list to be followed.
+  if (![nonAllowedRedirectionCodes containsObject:@(response.statusCode)]) {
+    completionHandler(request);
+    return;
+  }
+
+  // Do not allow redirection if the response code is in the non-allowed list.
+  NSURLRequest *newRequest = request;
+
+  if (response) {
+    newRequest = nil;
+  }
+
+  completionHandler(newRequest);
+}
+
+#pragma mark - Helper Methods
+
+- (void)callCompletionHandler:(GULNetworkURLSessionCompletionHandler)handler
+                 withResponse:(NSHTTPURLResponse *)response
+                         data:(NSData *)data
+                        error:(NSError *)error {
+  if (error) {
+    [_loggerDelegate GULNetwork_logWithLevel:kGULNetworkLogLevelError
+                                 messageCode:kGULNetworkMessageCodeURLSession017
+                                     message:@"Encounter network error. Code, error"
+                                    contexts:@[ @(error.code), error ]];
+  }
+
+  if (handler) {
+    dispatch_async(dispatch_get_main_queue(), ^{
+      handler(response, data, self->_sessionID, error);
+    });
+  }
+}
+
+- (void)populateSessionConfig:(NSURLSessionConfiguration *)sessionConfig
+                  withRequest:(NSURLRequest *)request {
+  sessionConfig.HTTPAdditionalHeaders = request.allHTTPHeaderFields;
+  sessionConfig.timeoutIntervalForRequest = request.timeoutInterval;
+  sessionConfig.timeoutIntervalForResource = request.timeoutInterval;
+  sessionConfig.requestCachePolicy = request.cachePolicy;
+}
+
+@end
diff --git a/GoogleUtilities/Network/Private/GULMutableDictionary.h b/GoogleUtilities/Network/Private/GULMutableDictionary.h
new file mode 100644
index 0000000..a8cc45b
--- /dev/null
+++ b/GoogleUtilities/Network/Private/GULMutableDictionary.h
@@ -0,0 +1,46 @@
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+/// A mutable dictionary that provides atomic accessor and mutators.
+@interface GULMutableDictionary : NSObject
+
+/// Returns an object given a key in the dictionary or nil if not found.
+- (id)objectForKey:(id)key;
+
+/// Updates the object given its key or adds it to the dictionary if it is not in the dictionary.
+- (void)setObject:(id)object forKey:(id<NSCopying>)key;
+
+/// Removes the object given its session ID from the dictionary.
+- (void)removeObjectForKey:(id)key;
+
+/// Removes all objects.
+- (void)removeAllObjects;
+
+/// Returns the number of current objects in the dictionary.
+- (NSUInteger)count;
+
+/// Returns an object given a key in the dictionary or nil if not found.
+- (id)objectForKeyedSubscript:(id<NSCopying>)key;
+
+/// Updates the object given its key or adds it to the dictionary if it is not in the dictionary.
+- (void)setObject:(id)obj forKeyedSubscript:(id<NSCopying>)key;
+
+/// Returns the immutable dictionary.
+- (NSDictionary *)dictionary;
+
+@end
diff --git a/GoogleUtilities/Network/Private/GULNetwork.h b/GoogleUtilities/Network/Private/GULNetwork.h
new file mode 100644
index 0000000..0e75ae5
--- /dev/null
+++ b/GoogleUtilities/Network/Private/GULNetwork.h
@@ -0,0 +1,87 @@
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import "GULNetworkConstants.h"
+#import "GULNetworkLoggerProtocol.h"
+#import "GULNetworkURLSession.h"
+
+/// Delegate protocol for GULNetwork events.
+@protocol GULNetworkReachabilityDelegate
+
+/// Tells the delegate to handle events when the network reachability changes to connected or not
+/// connected.
+- (void)reachabilityDidChange;
+
+@end
+
+/// The Network component that provides network status and handles network requests and responses.
+/// This is not thread safe.
+///
+/// NOTE:
+/// User must add FIRAnalytics handleEventsForBackgroundURLSessionID:completionHandler to the
+/// AppDelegate application:handleEventsForBackgroundURLSession:completionHandler:
+@interface GULNetwork : NSObject
+
+/// Indicates if network connectivity is available.
