Question

I have a pinch gesture recognizer attached to an imageview from which I use pinches to enlarge and minimize the photo. Below is the code that I’m using in the delegate method:

- (void)scale:(UIPinchGestureRecognizer *)sender 
{
    if([sender state] == UIGestureRecognizerStateBegan) 
    {
        lastScale = [sender scale];
    }

    if ([sender state] == UIGestureRecognizerStateBegan || 
        [sender state] == UIGestureRecognizerStateChanged) 
    {

        CGFloat currentScale = [[[sender view].layer valueForKeyPath:@"transform.scale"] floatValue];


        CGFloat newScale = 1 -  (lastScale - [sender scale]) * (UIComicImageViewPinchSpeed); 
        newScale = MIN(newScale, minScale / currentScale);   
        newScale = MAX(newScale, maxScale / currentScale);
        CGAffineTransform transform = CGAffineTransformScale([[sender view] transform], newScale, newScale);
        [sender view].transform = transform;

        lastScale = [sender scale];   
    } 
}

I need to determine where the new center of the imageview frame will be before I actually perform the transformation. Is there anyway to determine this? Basically, I’m trying to halt the scaling if it’s about to move the image off the screen or close to it.

UPDATE

Thanks to Robin below for suggesting that method to figure out the transformed frame. The problem I’m running into now is that the frame becomes invalid after the transform is performed, and I need to keep track of the most recent frame in order to figure out the boundary of my image. Obviously, I can do this manually and store it in an instance variable, but wondering if there is a more “elegant” way to accomplish this?

Answer 1

Use CGRectApplyAffineTransform like this:

CGRect currentFrame = ....;

CGRect newFrame = CGRectApplyAffineTransform(currentFrame, transform);

// Then test if newFrame is within the limits you want