Problem with collision images, collision before touch - iphone

Question

Hi everyone I'm french so scuse me for my english.My problem is the following: I have an image in the center of the screen called viewToRotate, then I have an image called flakeView that is created outside of the screen and then it is moving to the center, and every second a Timer does this(create a flakeView outside the screen and then move it to the center of the screen).
What I wanted to do was : if flakeView and viewToRotate collide reduce the alpha of viewToRotate to 0.5. But when flakeView appears on the screen the action of reducing the alpha is called without the collision of viewToRotate and flakeView, so they collide before they touches. I don't know why. How can I solve this please . Here is the code :
UIImageView* flakeView = [[[UIImageView alloc] initWithImage:flakeImage] autorelease];
// use the random() function to randomize up our flake attributes
int startY = round(random() % 320);
// set the flake start position
flakeView.center = CGPointMake(490, startY);
flakeView.alpha = 1;
// put the flake in our main view
[self.view addSubview:flakeView];
[UIView animateWithDuration:7
animations:^{
// set the postion where flake will move to
flakeView.center = viewToRotate.center;
}];
}
-(void)checkCollision{
if(CGRectIntersectsRect(flakeView.frame, viewToRotate.frame) == 1)
{
viewToRotate.alpha=0.5;
}
}

Answer 1

I would not suggest using a timer on viewDidLoad. I believe that you should set the timer at some other function. Even if you do not use viewDidLoad for anything else.
The other thing is the UIView animations. Even if you have a timer at every 60fps the attributes of the animated objects are not changed by the animation. So before you create the animation you set those properties once. And when you create the animation you set the same properties for the second time. And that's it. From now on while the animation is running if you do checks on the properties you will always get the second values.
To have a working code one option is to create the UIView animations (and perform the property checks) for every step at a certain time interval. Or use OpenGL.

Answer 2

Before getting too deep into this issue, please also make sure that your viewToRotate.frame is actually sized the way you expect it (see my comment).
Once you are certain about those attributes, you might want to check the collision on the current presentationLayer of your animated objects and not on their original state.
UIView.layer.presentationLayer
Returns a copy of
the layer containing all properties as
they were at the start of the current
transaction, with any active
animations applied.
So you might adapt your collision detection like this:
-(void)checkCollision
{
if(CGRectIntersectsRect(flakeView.layer.presentationLayer.frame, viewToRotate.layer.presentationLayer.frame) == 1)
{
viewToRotate.alpha=0.5;
}
}
Also consider using the CoreAnimation Layer method hitTest: instead of your handmade collision detection.
hitTest: Returns the farthest
descendant of the receiver in the
layer hierarchy (including itself)
that contains a specified point.

Answer 3

These are the following things you would want to check.
Dont init with image, use init with frame and then do
flakeView.image = ----; Because your image could be wide enough
initially , and hence colliding from the time you initialize.
Try alternatives to .center, like .frame.size.x , because I have had
problems with .center as well
You could probably NSLog the bounds of both ur imageviews before and
after reducing alpha