UIPinchGestureRecognizer position the pinched view between the two fingers - ipad

Question

I successfully implemented a pinch a zoom of a view. However, the view doesn't position itself where I wished it to be. For the stackoverflowers with an iPad, I would like my view to be centered like on the iPad Photos.app : when you pinch&zoom on an album, the photos present themselves in a view that is expanding. This view is approximately centered with the top right hand corner on the first finger and the bottom left hand finger on the other finger. I mixed it with a pan recognizer, but this way the user always has to pinch, and then pan to adjust.
Here are so graphic explanation, I could post a video of my app if that's unclear (no secret, i'm trying to reproduce the Photos.app of the iPad...)
So for an initial position of the fingers, begining zooming :
This is the actual "zoomed" frame for now. The square is bigger, but the position is below the fingers
Here is what I would like to have : same size, but different origin.x and y :
(sorry about my poor photoshop skills ^^)

Answer 1

You can get the CGPoint of the midpoint between two fingers via the following code in the method handlingPinchGesture.
CGPoint point = [sender locationInView:self];
My whole handlePinchGesture method is below.
/*
instance variables
CGFloat lastScale;
CGPoint lastPoint;
*/
- (void)handlePinchGesture:(UIPinchGestureRecognizer *)sender {
if ([sender numberOfTouches] < 2)
return;
if (sender.state == UIGestureRecognizerStateBegan) {
lastScale = 1.0;
lastPoint = [sender locationInView:self];
}
// Scale
CGFloat scale = 1.0 - (lastScale - sender.scale);
[self.layer setAffineTransform:
CGAffineTransformScale([self.layer affineTransform],
scale,
scale)];
lastScale = sender.scale;
// Translate
CGPoint point = [sender locationInView:self];
[self.layer setAffineTransform:
CGAffineTransformTranslate([self.layer affineTransform],
point.x - lastPoint.x,
point.y - lastPoint.y)];
lastPoint = [sender locationInView:self];
}

Answer 2

Have a look at the Touches sample project. Specifically these methods could help you:
// scale and rotation transforms are applied relative to the layer's anchor point
// this method moves a gesture recognizer's view's anchor point between the user's fingers
- (void)adjustAnchorPointForGestureRecognizer:(UIGestureRecognizer *)gestureRecognizer {
if (gestureRecognizer.state == UIGestureRecognizerStateBegan) {
UIView *piece = gestureRecognizer.view;
CGPoint locationInView = [gestureRecognizer locationInView:piece];
CGPoint locationInSuperview = [gestureRecognizer locationInView:piece.superview];
piece.layer.anchorPoint = CGPointMake(locationInView.x / piece.bounds.size.width, locationInView.y / piece.bounds.size.height);
piece.center = locationInSuperview;
}
}
// scale the piece by the current scale
// reset the gesture recognizer's rotation to 0 after applying so the next callback is a delta from the current scale
- (void)scalePiece:(UIPinchGestureRecognizer *)gestureRecognizer
{
[self adjustAnchorPointForGestureRecognizer:gestureRecognizer];
if ([gestureRecognizer state] == UIGestureRecognizerStateBegan || [gestureRecognizer state] == UIGestureRecognizerStateChanged) {
[gestureRecognizer view].transform = CGAffineTransformScale([[gestureRecognizer view] transform], [gestureRecognizer scale], [gestureRecognizer scale]);
[gestureRecognizer setScale:1];
}
}