ObjC: Always dispatch async in UIApplication status observer.

webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "RTCUIApplicationStatusObserver.h"
 
 #if defined(WEBRTC_IOS)
 
 #import <UIKit/UIKit.h>
 
 @implementation RTCUIApplicationStatusObserver {
+  BOOL _initialized;
+  dispatch_block_t _initializeBlock;
   UIApplicationState _state;
 }
 
 + (instancetype)sharedInstance {
   static id sharedInstance;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
     sharedInstance = [[self alloc] init];
   });
 
@@ skipping 10 matching lines @@
                       _state = [UIApplication sharedApplication].applicationState;
                     }];
 
     [center addObserverForName:UIApplicationDidEnterBackgroundNotification
                         object:nil
                          queue:[NSOperationQueue mainQueue]
                     usingBlock:^(NSNotification *note) {
                       _state = [UIApplication sharedApplication].applicationState;
                     }];
 
-    dispatch_block_t initializeBlock = ^{
+    _initialized = NO;
+    _initializeBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
       _state = [UIApplication sharedApplication].applicationState;
-    };
+      _initialized = YES;
+    });
 
-    if ([NSThread isMainThread]) {
-      initializeBlock();
-    } else {
-      dispatch_sync(dispatch_get_main_queue(), initializeBlock);
-    }
+    dispatch_async(dispatch_get_main_queue(), _initializeBlock);
   }
 
   return self;
 }
 
 - (BOOL)isApplicationActive {
+  if (!_initialized) {
+    dispatch_block_wait(_initializeBlock, DISPATCH_TIME_FOREVER);

[daniela-webrtc, 2017/08/23 08:54:10]

This will make the 'isApplicationActive' a blocking call.  Will it have
implications in other parts of the code?

[andersc, 2017/08/23 09:32:26]

Before, we blocked on initialization by doing a dispatch_sync to the main queue,
which was problematic. Now instead we block the calling queue if
`isApplicationActive` is called before the ApplicationStatusObserver is fully
initialized. It will block at most once, when calling it directly after
initialization, before the _initializationBlock has completed, but some blocking
is needed since we have to wait for the initialization to run on the main queue.

[kthelgason, 2017/08/23 11:37:39]

Maybe instead of blocking forever we should add a timeout and a RTC_CHECK so
that if this ever causes problems it will be easier to find than a deadlock.

[andersc, 2017/08/23 12:14:22]

That sounds like a good idea! Done.

+  }
   return _state == UIApplicationStateActive;
 }
 
 @end
 
 #endif  // WEBRTC_IOS