+@property(nonatomic, readonly, getter=isNetworkConnected) BOOL networkConnected;
+
+/// Indicates if there are any uploads in progress.
+@property(nonatomic, readonly, getter=hasUploadInProgress) BOOL uploadInProgress;
+
+/// An optional delegate that can be used in the event when network reachability changes.
+@property(nonatomic, weak) id<GULNetworkReachabilityDelegate> reachabilityDelegate;
+
+/// An optional delegate that can be used to log messages, warnings or errors that occur in the
+/// network operations.
+@property(nonatomic, weak) id<GULNetworkLoggerDelegate> loggerDelegate;
+
+/// Indicates whether the logger should display debug messages.
+@property(nonatomic, assign) BOOL isDebugModeEnabled;
+
+/// The time interval in seconds for the network request to timeout.
+@property(nonatomic, assign) NSTimeInterval timeoutInterval;
+
+/// Initializes with the default reachability host.
+- (instancetype)init;
+
+/// Initializes with a custom reachability host.
+- (instancetype)initWithReachabilityHost:(NSString *)reachabilityHost;
+
+/// Handles events when background session with the given ID has finished.
++ (void)handleEventsForBackgroundURLSessionID:(NSString *)sessionID
+                            completionHandler:(GULNetworkSystemCompletionHandler)completionHandler;
+
+/// Compresses and sends a POST request with the provided data to the URL. The session will be
+/// background session if usingBackgroundSession is YES. Otherwise, the POST session is default
+/// session. Returns a session ID or nil if an error occurs.
+- (NSString *)postURL:(NSURL *)url
+                   payload:(NSData *)payload
+                     queue:(dispatch_queue_t)queue
+    usingBackgroundSession:(BOOL)usingBackgroundSession
+         completionHandler:(GULNetworkCompletionHandler)handler;
+
+/// Sends a GET request with the provided data to the URL. The session will be background session
+/// if usingBackgroundSession is YES. Otherwise, the GET session is default session. Returns a
+/// session ID or nil if an error occurs.
+- (NSString *)getURL:(NSURL *)url
+                   headers:(NSDictionary *)headers
+                     queue:(dispatch_queue_t)queue
+    usingBackgroundSession:(BOOL)usingBackgroundSession
+         completionHandler:(GULNetworkCompletionHandler)handler;
+
+@end
diff --git a/GoogleUtilities/Network/Private/GULNetworkConstants.h b/GoogleUtilities/Network/Private/GULNetworkConstants.h
new file mode 100644
index 0000000..44d440b
--- /dev/null
+++ b/GoogleUtilities/Network/Private/GULNetworkConstants.h
@@ -0,0 +1,79 @@
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+#import <GoogleUtilities/GULLogger.h>
+
+/// Error codes in Firebase Network error domain.
+/// Note: these error codes should never change. It would make it harder to decode the errors if
+/// we inadvertently altered any of these codes in a future SDK version.
+typedef NS_ENUM(NSInteger, GULNetworkErrorCode) {
+  /// Unknown error.
+  GULNetworkErrorCodeUnknown = 0,
+  /// Error occurs when the request URL is invalid.
+  GULErrorCodeNetworkInvalidURL = 1,
+  /// Error occurs when request cannot be constructed.
+  GULErrorCodeNetworkRequestCreation = 2,
+  /// Error occurs when payload cannot be compressed.
+  GULErrorCodeNetworkPayloadCompression = 3,
+  /// Error occurs when session task cannot be created.
+  GULErrorCodeNetworkSessionTaskCreation = 4,
+  /// Error occurs when there is no response.
+  GULErrorCodeNetworkInvalidResponse = 5
+};
+
+#pragma mark - Network constants
+
+/// The prefix of the ID of the background session.
+extern NSString *const kGULNetworkBackgroundSessionConfigIDPrefix;
+
+/// The sub directory to store the files of data that is being uploaded in the background.
+extern NSString *const kGULNetworkApplicationSupportSubdirectory;
+
+/// Name of the temporary directory that stores files for background uploading.
+extern NSString *const kGULNetworkTempDirectoryName;
+
+/// The period when the temporary uploading file can stay.
