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authorGravatar Muxi Yan <muxi@users.noreply.github.com>2018-02-28 16:53:41 -0800
committerGravatar GitHub <noreply@github.com>2018-02-28 16:53:41 -0800
commit07767950c82e3eed7c6ad506a0bfd2ce1c19c871 (patch)
treee1949f247677336942ceaf8ddd4536adae024ddd /src/objective-c/GRPCClient
parent1294d20b6b82ce8e1943d55abe34228a4cce88e9 (diff)
Revert "Revert "Refactor connectivity monitor on iOS""
Diffstat (limited to 'src/objective-c/GRPCClient')
-rw-r--r--src/objective-c/GRPCClient/GRPCCall.m136
-rw-r--r--src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h58
-rw-r--r--src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m199
-rw-r--r--src/objective-c/GRPCClient/private/GRPCHost.m25
4 files changed, 165 insertions, 253 deletions
diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index ac4596da25..02492607cd 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -108,6 +108,9 @@ static NSString * const kBearerPrefix = @"Bearer ";
// The dispatch queue to be used for enqueuing responses to user. Defaulted to the main dispatch
// queue
dispatch_queue_t _responseQueue;
+
+ // Whether the call is finished. If it is, should not call finishWithError again.
+ BOOL _finished;
}
@synthesize state = _state;
@@ -206,6 +209,8 @@ static NSString * const kBearerPrefix = @"Bearer ";
} else {
[_responseWriteable enqueueSuccessfulCompletion];
}
+
+ [GRPCConnectivityMonitor unregisterObserver:self];
}
- (void)cancelCall {
@@ -214,9 +219,10 @@ static NSString * const kBearerPrefix = @"Bearer ";
}
- (void)cancel {
- [self finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
- code:GRPCErrorCodeCancelled
- userInfo:@{NSLocalizedDescriptionKey: @"Canceled by app"}]];
+ [self maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+ code:GRPCErrorCodeCancelled
+ userInfo:@{NSLocalizedDescriptionKey: @"Canceled by app"}]];
+
if (!self.isWaitingForToken) {
[self cancelCall];
} else {
@@ -224,6 +230,19 @@ static NSString * const kBearerPrefix = @"Bearer ";
}
}
+- (void)maybeFinishWithError:(NSError *)errorOrNil {
+ BOOL toFinish = NO;
+ @synchronized(self) {
+ if (_finished == NO) {
+ _finished = YES;
+ toFinish = YES;
+ }
+ }
+ if (toFinish == YES) {
+ [self finishWithError:errorOrNil];
+ }
+}
+
- (void)dealloc {
__block GRPCWrappedCall *wrappedCall = _wrappedCall;
dispatch_async(_callQueue, ^{
@@ -250,11 +269,13 @@ static NSString * const kBearerPrefix = @"Bearer ";
if (self.state == GRXWriterStatePaused) {
return;
}
- __weak GRPCCall *weakSelf = self;
- __weak GRXConcurrentWriteable *weakWriteable = _responseWriteable;
dispatch_async(_callQueue, ^{
- [weakSelf startReadWithHandler:^(grpc_byte_buffer *message) {
+ __weak GRPCCall *weakSelf = self;
+ __weak GRXConcurrentWriteable *weakWriteable = self->_responseWriteable;
+ [self startReadWithHandler:^(grpc_byte_buffer *message) {
+ __strong GRPCCall *strongSelf = weakSelf;
+ __strong GRXConcurrentWriteable *strongWriteable = weakWriteable;
if (message == NULL) {
// No more messages from the server
return;
@@ -266,14 +287,14 @@ static NSString * const kBearerPrefix = @"Bearer ";
// don't want to throw, because the app shouldn't crash for a behavior
// that's on the hands of any server to have. Instead we finish and ask
// the server to cancel.
