WPF OnRender不起作用

Question

I write a control that derived a class from the FrameworkElement and overrode its OnRender method. Inside this method, there is a DrawLine animation by providing an AnimationClock instance to drawingContext.DrawLine method.

It works well if there are only a few lines, and it doesn't work any more if there are hundreds of lines. AffectsRender and InvalideVisual() method also doesn't work, it can't call the OnRender method. If there are a middling number of lines, it doesn't work, but if I resize the window, it works! And I found that if I change the DependencyProperty (which has the AffectsRender flag) and add a new visual at the same time, it works no matter how many lines are in the window.

I have looked up some articles that said that WPF is a retained graphics and the OnRender method is invoked by WPF itself. It is WPF that determine when the OnRender need to be invoked. How can I do to tell WPF to invoke OnRender?

Here are the codes:

class PipeLine:FrameworkElement
{                
    static PipeLine()
    {    
        SignalInputProperty = DependencyProperty.Register("SignalInput", typeof(int), typeof(PipeLine),
            new FrameworkPropertyMetadata(0, FrameworkPropertyMetadataOptions.AffectsRender));
        SignalOutputProperty = DependencyProperty.Register("SignalOutput", typeof(bool), typeof(PipeLine),
            new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.AffectsRender));            
    }       

    public static readonly DependencyProperty SignalInputProperty;
    public int SignalInput
    {
        get { return (int)GetValue(SignalInputProperty); }
        set
        {
            SetValue(SignalInputProperty, value);
        }
    }

    public static readonly DependencyProperty SignalOutputProperty;
    public bool SignalOutput
    {
        get { return (bool)GetValue(SignalOutputProperty); }
        set { SetValue(SignalOutputProperty, value); }
    }


    protected override void OnRender(System.Windows.Media.DrawingContext drawingContext)
    {
        base.OnRender(drawingContext);

        Point startPH = new Point(0, ActualHeight / 2);
        Point endPH = new Point(ActualWidth, ActualHeight / 2);


        Point startPH1 = new Point(-this.ActualHeight / 2, this.ActualHeight / 2);
        Point endPH1 = new Point(this.ActualWidth + this.ActualHeight / 2, this.ActualHeight / 2);


        PointAnimation animation = new PointAnimation();
        animation.From = startPH;
        animation.To = endPH;
        animation.Duration = new Duration(TimeSpan.FromSeconds(2.0));
        AnimationClock clock = animation.CreateClock();

        Pen pen1 = new Pen(Brushes.DarkGray, ActualHeight);
        Pen pen2 = new Pen(Brushes.LightSkyBlue, ActualHeight);

        if (SignalInput == 2  && !SignalOutput)
        {
            drawingContext.DrawLine(pen1, startPH, endPH);
            drawingContext.DrawLine(pen2, startPH, null, endPH, clock);
        }
        else 
        {
            drawingContext.DrawLine(pen1, startPH, endPH);
            SignalOutput = false;
        }
    }

    void clock_Completed(object sender, EventArgs e)
    {
        this.SignalOutput = true;
    }
}

Answer 1

In WPF OnRender method is called when Arrange or Measure occurs. So I guess this is the answer to second part of your question.

Answer 2

The problem is that SignalOutput value does not change . It gets True on the first tick . And then it gets true again and again .

A dependency property isn't updated if OldValue == NewValue.

So the DP does not affect the Render because it was not effected itself.

void clock_Completed(object sender, EventArgs e)
{
    this.InvalidateVisual();
}