[CRD iOS] Blur remote host content when going to background

This CL adds a blur view to the HostView when the app becomes inactive
(transitioning to background, etc.) and removes it when the app becomes
active again.

Screenshot: https://drive.google.com/open?id=1UrWMUc-j0rRS-DQJ5zLLZ4_dTGgLohVr

Bug: 791112
Change-Id: Ic66c57c939bb607ad7b740cb8112bd449cd20d45
Reviewed-on: https://chromium-review.googlesource.com/807402
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521567}

remoting/ios/app/host_view_controller.mm

@@ -47,6 +47,9 @@ static const CGFloat kMoveFABAnimationTime = 0.3;
   BOOL _surfaceCreated;
   HostSettings* _settings;
 
+  // Used to blur the content when the app enters background.
+  UIView* _blurView;
+
   // Only change this by calling setFabIsRight:.
   BOOL _fabIsRight;
   NSArray<NSLayoutConstraint*>* _fabLeftConstraints;
@@ -214,6 +217,7 @@ static const CGFloat kMoveFABAnimationTime = 0.3;
         [[ClientGestures alloc] initWithView:_hostView client:_client];
     _clientGestures.delegate = self;
   }
+
   [[NSNotificationCenter defaultCenter]
       addObserver:self
          selector:@selector(keyboardWillShow:)
@@ -226,6 +230,25 @@ static const CGFloat kMoveFABAnimationTime = 0.3;
              name:UIKeyboardWillHideNotification
            object:nil];
 
+  [NSNotificationCenter.defaultCenter
+      addObserver:self
+         selector:@selector(applicationDidBecomeActive:)
+             name:UIApplicationDidBecomeActiveNotification
+           object:nil];
+
+  [NSNotificationCenter.defaultCenter
+      addObserver:self
+         selector:@selector(applicationWillResignActive:)
+             name:UIApplicationWillResignActiveNotification
+           object:nil];
+
+  // If the host view is presented when the app is inactive, synthesize an
+  // initial UIApplicationWillResignActiveNotification event.
+  if (UIApplication.sharedApplication.applicationState !=
+      UIApplicationStateActive) {
+    [self applicationWillResignActive:UIApplication.sharedApplication];
+  }
+
   _surfaceSizeAnimationLink =
       [CADisplayLink displayLinkWithTarget:self
                                   selector:@selector(animateHostSurfaceSize:)];
@@ -602,4 +625,31 @@ static const CGFloat kMoveFABAnimationTime = 0.3;
   }
 }
 
+- (void)applicationDidBecomeActive:(UIApplication*)application {
+  if (!_blurView) {
+    LOG(DFATAL) << "Blur view does not exist.";
+    return;
+  }
+  [_blurView removeFromSuperview];
+  _blurView = nil;
+}
+
+- (void)applicationWillResignActive:(UIApplication*)application {
+  if (_blurView) {
+    LOG(DFATAL) << "Blur view already exists.";
+    return;
+  }
+  UIBlurEffect* effect =
+      [UIBlurEffect effectWithStyle:UIBlurEffectStyleRegular];
+  _blurView = [[UIVisualEffectView alloc] initWithEffect:effect];
+  _blurView.translatesAutoresizingMaskIntoConstraints = NO;
+  [self.view insertSubview:_blurView aboveSubview:_hostView];
+  [NSLayoutConstraint activateConstraints:@[
+    [_blurView.leadingAnchor constraintEqualToAnchor:_hostView.leadingAnchor],
+    [_blurView.trailingAnchor constraintEqualToAnchor:_hostView.trailingAnchor],
+    [_blurView.topAnchor constraintEqualToAnchor:_hostView.topAnchor],
+    [_blurView.bottomAnchor constraintEqualToAnchor:_hostView.bottomAnchor],
+  ]];
+}
+
 @end