- [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
- code:GRPCErrorCodeResourceExhausted
- userInfo:@{NSLocalizedDescriptionKey: @"Client does not have enough memory to hold the server response."}]];
- [weakSelf cancelCall];
+ [strongSelf maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+ code:GRPCErrorCodeResourceExhausted
+ userInfo:@{NSLocalizedDescriptionKey: @"Client does not have enough memory to hold the server response."}]];
+ [strongSelf cancelCall];
return;
}
- [weakWriteable enqueueValue:data completionHandler:^{
- [weakSelf startNextRead];
+ [strongWriteable enqueueValue:data completionHandler:^{
+ [strongSelf startNextRead];
}];
}];
});
@@ -333,12 +354,17 @@ static NSString * const kBearerPrefix = @"Bearer ";
_requestWriter.state = GRXWriterStatePaused;
}
- __weak GRPCCall *weakSelf = self;
dispatch_async(_callQueue, ^{
- [weakSelf writeMessage:value withErrorHandler:^{
- [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
- code:GRPCErrorCodeInternal
- userInfo:nil]];
+ __weak GRPCCall *weakSelf = self;
+ [self writeMessage:value withErrorHandler:^{
+ __strong GRPCCall *strongSelf = weakSelf;
+ if (strongSelf != nil) {
+ [strongSelf maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+ code:GRPCErrorCodeInternal
+ userInfo:nil]];
+ // Wrapped call must be canceled when error is reported to upper layers
+ [strongSelf cancelCall];
+ }
}];
});
}
@@ -360,12 +386,15 @@ static NSString * const kBearerPrefix = @"Bearer ";
if (errorOrNil) {
[self cancel];
} else {
- __weak GRPCCall *weakSelf = self;
dispatch_async(_callQueue, ^{
- [weakSelf finishRequestWithErrorHandler:^{
- [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
- code:GRPCErrorCodeInternal
- userInfo:nil]];
+ __weak GRPCCall *weakSelf = self;
+ [self finishRequestWithErrorHandler:^{
+ __strong GRPCCall *strongSelf = weakSelf;
+ [strongSelf maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+ code:GRPCErrorCodeInternal
+ userInfo:nil]];
+ // Wrapped call must be canceled when error is reported to upper layers
+ [strongSelf cancelCall];
}];
});
}
@@ -387,30 +416,37 @@ static NSString * const kBearerPrefix = @"Bearer ";
}
- (void)invokeCall {
+ __weak GRPCCall *weakSelf = self;
[self invokeCallWithHeadersHandler:^(NSDictionary *headers) {
// Response headers received.
- self.responseHeaders = headers;
- [self startNextRead];
+ __strong GRPCCall *strongSelf = weakSelf;
+ if (strongSelf) {
+ strongSelf.responseHeaders = headers;
+ [strongSelf startNextRead];
+ }
} completionHandler:^(NSError *error, NSDictionary *trailers) {
- self.responseTrailers = trailers;
+ __strong GRPCCall *strongSelf = weakSelf;
+ if (strongSelf) {
+ strongSelf.responseTrailers = trailers;
- if (error) {
- NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
- if (error.userInfo) {
- [userInfo addEntriesFromDictionary:error.userInfo];
- }
- userInfo[kGRPCTrailersKey] = self.responseTrailers;
- // TODO(jcanizales): The C gRPC library doesn't guarantee that the headers block will be
- // called before this one, so an error might end up with trailers but no headers. We
- // shouldn't call finishWithError until ater both blocks are called. It is also when this is
- // done that we can provide a merged view of response headers and trailers in a thread-safe
- // way.
- if (self.responseHeaders) {
- userInfo[kGRPCHeadersKey] = self.responseHeaders;
+ if (error) {
+ NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
+ if (error.userInfo) {
+ [userInfo addEntriesFromDictionary:error.userInfo];
+ }
+ userInfo[kGRPCTrailersKey] = strongSelf.responseTrailers;
+ // TODO(jcanizales): The C gRPC library doesn't guarantee that the headers block will be
+ // called before this one, so an error might end up with trailers but no headers. We
+ // shouldn't call finishWithError until ater both blocks are called. It is also when this is
+ // done that we can provide a merged view of response headers and trailers in a thread-safe
+ // way.
