#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;
        }

#743 – Using Touch Manipulation Events to Scale an Element

In the previous post, we used a ManipulationDelta object in the ManipulationDelta event handler to both translate and rotate a user interface element.  The user’s touch gestures for translation (sliding finger) and rotation (rotating two fingers) were automatically captured and available in the Translation and Rotation properties of the ManipulationDelta object.

We can also support scaling of an element using the ManipulationDelta event.  The ManipulationDelta object also contains a Scale property, which stores a Vector that indicates the target scale for the object (e.g. scale of 0.5 indicates 1/2 size).  This property is automatically set when a user uses two fingers on an element in a pinch or spread gesture, indicating that they want to zoom in or out of the element.

The sample code below supports all translation, rotation and scaling of an Image element.

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

Below is the code-behind for this sample.  Note that we now apply translation, rotation and scaling to the underlying matrix.

    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;
            Vector scale = md.Scale;

            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);
            m.ScaleAt(scale.X, scale.Y, 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));
        }
    }

#741 – Using Touch Manipulation Events to Translate an Element

You can use the touch manipulation events to translate an element, that is, to move it around on the screen.

To start with, you set the IsManipulationEnabled property of the element to true.  This allows the manipulation events to be fired.  You also handle both the ManipulationStarting and ManipulationDelta events.

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

Below is the code that allows the image to be moved around (translated) as the user moves their finger.  We bind the image’s render transform to a MatrixTransform object, which contains a matrix that allows us to scale, rotate or translate the image.  In our case, we modify the translation part of the matrix with the translation vector returned in the ManipulationDelta event.

    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;

            // Update matrix to reflect translation
            Matrix m = imageTransform.Matrix;
            m.Translate(trans.X, trans.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));
        }
    }