When WPF Commands update their CanExecute states?

When writing Command="{Binding WalterlvCommand}" into your XAML code and your button or other controls can automatically execute command and updating the command states, such as enabling or disabling the button.

We’ll talk about when the UI commands will refresh their can-execute states and how to force updating the states.

This post is written in multiple languages. Please select yours:

This post is written for my Stack Overflow answer:

Why C# WPF button binding command won’t change view after using simple injector? - Stack Overflow

A simple sample

  
<Button x:Name="TestCommand" Command="{Binding WalterlvCommand}" />

  
public class Walterlv
{
    // Assume that I've initialized this command.
    public WalterlvCommand WalterlvCommand { get; }
}

public class WalterlvCommand : ICommand
{
    public bool SomeFlag { get; set; }

    bool ICommand.CanExecute(object parameter)
    {
        // Return the real can execution state.
        return SomeFlag;
    }

    void ICommand.Execute(object parameter)
    {
        // The actual executing procedure.
    }
}

See this code below. After 5 seconds, the button will still be disabled even that we set the SomeFlat to true.

  
var walterlv = new Walterlv();
TestCommand.DataContext = walterlv;

await Task.Delay(5000);
walterlv.WalterlvCommand.SomeFlag = true;

How to update manually?

Call this method after you want to update your command states if it won’t update:

  
CommandManager.InvalidateRequerySuggested();

When do the commands update their states?

Commands only update when these general events happen:

KeyUp
MouseUp
GotKeyboardFocus
LostKeyboardFocus

You can see the code here:

CommandDevice.PostProcessInput

And the key code is here:

  
if (e.StagingItem.Input.RoutedEvent == Keyboard.KeyUpEvent ||
    e.StagingItem.Input.RoutedEvent == Mouse.MouseUpEvent ||
    e.StagingItem.Input.RoutedEvent == Keyboard.GotKeyboardFocusEvent ||
    e.StagingItem.Input.RoutedEvent == Keyboard.LostKeyboardFocusEvent)
{
    CommandManager.InvalidateRequerySuggested();
}

Actually, not only those events above but also these methods below refresh the command states:

When opening a submenu of a MenuItem. See MenuItem.OnIsSubmenuOpenChanged
When pressing and holding a RepeatButton in a Tracker. Tracker.DecreaseRepeatButton
When change the readonly property of DataGridCell. DataGridCell.OnNotifyIsReadOnlyChanged
When doing many operations in a DataGrid. DataGrid
When navigating back in a JournalNavigationScope. JournalNavigationScope.OnBackForwardStateChange
And others, you can find references of InvalidateRequerySuggested: InvalidateRequerySuggested

本文会经常更新，请阅读原文： https://blog.walterlv.com/post/when-wpf-commands-update-their-states-en.html ，以避免陈旧错误知识的误导，同时有更好的阅读体验。

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