Dragging an image in WPF

Question

I\'m trying to create a WPF application where I can drag an image around.

Currently I have an image placed in the center of the window, and I\'m thinking of using th

Answer 1

ok, here's an attached property "behaviour" that you can use to make any element draggable provided it's on a canvas:

public class DraggableExtender : DependencyObject
{
    // This is the dependency property we're exposing - we'll 
    // access this as DraggableExtender.CanDrag="true"/"false"
    public static readonly DependencyProperty CanDragProperty =
        DependencyProperty.RegisterAttached("CanDrag",
        typeof(bool),
        typeof(DraggableExtender),
        new UIPropertyMetadata(false, OnChangeCanDragProperty));

    // The expected static setter
    public static void SetCanDrag(UIElement element, bool o)
    {
        element.SetValue(CanDragProperty, o);
    }

    // the expected static getter
    public static bool GetCanDrag(UIElement element)
    {
        return (bool) element.GetValue(CanDragProperty);
    }

    // This is triggered when the CanDrag property is set. We'll
    // simply check the element is a UI element and that it is
    // within a canvas. If it is, we'll hook into the mouse events
    private static void OnChangeCanDragProperty(DependencyObject d, 
              DependencyPropertyChangedEventArgs e)
    {
        UIElement element = d as UIElement;
        if (element == null) return;

        if (e.NewValue != e.OldValue)
        {
            if ((bool)e.NewValue)
            {
                element.PreviewMouseDown += element_PreviewMouseDown;
                element.PreviewMouseUp += element_PreviewMouseUp;
                element.PreviewMouseMove += element_PreviewMouseMove;
            }
            else
            {
                element.PreviewMouseDown -= element_PreviewMouseDown;
                element.PreviewMouseUp -= element_PreviewMouseUp;
                element.PreviewMouseMove -= element_PreviewMouseMove;
            }
        }
    }

    // Determine if we're presently dragging
    private static bool _isDragging = false;
    // The offset from the top, left of the item being dragged 
    // and the original mouse down
    private static Point _offset;

    // This is triggered when the mouse button is pressed 
    // on the element being hooked
    static void element_PreviewMouseDown(object sender,
            System.Windows.Input.MouseButtonEventArgs e)
    {
        // Ensure it's a framework element as we'll need to 
        // get access to the visual tree
        FrameworkElement element = sender as FrameworkElement;
        if (element == null) return;

        // start dragging and get the offset of the mouse 
        // relative to the element
        _isDragging = true;
        _offset = e.GetPosition(element);
    }

    // This is triggered when the mouse is moved over the element
    private static void element_PreviewMouseMove(object sender, 
              MouseEventArgs e)
    {
        // If we're not dragging, don't bother - also validate the element
        if (!_isDragging) return;

        FrameworkElement element = sender as FrameworkElement;
        if (element == null) return;

        Canvas canvas = element.Parent as Canvas;
        if( canvas == null ) return;

        // Get the position of the mouse relative to the canvas
        Point mousePoint = e.GetPosition(canvas);

        // Offset the mouse position by the original offset position
        mousePoint.Offset(-_offset.X, -_offset.Y);

        // Move the element on the canvas
        element.SetValue(Canvas.LeftProperty, mousePoint.X);
        element.SetValue(Canvas.TopProperty, mousePoint.Y);
    }

    // this is triggered when the mouse is released
    private static void element_PreviewMouseUp(object sender, 
            MouseButtonEventArgs e)
    {
        _isDragging = false;
    }

}

You can then use this in your XAML by importing the namespace your class is contained in (something like this:)

<Window x:Class="WPFFunWithDragging.Window1"
        xmlns:local="clr-namespace:WPFFunWithDragging" .. >

And then you can just set DraggableExtender.CanDrag="true" on elements to drag around:

<Canvas>
    <Image Source="Garden.jpg" 
           Width="50" 
           Canvas.Left="10" Canvas.Top="10" 
           local:DraggableExtender.CanDrag="true"/>
</Canvas>

Hope this is of some use :)

Answer 2

The solution that deepcode.co.uk provided is ok; however, I missed some functonalities when imlementing it, therefore I'm expanding on his solution.

When there are 2 or more elements within the canvas, it is necessary to also set:

    private void Element_PreviewMouseDown(object sender, MouseEventArgs e)
    {
        element.CaptureMouse();
    }

    private void Element_PreviewMouseUp(object sender, MouseButtonEventArgs e)
    {
        element.ReleaseMouseCapture();
    }

So that the dragging action would influence only the currently active element.

One may also not want an element to go beyond the borders of the canvas when dragging:

    if (mousePoint.X > 0 && mousePoint.X + element.ActualWidth <= canvas.ActualWidth &&
        mousePoint.Y > 0 && mousePoint.Y + element.ActualHeight <= canvas.ActualHeight)
    {
        element.SetValue(Canvas.LeftProperty, mousePoint.X);
        element.SetValue(Canvas.TopProperty, mousePoint.Y);
    }

And in case you want the active element to always appear on top:

    private void Element_PreviewMouseDown(object sender, MouseEventArgs e)
    {
        Panel.SetZIndex(element, 1);
    }

    private void Element_PreviewMouseUp(object sender, MouseButtonEventArgs e)
    {
        Panel.SetZIndex(element, 0);
    }