How to resume a CAAnimation after coming back from multitasking

Question

in my app i have an array of CALayer that I have animated along a bezierPath. When I close and reopen the app my layers are not animating and not in the same position as before closing the app. I have implemented two methods, pauseLayer and resumeLayer that works when I trigger them with two buttons inside my app but they won't work after closing the app. The code is the following

   - (void)pauseLayers{

    for(int y=0; y<=end;y++)
    {



        CFTimeInterval pausedTime = [car[y] convertTime:CACurrentMediaTime() fromLayer:nil];
        car[y].speed = 0.0;
        car[y].timeOffset = pausedTime;

         standardUserDefaults[y] = [NSUserDefaults standardUserDefaults];


        if (standardUserDefaults[y]) {
            [standardUserDefaults[y] setDouble:pausedTime forKey:@"pausedTime"];
            [standardUserDefaults[y] synchronize];
        }


        NSLog(@"saving positions");


        }


}

-(void)resumeLayers

{  




    for(int y=0; y<=end;y++)
    {




        standardUserDefaults[y] = [NSUserDefaults standardUserDefaults];     
        car[y].timeOffset = [standardUserDefaults[y] doubleForKey:@"pausedTime"];

    CFTimeInterval pausedTime = [car[y] timeOffset];
    car[y].speed = 1.0;
    car[y].timeOffset = 0.0;
    car[y].beginTime = 0.0;

    CFTimeInterval timeSincePause = [car[y] convertTime:CACurrentMediaTime() fromLayer:nil] - pausedTime;
    car[y].beginTime = timeSincePause;
        }


}

Answer 1

- (void)applicationDidEnterBackground:(UIApplication *)application {

 mosquitosViewController *mvc = [[mosquitosViewController alloc] init];
  [mvc pauseLayers];

  }

The problem with what you are trying to do above is that you are creating a completely new instance of your view controller, which is not the one that was showing onscreen. That's why nothing happens when you send the pauseLayers message.

What you should do is register to receive notifications for when your app goes to and comes from the background and call the appropriate methods (pauseLayers and resumeLayers) when that notification arrives.

You should add the following code somewhere in your mosquitosViewController implementation (I usually do so in viewDidLoad):

// Register for notification that app did enter background
[[NSNotificationCenter defaultCenter] addObserver:self 
                                      selector:@selector(pauseLayers)
                                      name:UIApplicationDidEnterBackgroundNotification 
                                      object:[UIApplication sharedApplication]];

// Register for notification that app did enter foreground
[[NSNotificationCenter defaultCenter] addObserver:self 
                                      selector:@selector(resumeLayers) 
                                      name:UIApplicationWillEnterForegroundNotification 
                                      object:[UIApplication sharedApplication]];