Pause and save animations when app going into background, resume when going foreground.

CALayer+persist.m

@implementation CALayer (persist)

- (NSArray*)pauseAndSaveAnimations {
    NSMutableArray *array = [NSMutableArray new];
    
    __unsafe_unretained __block void (^recurse2)(CALayer *);
    void (^recurse)(CALayer *) = ^(CALayer *layer) {
        for(NSString *key in layer.animationKeys) {
            NSArray *a = @[layer,key,[[layer animationForKey:key] copy]];
            [array addObject:a];
        }
        for(CALayer *sublayer in layer.sublayers) {
            recurse2(sublayer);
        }
    };
    recurse2 = recurse;
    recurse(self);

    [self pauseAnimations];

    return array;
}

+ (void)resumeSavedAnimations:(NSArray *)anims {
    CALayer *superLayer;
    for(NSArray *a in anims) {
        CALayer *layer = a[0];
        NSString *key = a[1];
        CAAnimation *anim = a[2];
        [layer addAnimation:anim forKey:key];
        if(!superLayer || ![superLayer.sublayers containsObject:layer]) {
            superLayer = layer;
            [superLayer resumeAnimations];
        }
    }
}

- (void)pauseAnimations {
    CFTimeInterval pausedTime = [self convertTime:CACurrentMediaTime() fromLayer:nil];
    self.speed = 0.0;
    self.timeOffset = pausedTime;
}

- (void)resumeAnimations {
    CFTimeInterval pausedTime = [self timeOffset];
    self.speed = 1.0;
    self.timeOffset = 0.0;
    self.beginTime = 0.0;
    CFTimeInterval timeSincePause = [self convertTime:CACurrentMediaTime() fromLayer:nil] - pausedTime;
    self.beginTime = timeSincePause;
}