EyeballAnimation Demo App

initializing_animation_timers.m

- (void) initializeTimerWithNSTimer
{
    CGFloat interval = 1.0f / 50.0f;
    [NSTimer scheduledTimerWithTimeInterval:interval
                                     target:self
                                   selector:@selector(animateBallNSTimer:)
                                   userInfo:nil
                                    repeats:YES];
}

- (void) initializeTimerWithCADisplayLink
{
    _timer = [CADisplayLink displayLinkWithTarget:self selector:@selector(animateBallCADisplayLink)];
    _timer.frameInterval = 2;
    [_timer addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}

performing_animation_steps.m

- (void) performAnimationStep
{
    //
    // reposition the ball
    //
    CGPoint ballCenter = self.ballImageView.center;
    CGFloat newX = ballCenter.x + _ballMovement.x;
    CGFloat newY = ballCenter.y + _ballMovement.y;
    self.ballImageView.center = CGPointMake(newX, newY);

    //
    // rotate the ball
    //
    self.ballImageView.transform =
        CGAffineTransformRotate(self.ballImageView.transform, DegreesToRadians(_ballRotation));

    //
    // change the opacity of the ball
    //
    CGFloat newAlpha = self.ballImageView.alpha + _ballAlphaChange;
    self.ballImageView.alpha = newAlpha;

    //
    // change move direction and rotation direction when
    // the ball collides one of the borders of the screen
    //
    ballCenter = self.ballImageView.center;
    if ((ballCenter.x > 300) || (ballCenter.x < 20)) {
        _ballMovement.x *= -1;
        _ballRotation *= -1;
    }
    if ((ballCenter.y > 440) || (ballCenter.y < 20)) {
        _ballMovement.y *= -1;
        _ballRotation *= -1;
    }
    
    //
    // check bounds and reverse the opacity change direction
    //
    if ((newAlpha >= 1.0f) || newAlpha <= 0.15f) {
        _ballAlphaChange *= -1;
    }
}