Resume animation rotation on leaving the background state to becoming the active app.

resumeRotationOnEnterForeground.m

- (void)registerForAppStateNotifications
{
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidEnterBackground) name:UIApplicationDidEnterBackgroundNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillEnterForeground) name:UIApplicationWillEnterForegroundNotification object:nil];
}

- (void)applicationDidEnterBackground
{
    self.suspendedRotation = [self.imageView.layer.presentationLayer valueForKeyPath:@"transform.rotation.z"];
}

- (void)applicationWillEnterForeground
{
    // restart animation from where we left off
    CABasicAnimation* rotateAnimation = [CABasicAnimation animationWithKeyPath:@"transform.rotation.z"];
    rotateAnimation.fromValue = self.suspendedRotation;
    rotateAnimation.byValue = @(M_PI * 2);
    
    [self.imageView.layer addAnimation:rotateAnimation forKey:@"rotate"];

}