iPhone Game Development, collision detection that creates a wall effect
-(void)goDown{
mainSprite.center = CGPointMake(mainSprite.center.x, mainSprite.center.y+5);
}

-(IBAction)down{
    [self downAnimation];
    goDown = [NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector(goDown) userInfo:nil repeats:YES];
    if (goDown == nil) {
        goUp = [NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector(goDown) userInfo:nil repeats:YES];

    }
}

-(void)collision {
    if (CGRectIntersectsRect(mainSprite.frame, collisionImage.frame)) {

    }
}

- (void)goDown {
   float delta = [self checkCollision];
   mainSprite.center = CGPointMake(mainSprite.center.x, mainSprite.center.y + delta);
}

- (float)checkCollision {
   CGRect testFrame = CGRectMake(mainSprite.frame.origin.x, mainSprite.frame.origin.y + 5.0f, mainSprite.frame.size.width, mainSprite.frame.size.height);
   if (CGRectIntersectsRect(testFrame, collisionImage.frame)) {
       return 0.0f;
       // stop the sprite from moving anymore so it doesn't actually intersect, just stops "on top"
    }
    return 5.0f;
    // if they have not collided then just continue on your way