+ if (strongSelf.responseHeaders) {
+ userInfo[kGRPCHeadersKey] = strongSelf.responseHeaders;
+ }
+ error = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
}
- error = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
+ [strongSelf maybeFinishWithError:error];
}
- [self finishWithError:error];
}];
// Now that the RPC has been initiated, request writes can start.
@synchronized(_requestWriter) {
@@ -439,16 +475,8 @@ static NSString * const kBearerPrefix = @"Bearer ";
// TODO(jcanizales): Check this on init.
[NSException raise:NSInvalidArgumentException format:@"host of %@ is nil", _host];
}
- _connectivityMonitor = [GRPCConnectivityMonitor monitorWithHost:host];
- __weak typeof(self) weakSelf = self;
- void (^handler)(void) = ^{
- typeof(self) strongSelf = weakSelf;
- [strongSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
- code:GRPCErrorCodeUnavailable
- userInfo:@{ NSLocalizedDescriptionKey : @"Connectivity lost." }]];
- };
- [_connectivityMonitor handleLossWithHandler:handler
- wifiStatusChangeHandler:nil];
+ [GRPCConnectivityMonitor registerObserver:self
+ selector:@selector(connectivityChanged:)];
}
- (void)startWithWriteable:(id<GRXWriteable>)writeable {
@@ -512,4 +540,12 @@ static NSString * const kBearerPrefix = @"Bearer ";
}
}
+- (void)connectivityChanged:(NSNotification *)note {
+ [self maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+ code:GRPCErrorCodeUnavailable
+ userInfo:@{ NSLocalizedDescriptionKey : @"Connectivity lost." }]];
+ // Cancel underlying call upon this notification
+ [self cancelCall];
+}
+
@end
diff --git a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
index cb55e46d70..394d21792d 100644
--- a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
+++ b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
@@ -19,44 +19,30 @@
#import <Foundation/Foundation.h>
#import <SystemConfiguration/SystemConfiguration.h>
-@interface GRPCReachabilityFlags : NSObject
-
-+ (nonnull instancetype)flagsWithFlags:(SCNetworkReachabilityFlags)flags;
-
-/**
- * One accessor method to query each of the different flags. Example:
-
-@property(nonatomic, readonly) BOOL isCell;
-
- */
-#define GRPC_XMACRO_ITEM(methodName, FlagName) \
-@property(nonatomic, readonly) BOOL methodName;
-
-#include "GRPCReachabilityFlagNames.xmacro.h"
-#undef GRPC_XMACRO_ITEM
-
-@property(nonatomic, readonly) BOOL isHostReachable;
-@end
-
+typedef NS_ENUM(NSInteger, GRPCConnectivityStatus) {
+ GRPCConnectivityUnknown = 0,
+ GRPCConnectivityNoNetwork = 1,
+ GRPCConnectivityCellular = 2,
+ GRPCConnectivityWiFi = 3,
+};
+
+extern NSString * _Nonnull kGRPCConnectivityNotification;
+
+// This interface monitors OS reachability interface for any network status
+// change. Parties interested in these events should register themselves as
+// observer.
@interface GRPCConnectivityMonitor : NSObject
-+ (nullable instancetype)monitorWithHost:(nonnull NSString *)hostName;
-
- (nonnull instancetype)init NS_UNAVAILABLE;
-/**
- * Queue on which callbacks will be dispatched. Default is the main queue. Set it before calling
- * handleLossWithHandler:.
- */
-// TODO(jcanizales): Default to a serial background queue instead.
-@property(nonatomic, strong, null_resettable) dispatch_queue_t queue;
-
-/**
- * Calls handler every time the connectivity to this instance's host is lost. If this instance is
- * released before that happens, the handler won't be called.
- * Only one handler is active at a time, so if this method is called again before the previous
- * handler has been called, it might never be called at all (or yes, if it has already been queued).
