在触摸坐标处获取UI元素

Question

I'm sure I've read that there is a way of getting co-ordinates on TouchDown in a WPF app and finding out what UI elements are 'underneath' this touch. Can anyone help?

Answer 1

You have to do a hit test against the visual tree.

Here is an example with mouse click (but touch is more or less the same thing in this respect):

// Respond to the left mouse button down event by initiating the hit test.
private void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    // Retrieve the coordinate of the mouse position.
    Point pt = e.GetPosition((UIElement)sender);

    // Perform the hit test against a given portion of the visual object tree.
    HitTestResult result = VisualTreeHelper.HitTest(myCanvas, pt);

    if (result != null)
    {
        // Perform action on hit visual object.
    } 
}

Other overload of HitTest can give you multiple hit visuals.

Answer 2

Let your UI elements extend UIElement class like this:

class MyUIElement : UIElement
    {
        protected override void OnManipulationStarting(System.Windows.Input.ManipulationStartingEventArgs e)
        {
            base.OnManipulationStarting(e);
            UIElement involvedUIElement = e.Source as UIElement;

            // to cancel the touch manipulaiton:
            e.Cancel();
        }
    }

involvedUIElement should hold the UI elements that raised the touch event, if you need to cancel the manipulation for specific elements you just need to call e.Cancel();

I hope this will help!