iOS 限制UIImageView放大和缩小

编辑图片的时候通常需要设置图片的大小 

http://stackoverflow.com/a/5446348/2177332

- (void)pinchGestureDetected:(UIPinchGestureRecognizer *)recognizer{
    UIGestureRecognizerState state = [recognizer state];
    
    if(state == UIGestureRecognizerStateBegan) {
        // Reset the last scale, necessary if there are multiple objects with different scales
        //获取最后的比例
        _lastScale = [recognizer scale];
    }
    
    if (state == UIGestureRecognizerStateBegan ||
        state == UIGestureRecognizerStateChanged) {
        //获取当前的比例
        CGFloat currentScale = [[[recognizer view].layer valueForKeyPath:@"transform.scale"] floatValue];
        
        // Constants to adjust the max/min values of zoom
        //设置最大最小的比例
        const CGFloat kMaxScale = 3.0;
        const CGFloat kMinScale = 1.0;
        //设置
        //假设上次比例是1 这次比例是4 这是放大
        //newScale = 4
        //最小 min(4,3/4) 就是3/4
        //最大 max(3/4,1/4) 就是3/4   4*3/4 = 3
        
        //假设上次比例是3 这次比例是1/2 这是缩小
        //newScale = -3/2
        //最小 min(-3/2,3/1/2) = 就是-3/2
        //最大 max(-3/2,1/1/2) = 就是2  1/2*2 = 1
        //获取上次比例减去想去得到的比例
        CGFloat newScale = 1 -  (_lastScale - [recognizer scale]);
        newScale = MIN(newScale, kMaxScale / currentScale);
        newScale = MAX(newScale, kMinScale / currentScale);
        CGAffineTransform transform = CGAffineTransformScale([[recognizer view] transform], newScale, newScale);
        [recognizer view].transform = transform;
        // Store the previous scale factor for the next pinch gesture call
        //获取最后比例 下次再用
        _lastScale = [recognizer scale];
    }
}