Creating a boundary for objects - ios

Question

I am fairly new to objective-c and iOS programming in Xcode, and while making my first app I ran into the problem of creating barriers around the edge of the screen which the object cannot pass through. I tried placing objects at the boundary and using CGRectIntersectsRect, but the collision between the boundary and the main object is not registered. Does anyone know how I would go about in doing this?
this is the code that I have used for the object's movement:
const float rate= 0.025;
NSTimer *goLeft;
NSTimer *goRight;
- (IBAction)right {
goRight=[NSTimer scheduledTimerWithTimeInterval:rate
target:self
selector:#selector(goRight)
userInfo:nil
repeats:YES];
if (goRight == nil){
goRight=[NSTimer scheduledTimerWithTimeInterval:rate target:self selector:#selector(goRight) userInfo:nil repeats:YES];
}
}
- (IBAction)left{
goLeft=[NSTimer scheduledTimerWithTimeInterval:rate
target:self
selector:#selector(goLeft)
userInfo:nil
repeats:YES];
if (goLeft == nil){
goLeft=[NSTimer scheduledTimerWithTimeInterval:rate target:self selector:#selector(goLeft) userInfo:nil repeats:YES];
}
}
-(IBAction)stopLeft{
[goLeft invalidate];
goLeft=nil;
}
-(IBAction)stopRight{
[goRight invalidate];
goRight=nil;
}
-(void)goLeft{
_userToken.center =CGPointMake(_userToken.center.x -20, _userToken.center.y);
}
-(void)goRight{
_userToken.center =CGPointMake(_userToken.center.x +20, _userToken.center.y);
}
And for collision with object at left border (userToken and friendly are both UIImageView). However when they collide NSLog is not executed.
-(void)checkForCollision{
if (CGRectIntersectsRect(_userToken.frame, _friendly.frame)){
NSLog(#"Left Intersect");
}
}

Answer 1

You don't say what _userToken.frame and _friendly.frame are set to when you test - it's a good idea to log them while testing so you can manually verify.
An alternative approach, if the valid screen area is a rect, is to check the intersection of the token and the valid area and verify that it is equal to the token frame:
if (CGRectEqualToRect(CGRectIntersection(_userToken.frame, validFrame), _userToken.frame)) { ...
It's important when checking by any method that the frames are in the same coordinate space (that the views have the same superview, or that you convert them to the same common superview) - again, logging will help you to verify this.
Aside:
Trying to recreate the timer if it just failed is unlikely to help. And you should really check for an existing timer and invalidate it before creating a new timer (otherwise you could end up with multiple timers running and no way to stop them).