#744 – Keeping an Element within Window During Touch Manipulation

You can use the ManipulationDelta event handler to translate a user interface element in response to the user touching and dragging it.  In the previous code, there was nothing preventing the user from sliding the element off of the screen.

We can make the element stop when it hits a window boundary by checking its bounds against the bounds of its parent.  Below is the update code for the ManipulationDelta event handler that does the checking.  (See earlier example for the full code sample).

        private void Image_ManipulationDelta(object sender, ManipulationDeltaEventArgs e)
        {
            ManipulationDelta md = e.DeltaManipulation;
            Vector trans = md.Translation;

            Matrix m = imageTransform.Matrix;

            // Find center of element and then transform to get current location of center
            FrameworkElement fe = e.Source as FrameworkElement;
            Point center = new Point(fe.ActualWidth / 2, fe.ActualHeight / 2);
            center = m.Transform(center);

            // Check to see if element is at one of the edges of the window
            FrameworkElement feParent = fe.Parent as FrameworkElement;
            bool atEdge = false;
            if (feParent != null)
            {
                Rect feRect = fe.TransformToAncestor(feParent).TransformBounds(
                    new Rect(0.0, 0.0, fe.ActualWidth, fe.ActualHeight));
                atEdge = (feRect.Right + trans.X) > feParent.ActualWidth ||
                    (feRect.Bottom + trans.Y) > feParent.ActualHeight ||
                    (feRect.Left + trans.X) < 0 ||
                    (feRect.Top + trans.Y) < 0;
            }

            // Update matrix to reflect translation
            if (!atEdge)
                m.Translate(trans.X, trans.Y);

            imageTransform.Matrix = m;
            RaisePropertyChanged("ImageTransform");

            e.Handled = true;
        }

About Sean
Software developer in the Twin Cities area, passionate about .NET technologies. Equally passionate about my own personal projects related to family history and preservation of family stories and photos.

One Response to #744 – Keeping an Element within Window During Touch Manipulation

  1. Pingback: Dew Drop – January 29, 2013 (#1,489) | Alvin Ashcraft's Morning Dew

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