- */
-- (void)handleLossWithHandler:(nullable void (^)(void))lossHandler
- wifiStatusChangeHandler:(nullable void (^)(void))wifiStatusChangeHandler;
+// Register an object as observer of network status change. \a observer
+// must have a notification method with one parameter of type
+// (NSNotification *) and should pass it to parameter \a selector. The
+// parameter of this notification method is not used for now.
++ (void)registerObserver:(_Nonnull id)observer
+ selector:(_Nonnull SEL)selector;
+
+// Ungegister an object from observers of network status change.
++ (void)unregisterObserver:(_Nonnull id)observer;
+
@end
diff --git a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
index c8e10dd75f..7f31c7e23e 100644
--- a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
+++ b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
@@ -18,175 +18,74 @@
#import "GRPCConnectivityMonitor.h"
-#pragma mark Flags
+#include <netinet/in.h>
-@implementation GRPCReachabilityFlags {
- SCNetworkReachabilityFlags _flags;
-}
+NSString *kGRPCConnectivityNotification = @"kGRPCConnectivityNotification";
-+ (instancetype)flagsWithFlags:(SCNetworkReachabilityFlags)flags {
- return [[self alloc] initWithFlags:flags];
-}
+static SCNetworkReachabilityRef reachability;
+static GRPCConnectivityStatus currentStatus;
-- (instancetype)initWithFlags:(SCNetworkReachabilityFlags)flags {
- if ((self = [super init])) {
- _flags = flags;
+// Aggregate information in flags into network status.
+GRPCConnectivityStatus CalculateConnectivityStatus(SCNetworkReachabilityFlags flags) {
+ GRPCConnectivityStatus result = GRPCConnectivityUnknown;
+ if (((flags & kSCNetworkReachabilityFlagsReachable) == 0) ||
+ ((flags & kSCNetworkReachabilityFlagsConnectionRequired) != 0)) {
+ return GRPCConnectivityNoNetwork;
}
- return self;
-}
-
-/*
- * One accessor method implementation per flag. Example:
-
-- (BOOL)isCell { \
- return !!(_flags & kSCNetworkReachabilityFlagsIsWWAN); \
-}
-
- */
-#define GRPC_XMACRO_ITEM(methodName, FlagName) \
-- (BOOL)methodName { \
- return !!(_flags & kSCNetworkReachabilityFlags ## FlagName); \
-}
-#include "GRPCReachabilityFlagNames.xmacro.h"
-#undef GRPC_XMACRO_ITEM
-
-- (BOOL)isHostReachable {
- // Note: connectionOnDemand means it'll be reachable only if using the CFSocketStream API or APIs
- // on top of it.
- // connectionRequired means we can't tell until a connection is attempted (e.g. for VPN on
- // demand).
- return self.reachable && !self.interventionRequired && !self.connectionOnDemand;
-}
-
-- (NSString *)description {
- NSMutableArray *activeOptions = [NSMutableArray arrayWithCapacity:9];
-
- /*
- * For each flag, add its name to the array if it's ON. Example:
-
- if (self.isCell) {
- [activeOptions addObject:@"isCell"];
+ result = GRPCConnectivityWiFi;
+#if TARGET_OS_IPHONE
+ if (flags & kSCNetworkReachabilityFlagsIsWWAN) {
+ return result = GRPCConnectivityCellular;
}
-
- */
- #define GRPC_XMACRO_ITEM(methodName, FlagName) \
- if (self.methodName) { \
- [activeOptions addObject:@ #methodName]; \
- }
- #include "GRPCReachabilityFlagNames.xmacro.h"
- #undef GRPC_XMACRO_ITEM
-
- return activeOptions.count == 0 ? @"(none)" : [activeOptions componentsJoinedByString:@", "];
-}
-
-- (BOOL)isEqual:(id)object {
- return [object isKindOfClass:[GRPCReachabilityFlags class]] &&
- _flags == ((GRPCReachabilityFlags *)object)->_flags;
-}
-
-- (NSUInteger)hash {
- return _flags;
-}
-@end
-
-#pragma mark Connectivity Monitor
-
-// Assumes the third argument is a block that accepts a GRPCReachabilityFlags object, and passes the
-// received ones to it.
