Access UIApplication on main thread

Track UIApplication applicationState changes from a C++ class. Uses
NSNotificationCenter to access changes on the main thread and exposes
a local variable that can be checked from any thread.

This fixes a runtime warning on iOS 11 beta.

My Objective-C++ is a little rusty so please check if this follows
the conventions for C++ code in the project. It also changes the
interface exposed by RTCUIApplication.h, not sure if that has impact
on any public APIs that needs to be documented somewhere?

Bug: webrtc:7773
Change-Id: I9c8ba090ef9f28d812114026a906cef742192c39
Reviewed-on: https://chromium-review.googlesource.com/527442
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Kári Tristan Helgason <kthelgason@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18558}

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

@@ -0,0 +1,68 @@
+/*
+ *  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 {
+  UIApplicationState _state;
+}
+
++ (instancetype)sharedInstance {
+  static id sharedInstance;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    sharedInstance = [[self alloc] init];
+  });
+
+  return sharedInstance;
+}
+
+- (id)init {
+  if (self = [super init]) {
+    NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
+    [center addObserverForName:UIApplicationDidBecomeActiveNotification
+                        object:nil
+                         queue:[NSOperationQueue mainQueue]
+                    usingBlock:^(NSNotification *note) {
+                      _state = [UIApplication sharedApplication].applicationState;
+                    }];
+
+    [center addObserverForName:UIApplicationDidEnterBackgroundNotification
+                        object:nil
+                         queue:[NSOperationQueue mainQueue]
+                    usingBlock:^(NSNotification *note) {
+                      _state = [UIApplication sharedApplication].applicationState;
+                    }];
+
+    dispatch_block_t initializeBlock = ^{
+      _state = [UIApplication sharedApplication].applicationState;
+    };
+
+    if ([NSThread isMainThread]) {
+      initializeBlock();
+    } else {
+      dispatch_sync(dispatch_get_main_queue(), initializeBlock);
+    }
+  }
+
+  return self;
+}
+
+- (BOOL)isApplicationActive {
+  return _state == UIApplicationStateActive;
+}
+
+@end
+
+#endif  // WEBRTC_IOS