How to resume a CAAnimation after coming back from multitasking

in my app i have an array of CALayer that I have animated along a be开发者_如何学PythonzierPath. 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;
        }


}

---

- (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]];

---

I write a Swift 4 version extension based on @cclogg and @Matej Bukovinski answers from this thread. All you need is to call layer.makeAnimationsPersistent()

Full Gist here: CALayer+AnimationPlayback.swift, CALayer+PersistentAnimations.swift

Core part:

public extension CALayer {
    static private var persistentHelperKey = "CALayer.LayerPersistentHelper"

    public func makeAnimationsPersistent() {
        var object = objc_getAssociatedObject(self, &CALayer.persistentHelperKey)
        if object == nil {
            object = LayerPersistentHelper(with: self)
            let nonatomic = objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC
            objc_setAssociatedObject(self, &CALayer.persistentHelperKey, object, nonatomic)
        }
    }
}

public class LayerPersistentHelper {
    private var persistentAnimations: [String: CAAnimation] = [:]
    private var persistentSpeed: Float = 0.0
    private weak var layer: CALayer?

    public init(with layer: CALayer) {
        self.layer = layer
        addNotificationObservers()
    }

    deinit {
        removeNotificationObservers()
    }
}

private extension LayerPersistentHelper {
    func addNotificationObservers() {
        let center = NotificationCenter.default
        let enterForeground = NSNotification.Name.UIApplicationWillEnterForeground
        let enterBackground = NSNotification.Name.UIApplicationDidEnterBackground
        center.addObserver(self, selector: #selector(didBecomeActive), name: enterForeground, object: nil)
        center.addObserver(self, selector: #selector(willResignActive), name: enterBackground, object: nil)
    }

    func removeNotificationObservers() {
        NotificationCenter.default.removeObserver(self)
    }

    func persistAnimations(with keys: [String]?) {
        guard let layer = self.layer else { return }
        keys?.forEach { (key) in
            if let animation = layer.animation(forKey: key) {
                persistentAnimations[key] = animation
            }
        }
    }

    func restoreAnimations(with keys: [String]?) {
        guard let layer = self.layer else { return }
        keys?.forEach { (key) in
            if let animation = persistentAnimations[key] {
                layer.add(animation, forKey: key)
            }
        }
    }
}

@objc extension LayerPersistentHelper {
    func didBecomeActive() {
        guard let layer = self.layer else { return }
        restoreAnimations(with: Array(persistentAnimations.keys))
        persistentAnimations.removeAll()
        if persistentSpeed == 1.0 { // if layer was playing before background, resume it
            layer.resumeAnimations()
        }
    }

    func willResignActive() {
        guard let layer = self.layer else { return }
        persistentSpeed = layer.speed
        layer.speed = 1.0 // in case layer was paused from outside, set speed to 1.0 to get all animations
        persistAnimations(with: layer.animationKeys())
        layer.speed = persistentSpeed // restore original speed
        layer.pauseAnimations()
    }
}

---

- (void)applicationDidEnterBackground:(UIApplication *)application

{

NSLog(@"1");
mosquitosViewController *mvc = [[mosquitosViewController alloc] init];
[mvc pauseLayers];

/*
 Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later. 
 If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
 */

}

---

See my answer to this post for details on how to restart an animation after multitasking:

Restoring animation where it left off when app resumes from background