"PropertyChanged 后 ICommand CanExecute 不触发？"

Question

I got a WPF application that shows a button bound to a command like that:

<Button Command="{Binding Path=TestrunStartCommand}" Content="GO!">

The command is defined like that:

public ICommand TestrunStartCommand
{
    get { return new RelayCommand(TestrunStartExecute, () => !IsTestrunInProgress); }
}

public bool IsTestrunInProgress
{
    get{
        return _isTestrunInProgress;
    }
    set{
        _isTestrunInProgress = value;
        RaisePropertyChanged(IsTestrunInProgressPropertyName);
    }
}   

The problem is, the button won't be enabled immediately after I set IsTestrunInProgress to false, but only after I click inside the application window .

Could you help me understand this behaviour and show me how to fix this?

Further reading: wpf command pattern - when does it query canexecute

Answer 1

The ICommand interface exposes an event ICommand.CanExecuteChanged which is used to inform the UI when to re-determine the IsEnabled state of command driven UI components.

Depending upon the implementation of the RelayCommand you are using, you may need to raise this event; Many implementations expose a method such as RelayCommand.RaiseCanExecuteChanged() which you can invoke to force the UI to refresh.

Some implementations of the RelayCommand make use of CommandManager.RequerySuggested , in which case you will need to call CommandManager.InvalidateRequerySuggested() to force the UI to refresh.

Long story short, you will need to call one of these methods from your property setter.

Update

As the state of the button is being determined when the active focus is changing, I believe the CommandManager is being used. So in the setter of your property, after assigning the backing field, invoke CommandManager.InvalidateRequerySuggested() .

Update 2

The RelayCommand implementation is from the MVVM light toolkit. When consumed from WPF/.NET, the implementation wraps the methods and events exposed from the CommandManager . This will mean that these commands work automagically in the majority of situations (where the UI is altered, or the focused element is changed). But in a few cases, such as this one, you will need to manually force the command to re-query. The proper way to do this using this library would be to call the RaiseCanExecuteChanged() method on the RelayCommand .

Answer 2

This is so important and easy to miss, I am repeating what @Samir said in a comment. Mr Laurent Bugnion wrote in his blog :

In WPF 4 and WPF 4.5, however, there is a catch: The CommandManager will stop working after you upgrade MVVM Light to V5. What you will observe is that your UI elements (buttons, etc) will stop getting disabled/enabled when the RelayCommand's CanExecute delegate returns false.

If you are in a hurry, here is the fix: In any class that uses the RelayCommand, replace the line saying:

using GalaSoft.MvvmLight.Command;

with:

using GalaSoft.MvvmLight.CommandWpf;

Answer 3

You can try with CommandManager.InvalidateRequerySuggested .

Anyway this did not help me sometimes in the past. For me the best solution turned out to be to bind the boolean property to the Button.IsEnabled dependency property.

In your case something like

IsEnabled={Binding IsTestrunInProgress}

Answer 4

The issue is, the ICommand Property TestrunStartCommand is always returning a new command object whenever it is accessed.

A simple fix is to create the ICommand object once and use it again and again.

private ICommand _testRunCommand = null;
public ICommand TestrunStartCommand
{
    get 
    { 
        return _testRunCommand ?? (_testRunCommand = new RelayCommand(TestrunStartExecute, () => !IsTestrunInProgress)); 
    }
}

This was quite a simple fix and it worked for me.

Answer 5

Addition to Riegardt Steyn's answer above: https://stackoverflow.com/a/33503341/1964969

If you don't want to change Command to CommandWpf usage (as that two RelayCommand versions are not compatible inbetween), another workaround could be to not instantiate a command at the declaration place. Use constructor code instead:

public class SomeVMClass
{
    // CanExecute won't work:
    // Declaration and instantiation same place
    public RelayCommand MyCommand1 => new RelayCommand(MyBusinessLogic, MyCanExecuteValidator);

    // CanExecute will work
    // Declaration only
    public RelayCommand MyCommand2 { get; private set; }

    public SomeVMClass()
    {
        // Let's instantiate our declared command
        MyCommand2 = new RelayCommand(MyBusinessLogic, MyCanExecuteValidator);
       ...

Answer 6

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