捏手势变焦

private float minScale = 1f;
private float maxScale = 4f;

private void SetUpPinchGestureZoom()
{
    imageViewToZoom.ContentMode = UIViewContentMode.ScaleAspectFit;

    scrollView.MaximumZoomScale = maxScale;
    scrollView.MinimumZoomScale = minScale;

    scrollView.ViewForZoomingInScrollView += (UIScrollView sv) => { return imageViewToZoom; };
}