Implement Tap-to-Zoom

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This recipe shows how to load an image into a scroll view and allow the user to double-tap to zoom.

Recipe

To display an image inside a scroll view and support the double-tap to zoom gesture:

  1. Add the image to your Xamarin.iOS project and ensure the Build Action is set to BundleResource. The example code loads the image “halloween.jpg”

  2. Declare class level fields for a UIScrollView and UIImageVIew

UIScrollView scrollView;
UIImageView imageView;
  1. Create a UIScrollView and it to the View Controller:
scrollView = new UIScrollView (
new CGRect (0, 0, View.Frame.Width
, View.Frame.Height - NavigationController.NavigationBar.Frame.Height));
View.AddSubview (scrollView);
  1. Create a UIImageView and it to the ScrollView:
imageView = new UIImageView (UIImage.FromFile ("halloween.jpg"));
scrollView.ContentSize = imageView.Image.Size;
scrollView.AddSubview (imageView);
  1. Set the zoom properties:
scrollView.MaximumZoomScale = 3f;
scrollView.MinimumZoomScale = .1f;
scrollView.ViewForZoomingInScrollView += (UIScrollView sv) => { return imageView; };
  1. Create a UITapGestureRecognizer and configure it for two taps, then attach it to the Scroll View:
UITapGestureRecognizer doubletap = new UITapGestureRecognizer(OnDoubleTap) {
NumberOfTapsRequired = 2 // double tap
};
scrollView.AddGestureRecognizer(doubletap); // detect when the scrollView is double-tapped
  1. Implement the handler to change the zoom level when a double-tap is detected on the Scroll View:
private void OnDoubleTap (UIGestureRecognizer gesture) {
            if (scrollView.ZoomScale >= 1)
                scrollView.SetZoomScale(0.25f, true);
            else
                scrollView.SetZoomScale(2f, true);
        }

These screenshots show the image zoomed out and zoomed in:

Xamarin Workbook

If it's not already installed, install the Xamarin Workbooks app first. The workbook file should download automatically, but if it doesn't, just click to start the workbook download manually.