#742 – Using Touch Manipulation Events to Rotate an Element

In addition to using the touch manipulation events to handle translation of an element, we can use the same mechanisms to allow a user to rotate an element using touch.

We can do both translation and rotation in the same event handler.  The ManipulationDelta object gives us both a translation (vector) and a rotation (angle).  Both are automatically incorporated into the ManipulationDelta object, based on how the user is touching the screen.  The user can translate by sliding one finger around and can rotate by placing two fingers on the object and rotating it.

We transform the element by calling two different functions of the underlying Matrix, for both translation and rotation.

Here’s the XAML, containing a single Image control that we’ll interact with.

    <Canvas Name="canvMain" Background="Transparent">
        <Image Source="JamesII.jpg" Width="100"
               IsManipulationEnabled="True"
               RenderTransform="{Binding ImageTransform}"
               ManipulationStarting="Image_ManipulationStarting" ManipulationDelta="Image_ManipulationDelta"/>
    </Canvas>

Here is the source code, with the updated ManipulationDelta event handler.

    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;

            ImageTransform = new MatrixTransform();
        }

        private MatrixTransform imageTransform;
        public MatrixTransform ImageTransform
        {
            get { return imageTransform; }
            set
            {
                if (value != imageTransform)
                {
                    imageTransform = value;
                    RaisePropertyChanged("ImageTransform");
                }
            }
        }

        private void Image_ManipulationStarting(object sender, ManipulationStartingEventArgs e)
        {
            // Ask for manipulations to be reported relative to the canvas
            e.ManipulationContainer = canvMain;
        }

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

            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);

            // Update matrix to reflect translation/rotation
            m.Translate(trans.X, trans.Y);
            m.RotateAt(rotate, center.X, center.Y);

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

            e.Handled = true;
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void RaisePropertyChanged(string prop)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(prop));
        }
    }