+extern const NSTimeInterval kGULNetworkTempFolderExpireTime;
+
+/// The default network request timeout interval.
+extern const NSTimeInterval kGULNetworkTimeOutInterval;
+
+/// The host to check the reachability of the network.
+extern NSString *const kGULNetworkReachabilityHost;
+
+/// The key to get the error context of the UserInfo.
+extern NSString *const kGULNetworkErrorContext;
+
+#pragma mark - Network Status Code
+
+extern const int kGULNetworkHTTPStatusOK;
+extern const int kGULNetworkHTTPStatusNoContent;
+extern const int kGULNetworkHTTPStatusCodeMultipleChoices;
+extern const int kGULNetworkHTTPStatusCodeMovedPermanently;
+extern const int kGULNetworkHTTPStatusCodeFound;
+extern const int kGULNetworkHTTPStatusCodeNotModified;
+extern const int kGULNetworkHTTPStatusCodeMovedTemporarily;
+extern const int kGULNetworkHTTPStatusCodeNotFound;
+extern const int kGULNetworkHTTPStatusCodeCannotAcceptTraffic;
+extern const int kGULNetworkHTTPStatusCodeUnavailable;
+
+#pragma mark - Error Domain
+
+extern NSString *const kGULNetworkErrorDomain;
+
+/// The logger service for GULNetwork.
+extern GULLoggerService kGULLoggerNetwork;
diff --git a/GoogleUtilities/Network/Private/GULNetworkLoggerProtocol.h b/GoogleUtilities/Network/Private/GULNetworkLoggerProtocol.h
new file mode 100644
index 0000000..f1be590
--- /dev/null
+++ b/GoogleUtilities/Network/Private/GULNetworkLoggerProtocol.h
@@ -0,0 +1,51 @@
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import <GoogleUtilities/GULLoggerLevel.h>
+
+#import "GULNetworkMessageCode.h"
+
+/// The log levels used by GULNetworkLogger.
+typedef NS_ENUM(NSInteger, GULNetworkLogLevel) {
+  kGULNetworkLogLevelError = GULLoggerLevelError,
+  kGULNetworkLogLevelWarning = GULLoggerLevelWarning,
+  kGULNetworkLogLevelInfo = GULLoggerLevelInfo,
+  kGULNetworkLogLevelDebug = GULLoggerLevelDebug,
+};
+
+@protocol GULNetworkLoggerDelegate <NSObject>
+
+@required
+/// Tells the delegate to log a message with an array of contexts and the log level.
+- (void)GULNetwork_logWithLevel:(GULNetworkLogLevel)logLevel
+                    messageCode:(GULNetworkMessageCode)messageCode
+                        message:(NSString *)message
+                       contexts:(NSArray *)contexts;
+
+/// Tells the delegate to log a message with a context and the log level.
+- (void)GULNetwork_logWithLevel:(GULNetworkLogLevel)logLevel
+                    messageCode:(GULNetworkMessageCode)messageCode
+                        message:(NSString *)message
+                        context:(id)context;
+
+/// Tells the delegate to log a message with the log level.
+- (void)GULNetwork_logWithLevel:(GULNetworkLogLevel)logLevel
+                    messageCode:(GULNetworkMessageCode)messageCode
+                        message:(NSString *)message;
+
+@end
diff --git a/GoogleUtilities/Network/Private/GULNetworkMessageCode.h b/GoogleUtilities/Network/Private/GULNetworkMessageCode.h
new file mode 100644
index 0000000..ce78e60
--- /dev/null
+++ b/GoogleUtilities/Network/Private/GULNetworkMessageCode.h
@@ -0,0 +1,44 @@
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Make sure these codes do not overlap with any contained in the FIRAMessageCode enum.
