Support background timers on iOS via NSTimer

React/Modules/RCTTiming.m

@@ -96,6 +96,7 @@ @implementation RCTTiming
   NSMutableDictionary<NSNumber *, _RCTTimer *> *_timers;
   NSTimer *_sleepTimer;
   BOOL _sendIdleEvents;
+  BOOL _inBackground;
 }
 
 @synthesize bridge = _bridge;
@@ -110,20 +111,21 @@ - (void)setBridge:(RCTBridge *)bridge
 
   _paused = YES;
   _timers = [NSMutableDictionary new];
+  _inBackground = NO;
 
   for (NSString *name in @[UIApplicationWillResignActiveNotification,
                            UIApplicationDidEnterBackgroundNotification,
                            UIApplicationWillTerminateNotification]) {
     [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(stopTimers)
+                                             selector:@selector(appDidMoveToBackground)
                                                  name:name
                                                object:nil];
   }
 
   for (NSString *name in @[UIApplicationDidBecomeActiveNotification,
                            UIApplicationWillEnterForegroundNotification]) {
     [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(startTimers)
+                                             selector:@selector(appDidMoveToForeground)
                                                  name:name
                                                object:nil];
   }
@@ -148,8 +150,29 @@ - (void)invalidate
   _bridge = nil;
 }
 
+- (void)appDidMoveToBackground
+{
+  // Deactivate the CADisplayLink while in the background.
+  [self stopTimers];
+  _inBackground = YES;
+
+  // Issue one final timer callback, which will schedule a
+  // background NSTimer, if needed.
+  [self didUpdateFrame:nil];
+}
+
+- (void)appDidMoveToForeground
+{
+  _inBackground = NO;
+  [self startTimers];
+}
+
 - (void)stopTimers
 {
+  if (_inBackground) {
+    return;
+  }
+
   if (!_paused) {
     _paused = YES;
     if (_pauseCallback) {
@@ -160,7 +183,7 @@ - (void)stopTimers
 
 - (void)startTimers
 {
-  if (!_bridge || ![self hasPendingTimers]) {
+  if (!_bridge || _inBackground || ![self hasPendingTimers]) {
     return;
   }
 
@@ -225,7 +248,11 @@ - (void)didUpdateFrame:(RCTFrameUpdate *)update
   // Switch to a paused state only if we didn't call any timer this frame, so if
   // in response to this timer another timer is scheduled, we don't pause and unpause
   // the displaylink frivolously.
-  if (!_sendIdleEvents && timersToCall.count == 0) {
+  if (_inBackground) {
+    if (_timers.count) {
+      [self scheduleSleepTimer:nextScheduledTarget];
+    }
+  } else if (!_sendIdleEvents && timersToCall.count == 0) {
     // No need to call the pauseCallback as RCTDisplayLink will ask us about our paused
     // status immediately after completing this call
     if (_timers.count == 0) {
@@ -295,7 +322,10 @@ - (void)timerDidFire
                                                 targetTime:targetTime
                                                    repeats:repeats];
   _timers[callbackID] = timer;
-  if (_paused) {
+
+  if (_inBackground) {
+    [self scheduleSleepTimer:timer.target];
+  } else if (_paused) {
     if ([timer.target timeIntervalSinceNow] > kMinimumSleepInterval) {
       [self scheduleSleepTimer:timer.target];
     } else {