ipad - UIPinchGestureRecognizer position the pinched view between the two fingers

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];
}