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diff --git a/Firestore/Source/Remote/FSTOnlineStateTracker.h b/Firestore/Source/Remote/FSTOnlineStateTracker.h
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+++ b/Firestore/Source/Remote/FSTOnlineStateTracker.h
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+/*
+ * Copyright 2018 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 "Firestore/Source/Core/FSTTypes.h"
+
+@class FSTDispatchQueue;
+@protocol FSTOnlineStateDelegate;
+
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ * A component used by the FSTRemoteStore to track the FSTOnlineState (that is, whether or not the
+ * client as a whole should be considered to be online or offline), implementing the appropriate
+ * heuristics.
+ *
+ * In particular, when the client is trying to connect to the backend, we allow up to
+ * kMaxWatchStreamFailures within kOnlineStateTimeout for a connection to succeed. If we have too
+ * many failures or the timeout elapses, then we set the FSTOnlineState to Offline, and
+ * the client will behave as if it is offline (getDocument() calls will return cached data, etc.).
+ */
+@interface FSTOnlineStateTracker : NSObject
+
+- (instancetype)initWithWorkerDispatchQueue:(FSTDispatchQueue *)queue;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+/** A delegate to be notified on FSTOnlineState changes. */
+@property(nonatomic, weak) id<FSTOnlineStateDelegate> onlineStateDelegate;
+
+/**
+ * Called by FSTRemoteStore when a watch stream is started.
+ *
+ * It sets the FSTOnlineState to Unknown and starts the onlineStateTimer if necessary.
+ */
+- (void)handleWatchStreamStart;
+
+/**
+ * Called by FSTRemoteStore when a watch stream fails.
+ *
+ * Updates our FSTOnlineState as appropriate. The first failure moves us to FSTOnlineStateUnknown.
+ * We then may allow multiple failures (based on kMaxWatchStreamFailures) before we actually
+ * transition to FSTOnlineStateOffline.
+ */
+- (void)handleWatchStreamFailure;
+
+/**
+ * Explicitly sets the FSTOnlineState to the specified state.
+ *
+ * Note that this resets the timers / failure counters, etc. used by our Offline heuristics, so
+ * it must not be used in place of handleWatchStreamStart and handleWatchStreamFailure.
+ */
+- (void)updateState:(FSTOnlineState)newState;
+
+@end
+
+NS_ASSUME_NONNULL_END