WPF and Adorners

Question

Hi WPF users and developers.

I am new to WPF, I am actually a little bit confused by the adorners. so far I understand that the Adorner are rendered on Top of the element in a specific layer (AdornerLayer) which is defined in higher level of the visual tree.

Firstly

How can I create an AdornerLayer for a specific element, for example a shape.

Secondly

I guess I need to add the AdornerLayer to the visual children of the Adorned element in order to get valid hit testing on the Adorners in the AdornerLayer. How to do It?

Thank you in advance.

Answer 1

first of all you have to crate a class derived from Adorner like below. you can use thumb or thumbs to be rendered. here is code that I have implement before but I have changed a little bit for simplicity so there may be mistakes.

public class ResizingAdorner : Adorner
{
    Thumb topLeft;

    // To store and manage the adorner's visual children.
    VisualCollection visualChildren;

    // Initialize the ResizingAdorner.
    public ResizingAdorner(UIElement adornedElement)
        : base(adornedElement)
    {
        visualChildren = new VisualCollection(this);
        // Call a helper method to initialize the Thumbs
        BuildAdornerCorner(ref topLeft, Cursors.SizeNWSE);

        //these are events for the thumbs you may want it or not according to your needs//
        topLeft.PreviewMouseLeftButtonDown += new MouseButtonEventHandler(topLeft_PreviewMouseLeftButtonDown);
        topLeft.DragDelta += new DragDeltaEventHandler(HandleTopLeft);
    }
    //override this method for rendering
    protected override void OnRender(DrawingContext drawingContext)
    {
        Rect adornedElementRect = new Rect(this.AdornedElement.DesiredSize);
        // Some arbitrary drawing implements.
        SolidColorBrush RectBrush = new SolidColorBrush(Colors.Green);
        RectBrush.Opacity = 0.3;
        Pen renderPen = new Pen(RectBrush, 1.5);
        // Draw Rectangle
        drawingContext.DrawRectangle(null, renderPen, adornedElementRect);
    }

    // Arrange the Adorners.
    protected override Size ArrangeOverride(Size finalSize)
    {
        Rect adornedElementRect = new Rect(this.AdornedElement.DesiredSize);
        // desiredWidth and desiredHeight are the width and height of the element that's being adorned.  
        // These will be used to place the ResizingAdorner at the corners of the adorned element.  
        double desiredWidth = adornedElementRect.BottomRight.X;
        double desiredHeight = adornedElementRect.BottomRight.Y;
        // adornerWidth & adornerHeight are used for placement as well.
        double adornerWidth = adornedElementRect.TopLeft.X;
        double adornerHeight = adornedElementRect.TopLeft.Y;
        //Arrange PathPoints with the helper Methods
        topLeft.Arrange(new Rect(new Point(adornerWidth, adornerHeight), new Point(desiredWidth, desiredHeight)));
        // Return the final size.
        return finalSize;
    }
    // set some appearance properties, and add the elements to the visual tree//
    void BuildAdornerCorner(ref Thumb cornerThumb, Cursor customizedCursor)
    {
        Path adornered = AdornedElement as Path;
        if (cornerThumb != null) return;
        cornerThumb = new Thumb();
        // Set some arbitrary visual characteristics.
        cornerThumb.Cursor = customizedCursor;
        cornerThumb.Height = cornerThumb.Width = 10;
        cornerThumb.Opacity = 1;
        cornerThumb.Background = new SolidColorBrush(Colors.Black);

        visualChildren.Add(cornerThumb);
    }

    // UnScale Adorners. this part is optional
    public override GeneralTransform GetDesiredTransform(GeneralTransform transform)
    {
        if (this.visualChildren != null)
        {
            foreach (var thumb in this.visualChildren.OfType<Thumb>())
            {
                thumb.RenderTransform
                    = new ScaleTransform(1 / ScaleXC, 1 / ScaleYC);
                thumb.RenderTransformOrigin = new Point(0.5, 0.5);
            }
        }
        return base.GetDesiredTransform(transform);
    }
}

after that use this class as shown below to adorn element

    AdornerLayer ADL;  //consider this is as a public field somewhere in your class 
    ...
    ...
    ResizingAdorner A = new XamlEditor.ResizingAdorner(p); // p is an UIElement I my case
    ADL = AdornerLayer.GetAdornerLayer(p);
    ADL.Add(A);