iphone development: how to create an animation by using NSTimer - iphone

Question

In my app I have this code in viewWillAppear. It simply animates random object from top of the screen to the bottom. The problem is I need to detect collision and what I have learnt so far is it is impossible to retrieve the coordinates at any time for an animation. so how can I implement it by using NSTimer? or shall I use NSTimer? I could not figure it out. Any clue will be appreciated.
-(void)viewWillAppear:(BOOL)animated
{
for (int i = 0; i < 100; i++) {
p = arc4random_uniform(320)%4+1;
CGRect startFrame = CGRectMake(p*50, -50, 50, 50);
CGRect endFrame = CGRectMake(p*50, CGRectGetHeight(self.view.bounds) + 50,
50,
50);
animatedView = [[UIView alloc] initWithFrame:startFrame];
animatedView.backgroundColor = [UIColor redColor];
[self.view addSubview:animatedView];
[UIView animateWithDuration:2.f
delay:i * 0.5f
options:UIViewAnimationCurveLinear
animations:^{
animatedView.frame = endFrame;
} completion:^(BOOL finished) {
[animatedView removeFromSuperview];
}];
}

Answer 1

I would ditch NSTimer for CADisplayLink to send a message to your target. NSTimer can cause stuttering animations since it doesn't synchronize with the device's screen refresh rate. CADisplayLink does exactly that and makes your animations run like butter.
Documentation:
http://developer.apple.com/library/ios/#documentation/QuartzCore/Reference/CADisplayLink_ClassRef/Reference/Reference.html
Ultimately, using CADisplayLink looks something like this:
- (id) init {
// ...
CADisplayLink *displayLink = [CADisplayLink displayLinkWithTarget:self selector:#selector(update:)];
[displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
// ...
}
- (void) update {
[myView updatePositionOrSomethingElse];
}

Answer 2

Your statement:
is it is impossible to retrieve the coordinates at any time for an
animation
seems incorrect did you check this?
It looks like you can use the CALayer's presentationLayer property to extract coordinates info during an animation:
CGRect movingFrame = [[yourView.layer presentationLayer] frame];
I will use this info to check time to time if there is a collision during the animation. So I will use the timer to check the collision status not for animating the views.

Answer 3

You can have your objects animate in small increments (in a loop may be). Every time you complete your animation, you can get the coordinates.

Answer 4

I think you could use the beginAnimations - commitAnimations syntax in a for cycle it could count for eg. till 10, so after each cycle you can check for collision
CGRect incrementedViewFrame;
for(int i = 0; i < 10; ++i)
{
incrementedViewFrame = CGRectMake(/*calculate the coords here*/);
if(collision)
{
//do stuff
}
else
{
//do an other animation cycle
[UIView beginAnimations:nil context:NULL];
[UIView setAnimationBeginsFromCurrentState:YES];
[UIView setAnimationDuration:0.1f];
[[self viewToAnimate]setFrame:incrementedViewFrame];
[UIView commitAnimations];
}
}
I hope that helps!