Refresh WPF Command

Question

Does anyone know how I can force CanExecute to get called on a custom command (Josh Smith's RelayCommand)?

Typically, CanExecute is called whenever interaction occurs on the UI. If I click something, my commands are updated.

I have a situation where the condition for CanExecute is getting turned on/off by a timer behind the scenes. Because this is not driven by user interaction, CanExecute is not called until the user interacts with the UI. The end result is that my Button remains enabled/disabled until the user clicks on it. After the click, it is updated correctly. Sometimes the Button appears enabled, but when the user clicks it changes to disabled instead of firing.

How can I force an update in code when the timer changes the property that affects CanExecute? I tried firing PropertyChanged (INotifyPropertyChanged) on the property that affects CanExecute, but that did not help.

Example XAML:

<Button Content="Button" Command="{Binding Cmd}"/>

Example code behind:

private ICommand m_cmd;
public ICommand Cmd
{
    if (m_cmd == null)
        m_cmd = new RelayCommand(
            (param) => Process(),
            (param) => EnableButton);

    return m_cmd;
}

// Gets updated from a timer (not direct user interaction)
public bool EnableButton { get; set; }

Answer 1

Calling System.Windows.Input.CommandManager.InvalidateRequerySuggested() forces the CommandManager to raise the RequerySuggested event.

Remarks: The CommandManager only pays attention to certain conditions in determining when the command target has changed, such as change in keyboard focus. In situations where the CommandManager does not sufficiently determine a change in conditions that cause a command to not be able to execute, InvalidateRequerySuggested can be called to force the CommandManager to raise the RequerySuggested event.

Answer 2

I was aware of CommandManager.InvalidateRequerySuggested() a long time ago, and used it, but it wasn't working for me sometimes. I finally figured out why this was the case! Even though it doesn't throw like some other actions, you HAVE to call it on the main thread.

Calling it on a background thread will appear to work, but sometimes leave the UI disabled. I really hope this helps somebody, and saves them the hours I just wasted.

Answer 3

A workaround for that is binding IsEnabled to a property:

<Button Content="Button" Command="{Binding Cmd}" IsEnabled="{Binding Path=IsCommandEnabled}"/>

and then implement this property in your ViewModel. This also makes it a bit easier for the UnitTesting to work with the properties rather than commands to see if the command can be executed at a certain point of time.

I, personally, find it more convenient.