Discrepancies between bounds, frame, and frame calculated through CGAffineTransformScale - iphone

Question

I have a simple app which places an imageview on the screen, and allows the user to move it and scale it using the pinch and pan gesture recognizers. In my uipangesturerecognizer delegate method (scale), I'm trying to calculate the value of the transformed frame before I actually perform the transformation using CGAffineTransformScale. I'm not getting the correct value however, and am getting some weird result that isn't inline with what the transformed frame should be. Below is my 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]) * (CropThePhotoViewControllerPinchSpeed);
newScale = MIN(newScale, CropThePhotoViewControllerMaxScale / currentScale);
newScale = MAX(newScale, CropThePhotoViewControllerMinScale / currentScale);
NSLog(#"currentBounds: %#", NSStringFromCGRect([[sender view] bounds]));
NSLog(#"currentFrame: %#", NSStringFromCGRect([[sender view] frame]));
CGAffineTransform transform = CGAffineTransformScale([[sender view] transform], newScale, newScale);
CGRect nextFrame = CGRectApplyAffineTransform([[sender view] frame], transform);
NSLog(#"nextFrame: %#", NSStringFromCGRect(nextFrame));
//NSLog(#"nextBounds: %#", NSStringFromCGRect(nextBounds));
[sender view].transform = transform;
lastScale = [sender scale];
}
}
Here is a printed result I get:
/* currentBounds: {{0, 0}, {316, 236.013}}
currentFrame: {{-115.226,-53.4392}, {543.452, 405.891}}
nextFrame: {{-202.566, -93.9454}, {955.382, 713.551}} */
With these results, the currentBounds coordinates is obviously 0,0 like it always is, and the size and width are that of the original image before it ever gets transformed. This value seems to stay the same no matter how many transformations I do.
currentFrame is the correct coordinates and the correct size and width based on the current state of the image in it's transformed state.
nextFrame is incorrect, it should match up with the currentFrame from the next set of values that gets printed but it doesn't.
So I have some questions:
1) Why is currentFrame displaying the correct value for the frame? I thought the frame was invalid after you perform transformations? This set of values was displayed after many enlargements and minimizations I was doing on the image with my fingers. It seems like the height and width of currentBounds is what I'd expect for the currentFrame.
2) Why is my next frame value being calculated incorrectly, and how do I accurately calculate the value of the transformed frame before I actually implement the transformation?

Answer 1

You need to apply your new transform to the view's untransformed frame. Try this:
CGAffineTransform transform = CGAffineTransformScale([[sender view] transform], newScale, newScale);
CGAffineTransform iTransform = CGAffineTransformInvert([[sender view] transform]);
CGRect rawFrame = CGRectApplyAffineTransform([[sender view] frame], iTransform);
CGRect nextFrame = CGRectApplyAffineTransform(rawFrame, transform);