-static void PassFlagsToContextInfoBlock(SCNetworkReachabilityRef target,
- SCNetworkReachabilityFlags flags,
- void *info) {
- #pragma unused (target)
- // This can be called many times with the same info. The info is retained by SCNetworkReachability
- // while this function is being executed.
- void (^handler)(GRPCReachabilityFlags *) = (__bridge void (^)(GRPCReachabilityFlags *))info;
- handler([[GRPCReachabilityFlags alloc] initWithFlags:flags]);
+#endif
+ return result;
}
-@implementation GRPCConnectivityMonitor {
- SCNetworkReachabilityRef _reachabilityRef;
- GRPCReachabilityFlags *_previousReachabilityFlags;
-}
+static void ReachabilityCallback(
+ SCNetworkReachabilityRef target, SCNetworkReachabilityFlags flags, void* info) {
+ GRPCConnectivityStatus newStatus = CalculateConnectivityStatus(flags);
-- (nullable instancetype)initWithReachability:(nullable SCNetworkReachabilityRef)reachability {
- if (!reachability) {
- return nil;
+ if (newStatus != currentStatus) {
+ [[NSNotificationCenter defaultCenter] postNotificationName:kGRPCConnectivityNotification
+ object:nil];
+ currentStatus = newStatus;
}
- if ((self = [super init])) {
- _reachabilityRef = CFRetain(reachability);
- _queue = dispatch_get_main_queue();
- _previousReachabilityFlags = nil;
- }
- return self;
}
-+ (nullable instancetype)monitorWithHost:(nonnull NSString *)host {
- const char *hostName = host.UTF8String;
- if (!hostName) {
- [NSException raise:NSInvalidArgumentException
- format:@"host.UTF8String returns NULL for %@", host];
- }
- SCNetworkReachabilityRef reachability =
- SCNetworkReachabilityCreateWithName(NULL, hostName);
+@implementation GRPCConnectivityMonitor
- GRPCConnectivityMonitor *returnValue = [[self alloc] initWithReachability:reachability];
- if (reachability) {
- CFRelease(reachability);
- }
- return returnValue;
-}
++ (void)initialize {
+ if (self == [GRPCConnectivityMonitor self]) {
+ struct sockaddr_in addr = {0};
+ addr.sin_len = sizeof(addr);
+ addr.sin_family = AF_INET;
+ reachability = SCNetworkReachabilityCreateWithAddress(NULL, (struct sockaddr *)&addr);
+ currentStatus = GRPCConnectivityUnknown;
-- (void)handleLossWithHandler:(nullable void (^)(void))lossHandler
- wifiStatusChangeHandler:(nullable void (^)(void))wifiStatusChangeHandler {
- __weak typeof(self) weakSelf = self;
- [self startListeningWithHandler:^(GRPCReachabilityFlags *flags) {
- typeof(self) strongSelf = weakSelf;
- if (strongSelf) {
- if (lossHandler && !flags.reachable) {
- lossHandler();
-#if TARGET_OS_IPHONE
- } else if (wifiStatusChangeHandler &&
- strongSelf->_previousReachabilityFlags &&
- (flags.isWWAN ^
- strongSelf->_previousReachabilityFlags.isWWAN)) {
- wifiStatusChangeHandler();
-#endif
- }
- strongSelf->_previousReachabilityFlags = flags;
+ SCNetworkConnectionFlags flags;
+ if (SCNetworkReachabilityGetFlags(reachability, &flags)) {
+ currentStatus = CalculateConnectivityStatus(flags);
}
- }];
-}
-- (void)startListeningWithHandler:(void (^)(GRPCReachabilityFlags *))handler {
- // Copy to ensure the handler block is in the heap (and so can't be deallocated when this method
- // returns).
