Trouble with CGRectIntersectsRect action - objective-c

Question

.m:
-(void)accelerometer:(UIAccelerometer *)accelerometer
didAccelerate:(UIAcceleration *)acceleration
{
if(CGRectIntersectsRect(ball.bounds , fang.bounds))
{
UIImage *image = [UIImage imageNamed:#"guy.png"];
UIImageView *imageview = [[UIImageView alloc] initWithImage:image];
[self.view addSubView:imageview];
[imageview release];
}
NSLog(#"x : %g", acceleration.x);
NSLog(#"y : %g", acceleration.y);
NSLog(#"z : %g", acceleration.z);
delta.y = acceleration.y * 60;
delta.x = acceleration.x * 60;
ball.center = CGPointMake(ball.center.x + delta.x, ball.center.y + delta.y);
// Right
if(ball.center.x < 0) {
ball.center = CGPointMake(320, ball.center.y);
}
// Left
if(ball.center.x > 320) {
ball.center = CGPointMake(0, ball.center.y);
}
// Top
if(ball.center.y < 0) {
ball.center = CGPointMake(ball.center.x, 460);
}
// Bottom
if(ball.center.y > 460) {
ball.center = CGPointMake(ball.center.x, 0);
}
}
.h:
UIImageView *ball;
IBOutlet UIImageView *fang;
MY TROUBLES:
When I open the app, the image #"guy.png" reveals itself without anything touching the fang. I need help. ALSO//""**My accelerometer is SO choppy and barely works with this .m code :
if(CGRectIntersectsRect(ball.bounds , fang.bounds))
{
UIImage *image = [UIImage imageNamed:#"endScreenImage.png"];
UIImageView *imageview = [[UIImageView alloc] initWithImage:image];
[self.view addSubView:imageview];
[imageview release];
}
HELP PLEASE

Answer 1

A few issues:
CGRectIntersectsRect() is returning YES because you're comparing the bounds of your objects, not the frames. The bounds of an object is the rectangle describing how the object sees its own dimensions. It generally has its origin at (0,0), and the size is the size of the view. Since both views have a bounds starting at (0,0), the bounds rectangles do intersect.
What you really want is the frame of the views. The frame of a view represents the space occupied in that view, with respect to its superview. That is:
if (CGRectIntersectsRect(ball.frame, fang.frame)) { // etc. }
The frame represents the location of the view. The bounds represents the location of the content within the view itself. When you're comparing the position of two views, you almost always want to use frame.
As currently written, your code will add a new subview every time there's an intersection, but will never remove the old subview. Since the accelerometer can fire multiple times every second, this can result in a lot of views being added to the view hierarchy. That could significantly impact performance, and may be why you're seeing such poor performance. I would recommend instead creating a single UIImageView as an instance variable, and then use the hidden property to control whether it's visible or not.
Thus, I'd recommend you modify your code as follows:
if (CGRectIntersectsRect(ball.frame, fang.frame)) {
guyView.hidden = NO;
}
else {
guyView.hidden = YES;
}