DispatchQueue delayed callback improvements + testing (#784)

Basically a port of https://github.com/firebase/firebase-js-sdk/commit/a1e346ff93c6cbcc0a1b3b33f0fbc3a7b66e7e12 and https://github.com/firebase/firebase-js-sdk/commit/fce4168309f42aa038125f39818fbf654b65b05f

* Introduces a DelayedCallback helper class in FSTDispatchQueue to encapsulate delayed callback logic.
* Adds cancellation support.
* Updates the idle timer in FSTStream to use new cancellation support.
* Adds a FSTTimerId enum for identifying delayed operations on the queue and uses it to identify our existing backoff and idle timers.
* Added containsDelayedCallback: and runDelayedCallbacksUntil: methods to FSTDispatchQueue which can be used from tests to check for the presence of a callback or to schedule them to run early.
* Removes FSTTestDispatchQueue and changes idle tests to use new test methods.

1 files changed, 10 insertions, 2 deletions

diff --git a/Firestore/Source/Remote/FSTStream.h b/Firestore/Source/Remote/FSTStream.h
index c390dbb..6630083 100644
--- a/Firestore/Source/Remote/FSTStream.h
+++ b/Firestore/Source/Remote/FSTStream.h
@@ -17,6 +17,7 @@
 #import <Foundation/Foundation.h>
 
 #import "Firestore/Source/Core/FSTTypes.h"
+#import "Firestore/Source/Util/FSTDispatchQueue.h"
 
 #include "Firestore/core/src/firebase/firestore/core/database_info.h"
 
@@ -91,6 +92,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (instancetype)initWithDatabase:(const firebase::firestore::core::DatabaseInfo *)database
              workerDispatchQueue:(FSTDispatchQueue *)workerDispatchQueue
+               connectionTimerID:(FSTTimerID)connectionTimerID
+                     idleTimerID:(FSTTimerID)idleTimerID
                      credentials:(id<FSTCredentialsProvider>)credentials
             responseMessageClass:(Class)responseMessageClass NS_DESIGNATED_INITIALIZER;
 
@@ -142,8 +145,13 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)stop;
 
 /**
- * Initializes the idle timer. If no write takes place within one minute, the GRPC stream will be
- * closed.
+ * Marks this stream as idle. If no further actions are performed on the stream for one minute, the
+ * stream will automatically close itself and notify the stream's close handler. The stream will
+ * then be in a non-started state, requiring the caller to start the stream again before further
+ * use.
+ *
+ * Only streams that are in state 'Open' can be marked idle, as all other states imply pending
+ * network operations.
  */
 - (void)markIdle;