Timing: Fixes timer when app get into background (facebook#24649)

Summary:
Related facebook#23674, in that PR, we imported background timer support, but it's not sufficient, I think the reason that works is because it enable the `Background Modes` and do some background tasks, for the users who don't enable it, timer would pause immediately before goes into background.

To fix it, we can mark a background task when goes into background, it can keep app active for minutes, try best to support timing when in background.

cc. cpojer .

## Changelog

[iOS] [Fixed] - Timing: Fixes timer when app get into background
Pull Request resolved: facebook#24649

Differential Revision: D15554451

Pulled By: cpojer

fbshipit-source-id: a33f7afe6b63d1a4fefcb7098459aee0c09145da

React/Modules/RCTTiming.m

@@ -97,6 +97,7 @@ @implementation RCTTiming
   NSTimer *_sleepTimer;
   BOOL _sendIdleEvents;
   BOOL _inBackground;
+  UIBackgroundTaskIdentifier _backgroundTaskIdentifier;
 }
 
 @synthesize bridge = _bridge;
@@ -112,6 +113,7 @@ - (void)setBridge:(RCTBridge *)bridge
   _paused = YES;
   _timers = [NSMutableDictionary new];
   _inBackground = NO;
+  _backgroundTaskIdentifier = UIBackgroundTaskInvalid;
 
   for (NSString *name in @[UIApplicationWillResignActiveNotification,
                            UIApplicationDidEnterBackgroundNotification,
@@ -135,10 +137,35 @@ - (void)setBridge:(RCTBridge *)bridge
 
 - (void)dealloc
 {
+  [self markEndOfBackgroundTaskIfNeeded];
   [_sleepTimer invalidate];
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
+- (void)markStartOfBackgroundTaskIfNeeded
+{
+  if (_backgroundTaskIdentifier == UIBackgroundTaskInvalid) {
+    __weak typeof(self) weakSelf = self;
+    // Marks the beginning of a new long-running background task. We can run the timer in the background.
+    _backgroundTaskIdentifier = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
+      typeof(self) strongSelf = weakSelf;
+      if (!strongSelf) {
+        return;
+      }
+      // Mark the end of background task
+      [strongSelf markEndOfBackgroundTaskIfNeeded];
+    }];
+  }
+}
+
+- (void)markEndOfBackgroundTaskIfNeeded
+{
+  if (_backgroundTaskIdentifier != UIBackgroundTaskInvalid) {
+    [[UIApplication sharedApplication] endBackgroundTask:_backgroundTaskIdentifier];
+    _backgroundTaskIdentifier = UIBackgroundTaskInvalid;
+  }
+}
+
 - (dispatch_queue_t)methodQueue
 {
   return RCTJSThread;
@@ -163,6 +190,7 @@ - (void)appDidMoveToBackground
 
 - (void)appDidMoveToForeground
 {
+  [self markEndOfBackgroundTaskIfNeeded];
   _inBackground = NO;
   [self startTimers];
 }
@@ -260,6 +288,7 @@ - (void)didUpdateFrame:(RCTFrameUpdate *)update
   }
   if (_inBackground) {
     if (timerCount) {
+      [self markStartOfBackgroundTaskIfNeeded];
       [self scheduleSleepTimer:nextScheduledTarget];
     }
   } else if (!_sendIdleEvents && timersToCall.count == 0) {
@@ -338,6 +367,7 @@ - (void)timerDidFire
   }
 
   if (_inBackground) {
+    [self markStartOfBackgroundTaskIfNeeded];
     [self scheduleSleepTimer:timer.target];
   } else if (_paused) {
     if ([timer.target timeIntervalSinceNow] > kMinimumSleepInterval) {