Question

I am working with a form where some ComboBoxes can be created and removed programmatically.

When they are created, some triggers which target them are created and applied to a button:

    Dictionary<ComboBox, DataTrigger> triggers = new Dictionary<ComboBox, DataTrigger>();

    private void CreateTrigger(ComboBox box)
    {
        Style s = new Style(typeof(Button), MyButton.Style);
        foreach(TriggerBase aTrigger in MyButton.Style.Triggers)
            s.Triggers.Add(aTrigger);

        DataTrigger t = new DataTrigger 
          { 
            Binding = new Binding("SelectedItem") { Source = box }, 
            Value = null 
          };
        t.Setters.Add(new Setter(Button.IsEnabledProperty, false));
        s.Triggers.Add(t);

        triggers.Add(box, t);

        MyButton.Style = s;
    }

So far so good*. . . the problem is, what to do when the ComboBox is removed from the window. I need to remove the trigger from the button’s Style, since I no longer want the ComboBox to influence its behavior. I tried the most obvious option:

    private void RemoveTrigger(ComboBox box)
    {
        Style s = new Style(typeof(Button), MyButton.Style);
        foreach(TriggerBase aTrigger in MyButton.Style)
            if(aTrigger != triggers[box]) s.Triggers.Add(aTrigger);

        triggers.Remove(box);

        MyButton.Style = s;
    }

But this does not seem to do the job – if the trigger is removed while it is active, then the button stays disabled.

I had assumed that the button would re-evaluate its Style whenever it is given a new one. that seems to be happening when the trigger is added, but not when it’s being removed – what am I missing here?

EDIT: Changed code for adding/removing triggers as per the advice in H.B.’s comment. However, the problem in question remains.

EDIT 2: *Maybe not so far so good after all – I went on to try adding an additional ComboBox (and trigger) and discovered that adding a second trigger seems to break the first one. Using this code, only the most recently added trigger works. Should I be perhaps thinking of a FrameworkElement’s triggers as a write-once collection and finding a different way to achieve this kind of behavior?

Answer 1

It’s been a long time since this question was asked, but I figured I’d at least post how I resolved the issue for the sake of sharing:

I never did find a way to remove triggers that worked reliably. So instead, I added a property to my View which indicated how all the triggers would have evaluated, if they had existed, and hooked a DataTrigger up to this property.

public bool TriggerPoseur { get; set; }  // Actually notifies when it changes

In place of adding and removing triggers, created an handlers to watch the properties that the triggers would have watched:

public void ComboBoxDataContext_SelectedItemChanged(object sender, PropertyChangedEventArgs e) { //update TriggerPoseur }

This sidesteps all the hassle with creating and removing triggers. Instead there’s one trigger, and adding and removing event handlers works just fine.

(Hacky, yes.)