+typedef NS_ENUM(NSInteger, GULNetworkMessageCode) {
+  // GULNetwork.m
+  kGULNetworkMessageCodeNetwork000 = 900000,  // I-NET900000
+  kGULNetworkMessageCodeNetwork001 = 900001,  // I-NET900001
+  kGULNetworkMessageCodeNetwork002 = 900002,  // I-NET900002
+  kGULNetworkMessageCodeNetwork003 = 900003,  // I-NET900003
+  // GULNetworkURLSession.m
+  kGULNetworkMessageCodeURLSession000 = 901000,  // I-NET901000
+  kGULNetworkMessageCodeURLSession001 = 901001,  // I-NET901001
+  kGULNetworkMessageCodeURLSession002 = 901002,  // I-NET901002
+  kGULNetworkMessageCodeURLSession003 = 901003,  // I-NET901003
+  kGULNetworkMessageCodeURLSession004 = 901004,  // I-NET901004
+  kGULNetworkMessageCodeURLSession005 = 901005,  // I-NET901005
+  kGULNetworkMessageCodeURLSession006 = 901006,  // I-NET901006
+  kGULNetworkMessageCodeURLSession007 = 901007,  // I-NET901007
+  kGULNetworkMessageCodeURLSession008 = 901008,  // I-NET901008
+  kGULNetworkMessageCodeURLSession009 = 901009,  // I-NET901009
+  kGULNetworkMessageCodeURLSession010 = 901010,  // I-NET901010
+  kGULNetworkMessageCodeURLSession011 = 901011,  // I-NET901011
+  kGULNetworkMessageCodeURLSession012 = 901012,  // I-NET901012
+  kGULNetworkMessageCodeURLSession013 = 901013,  // I-NET901013
+  kGULNetworkMessageCodeURLSession014 = 901014,  // I-NET901014
+  kGULNetworkMessageCodeURLSession015 = 901015,  // I-NET901015
+  kGULNetworkMessageCodeURLSession016 = 901016,  // I-NET901016
+  kGULNetworkMessageCodeURLSession017 = 901017,  // I-NET901017
+  kGULNetworkMessageCodeURLSession018 = 901018,  // I-NET901018
+};
diff --git a/GoogleUtilities/Network/Private/GULNetworkURLSession.h b/GoogleUtilities/Network/Private/GULNetworkURLSession.h
new file mode 100644
index 0000000..81190c6
--- /dev/null
+++ b/GoogleUtilities/Network/Private/GULNetworkURLSession.h
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import "GULNetworkLoggerProtocol.h"
+
+typedef void (^GULNetworkCompletionHandler)(NSHTTPURLResponse *response,
+                                            NSData *data,
+                                            NSError *error);
+typedef void (^GULNetworkURLSessionCompletionHandler)(NSHTTPURLResponse *response,
+                                                      NSData *data,
+                                                      NSString *sessionID,
+                                                      NSError *error);
+typedef void (^GULNetworkSystemCompletionHandler)(void);
+
+/// The protocol that uses NSURLSession for iOS >= 7.0 to handle requests and responses.
+@interface GULNetworkURLSession
+    : NSObject <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDownloadDelegate>
+
+/// Indicates whether the background network is enabled. Default value is NO.
+@property(nonatomic, getter=isBackgroundNetworkEnabled) BOOL backgroundNetworkEnabled;
+
+/// The logger delegate to log message, errors or warnings that occur during the network operations.
+@property(nonatomic, weak) id<GULNetworkLoggerDelegate> loggerDelegate;
+
+/// Calls the system provided completion handler after the background session is finished.
++ (void)handleEventsForBackgroundURLSessionID:(NSString *)sessionID
+                            completionHandler:(GULNetworkSystemCompletionHandler)completionHandler;
+
+/// Initializes with logger delegate.
+- (instancetype)initWithNetworkLoggerDelegate:(id<GULNetworkLoggerDelegate>)networkLoggerDelegate
+    NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+/// Sends an asynchronous POST request and calls the provided completion handler when the request
+/// completes or when errors occur, and returns an ID of the session/connection.
+- (NSString *)sessionIDFromAsyncPOSTRequest:(NSURLRequest *)request
+                          completionHandler:(GULNetworkURLSessionCompletionHandler)handler;
+
+/// Sends an asynchronous GET request and calls the provided completion handler when the request
+/// completes or when errors occur, and returns an ID of the session.
+- (NSString *)sessionIDFromAsyncGETRequest:(NSURLRequest *)request
+                         completionHandler:(GULNetworkURLSessionCompletionHandler)handler;
+
+@end