- void (^copiedHandler)(GRPCReachabilityFlags *) = [handler copy];
- SCNetworkReachabilityContext context = {
- .version = 0,
- .info = (__bridge void *)copiedHandler,
- .retain = CFRetain,
- .release = CFRelease,
- };
- // The following will retain context.info, and release it when the callback is set to NULL.
- SCNetworkReachabilitySetCallback(_reachabilityRef, PassFlagsToContextInfoBlock, &context);
- SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, _queue);
-}
-
-- (void)stopListening {
- // This releases the block on context.info.
- SCNetworkReachabilitySetCallback(_reachabilityRef, NULL, NULL);
- SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, NULL);
+ SCNetworkReachabilityContext context = {0, (__bridge void *)(self), NULL, NULL, NULL};
+ if (!SCNetworkReachabilitySetCallback(reachability, ReachabilityCallback, &context) ||
+ !SCNetworkReachabilityScheduleWithRunLoop(
+ reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes)) {
+ NSLog(@"gRPC connectivity monitor fail to set");
+ }
+ }
}
-- (void)setQueue:(dispatch_queue_t)queue {
- _queue = queue ?: dispatch_get_main_queue();
++ (void)registerObserver:(_Nonnull id)observer
+ selector:(SEL)selector {
+ [[NSNotificationCenter defaultCenter] addObserver:observer
+ selector:selector
+ name:kGRPCConnectivityNotification
+ object:nil];
}
-- (void)dealloc {
- if (_reachabilityRef) {
- [self stopListening];
- CFRelease(_reachabilityRef);
- }
++ (void)unregisterObserver:(_Nonnull id)observer {
+ [[NSNotificationCenter defaultCenter] removeObserver:observer];
}
@end
diff --git a/src/objective-c/GRPCClient/private/GRPCHost.m b/src/objective-c/GRPCClient/private/GRPCHost.m
index 71b57cf1f6..8568e334dd 100644
--- a/src/objective-c/GRPCClient/private/GRPCHost.m
+++ b/src/objective-c/GRPCClient/private/GRPCHost.m
@@ -37,12 +37,6 @@ NS_ASSUME_NONNULL_BEGIN
static NSMutableDictionary *kHostCache;
-// This connectivity monitor flushes the host cache when connectivity status
-// changes or when connection switch between Wifi and Cellular data, so that a
-// new call will use a new channel. Otherwise, a new call will still use the
-// cached channel which is no longer available and will cause gRPC to hang.
-static GRPCConnectivityMonitor *connectivityMonitor = nil;
-
@implementation GRPCHost {
// TODO(mlumish): Investigate whether caching channels with strong links is a good idea.
GRPCChannel *_channel;
@@ -90,17 +84,7 @@ static GRPCConnectivityMonitor *connectivityMonitor = nil;
kHostCache[address] = self;
_compressAlgorithm = GRPC_COMPRESS_NONE;
}
- // Keep a single monitor to flush the cache if the connectivity status changes
- // Thread safety guarded by @synchronized(kHostCache)
- if (!connectivityMonitor) {
- connectivityMonitor =
- [GRPCConnectivityMonitor monitorWithHost:hostURL.host];
- void (^handler)(void) = ^{
- [GRPCHost flushChannelCache];
- };
- [connectivityMonitor handleLossWithHandler:handler
- wifiStatusChangeHandler:handler];
- }
+ [GRPCConnectivityMonitor registerObserver:self selector:@selector(connectivityChange:)];
}
return self;
}
@@ -281,6 +265,13 @@ static GRPCConnectivityMonitor *connectivityMonitor = nil;
}
}
+// Flushes the host cache when connectivity status changes or when connection switch between Wifi
+// and Cellular data, so that a new call will use a new channel. Otherwise, a new call will still
+// use the cached channel which is no longer available and will cause gRPC to hang.
+- (void)connectivityChange:(NSNotification *)note {
+ [GRPCHost flushChannelCache];
+}
+
@end
NS_ASSUME_NONNULL_END