WPF create event trigger programmatically

Question

I have a canvas where i add more shapes programmatically. And for one of the shapes (a Path) i want to add a fill color that will flicker at every second (change from red to blue and back). I found an example on how to do that from xaml:

<Ellipse Fill="Red">
<Ellipse.Triggers>
    <EventTrigger RoutedEvent="Ellipse.Loaded">
        <EventTrigger.Actions>
            <BeginStoryboard>
                <Storyboard>
                    <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Ellipse.Fill).(SolidColorBrush.Color)"
                                                  Duration="0:0:2"
                                                  FillBehavior="HoldEnd"
                                                  RepeatBehavior="Forever">
                        <ColorAnimationUsingKeyFrames.KeyFrames>
                            <DiscreteColorKeyFrame KeyTime="0:0:0" Value="Red"/>
                            <DiscreteColorKeyFrame KeyTime="0:0:1" Value="Blue"/>
                        </ColorAnimationUsingKeyFrames.KeyFrames>
                    </ColorAnimationUsingKeyFrames>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger.Actions>                    
    </EventTrigger>
</Ellipse.Triggers>

But when i do it from the code i receive an ArgumentNullException: "{"Value cannot be null.\r\nParameter name: routedEvent"}"

This is my code:

 var sheetPath = new Path
        {
            Stroke = Brushes.Black,
            Fill = !isSelectedSheet ? Brushes.MediumSlateBlue : GetInvertedColor(Brushes.MediumSlateBlue),
            StrokeThickness = _lineWidth,
            HorizontalAlignment = HorizontalAlignment.Left,
            VerticalAlignment = VerticalAlignment.Center,
            Data = CreatePathGeometry(contour, height)
        };

        var colorAnimationUsingKeyFrames = new ColorAnimationUsingKeyFrames
        {
            Duration = new Duration(new TimeSpan(0, 0, 0, 300)),
            RepeatBehavior = RepeatBehavior.Forever,
            FillBehavior = FillBehavior.HoldEnd
        };

        colorAnimationUsingKeyFrames.KeyFrames.Add(new DiscreteColorKeyFrame
        {
            KeyTime = KeyTime.FromTimeSpan(new TimeSpan(0, 0, 0, 0, 100)),
            Value = Colors.Red
        });
        colorAnimationUsingKeyFrames.KeyFrames.Add(new DiscreteColorKeyFrame
        {
            KeyTime = KeyTime.FromTimeSpan(new TimeSpan(0, 0, 0, 0, 200)),
            Value = Colors.Blue
        });

        var storyboard = new Storyboard();
        storyboard.Children.Add(colorAnimationUsingKeyFrames);

        Storyboard.SetTargetProperty(storyboard.Children[0], new PropertyPath("(Path.Fill).(SolidColorBrush.Color)"));
        var beginStoryboard = new BeginStoryboard();

        beginStoryboard.Storyboard = storyboard;

        var eventTrigger = new EventTrigger();
        eventTrigger.Actions.Add(beginStoryboard);

        sheetPath.Triggers.Add(eventTrigger);

        canvas.Children.Add(sheetPath);

"a canvas where i add more shapes programmatically" - don't do that. Use an ItemsControl instead, as shown e.g. here: stackoverflow.com/a/40190793/1136211 — Clemens
– Clemens, Commented Aug 29, 2018 at 13:32
I can't do that. I have to create them dynamically. I have also text, some of the shapes are overlapping, i have to parse some objects in order to create them... — XamWaZZ
– XamWaZZ, Commented Aug 29, 2018 at 13:38
Anyway, do you have a suggestion for my problem? Or at least what's the problem in my code and why i get that exception? — XamWaZZ
– XamWaZZ, Commented Aug 29, 2018 at 13:38
All that doesn't mean you can't use an ItemsControl. You can still add text and have overlapping shapes, and you can of course also parse view model objects. Sooner or later MVVM turns out to be the superior approach. — Clemens
– Clemens, Commented Aug 29, 2018 at 17:31
I had a problem with the dependency properties and that's the way i chose MPV pattern. — XamWaZZ
– XamWaZZ, Commented Aug 30, 2018 at 7:33

mm8 · Accepted Answer · 2018-08-29 13:42:13Z

1

Set the RoutedEvent property of the EventTrigger:

var eventTrigger = new EventTrigger();
eventTrigger.RoutedEvent = LoadedEvent;
eventTrigger.Actions.Add(beginStoryboard);

answered Aug 29, 2018 at 13:42

mm8

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3 Comments

XamWaZZ Over a year ago

But since i create everything from the code, i am not in a view context, so i don't have a loadedEvent. Is there another way in order to start the eventTrigger?

mm8 Over a year ago

@XamWaZZ: LoadedEvent is a static member of the FrameworkElement class; eventTrigger.RoutedEvent = FrameworkElement.LoadedEvent;

XamWaZZ Over a year ago

Ah, great. Thanks. Now i have only a small problem. I want to change the color for an infinite number of times. "RepeatBehavior = RepeatBehavior.Forever". This line is not responsible for that? Cause right now i have only 1 flicker, from red to blue and that's all.

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