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XAML reference control and properties in x:Array

Answer 1

+1 A:

The proper way to do this is indeed with a MultiBinding on the CommandParameter. You won't see it call your CanExecute method unless WPF is informed that the method could return a different value than it had already cached (via the CanExecuteChanged event).

Since you are relying on the parameter passed in to determine this, we have to raise the event when the parameter changes. Since we can't really determine that in the command, we can use another technique: tell WPF to poll our command anytime it polls UICommands. This is done by implementing your ICommand like so:

public class MyCommand : ICommand
{
    public void Execute(object parameter) { /* do stuff */ }
    public bool CanExecute(object parameter { /* determine if we can do stuff */ }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }
}

Obviously, this prevents you from using the Prism DelegateCommand, but this will respond to changes in the command parameters.

UPDATE

Another thing to consider is that the Document property on the RichTextBox isn't actually changing. Instead, when you type into it, the content of the FlowDocument changes. Since the property instances don't change, the converter won't get fired again, and the originally converted value will get stored in the CommandParameter property.

One of the ways to force the converter to be called again is to add a Binding to the MultiBinding that is bound to a property that will change every time the text of the RichTextBox changes.

A somewhat hacky solution would be to use the IsKeyboardFocusWithin property, as that will mimic the default binding behavior of TextBox.Text (i.e. when the TextBox loses focus, the Binding updates):

<MultiBinding Converter="{StaticResource FlowDocumentConverter}">
    <Binding ElementName="OrigText" Path="Document" />
    <Binding ElementName="ModifiedText" Path="Document" />
    <Binding ElementName="OrigText" Path="IsKeyboardFocusWithin" />
    <Binding ElementName="ModifiedText" Path="IsKeyboardFocusWithin" />
</MultiBinding>

Obviously, in your converter, you will need to ignore these additional values, as they aren't relevant to your conversion.

Abe Heidebrecht 2010-09-07 20:43:13

Abe, thanks for the feedback...so as a DelegateCommand via Prism it is not possible?

Aaron 2010-09-08 14:29:45

Added an edit up top to clear some things up...

Aaron 2010-09-08 15:41:04

My sample app originally used TextBoxes, as the converter was easier to write. I updated my answer to address the problem with RichTextBoxes.

Abe Heidebrecht 2010-09-08 19:53:17

Abe figured it out! Check the answer above in my initial post...marking you as the answer since you began dialog which can often times be the answer in and of itself. Thanks!

Aaron 2010-09-08 20:41:20

Ah, that does make sense. I can't remember where I read this, but I recall that the array is cleared after being passed to the converter. You can also just call the LINQ ToArray method, and have similar results. Glad I could offer some help!

Abe Heidebrecht 2010-09-08 21:19:37

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XAML reference control and properties in x:Array

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