为什么从 UI 中删除命令源后会调用 CanExecute？

我试图理解为什么在已从 UI 中删除的命令源上调用 CanExecute。这是一个简化的程序来演示：

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="350" Width="525">
    <StackPanel>
        <ListBox ItemsSource="{Binding Items}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <Button Content="{Binding Txt}" 
                                Command="{Binding Act}" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <Button Content="Remove first item" Click="Button_Click"  />
    </StackPanel>
</Window>

隐藏代码：

public partial class MainWindow : Window
{
    public class Foo
    {
        static int _seq = 0;
        int _txt = _seq++;
        RelayCommand _act;
        public bool Removed = false;

        public string Txt { get { return _txt.ToString(); } }

        public ICommand Act
        {
            get
            {
                if (_act == null) {
                    _act = new RelayCommand(
                        param => { },
                        param => {
                            if (Removed)
                                Console.WriteLine("Why is this happening?");
                            return true;
                        });
                }
                return _act;
            }
        }
    }

    public ObservableCollection<Foo> Items { get; set; }

    public MainWindow()
    {
        Items = new ObservableCollection<Foo>();
        Items.Add(new Foo());
        Items.Add(new Foo());
        Items.CollectionChanged += 
            new NotifyCollectionChangedEventHandler(Items_CollectionChanged);
        DataContext = this;
        InitializeComponent();
    }

    void Items_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
            foreach (Foo foo in e.OldItems) {
                foo.Removed = true;
                Console.WriteLine("Removed item marked 'Removed'");
            }
    }

    void Button_Click(object sender, RoutedEventArgs e)
    {
        Items.RemoveAt(0);
        Console.WriteLine("Item removed");
    }
}

当我单击“删除第一项”按钮一次时，我得到以下输出：

Removed item marked 'Removed'
Item removed
Why is this happening?
Why is this happening?

“为什么会这样？”每次我单击窗口的某些空白部分时，都会不断打印。

为什么会发生这种情况？我可以或应该做什么来防止在已删除的命令源上调用 CanExecute？

Note:可以找到RelayCommandhere http://msdn.microsoft.com/en-us/magazine/dd419663.aspx#id0090030.

迈克尔·伊登菲尔德问题的答案：

Q1:在删除的按钮上调用 CanExecute 时的调用堆栈：

WpfApplication1.exe!WpfApplication1.MainWindow.Foo.get_Act.AnonymousMethod__1(对象参数) 第 30 行 WpfApplication1.exe!WpfApplication1.RelayCommand.CanExecute(对象参数) 第 41 行 + 0x1a 字节 PresentationFramework.dll!MS.Internal.Commands.CommandHelpers.CanExecuteCommandSource(System.Windows.Input.ICommandSource commandSource) + 0x8a 字节 PresentationFramework.dll！System.Windows.Controls.Primitives.ButtonBase.UpdateCanExecute（）+ 0x18字节PresentationFramework.dll！System.Windows.Controls.Primitives.ButtonBase.OnCanExecuteChanged（对象发送者，System.EventArgs e）+ 0x5字节 PresentationCore.dll!System.Windows.Input.CommandManager.CallWeakReferenceHandlers(System.Collections.Generic.List 处理程序) + 0xac 字节 PresentationCore.dll!System.Windows.Input.CommandManager.RaiseRequerySuggested(对象 obj) + 0xf 字节

Q2:另外，如果您从列表中删除所有按钮（而不仅仅是第一个按钮），这种情况还会继续发生吗？

Yes.

问题是命令源（即按钮）没有取消订阅CanExecuteChanged它所绑定的命令的，以便每当CommandManager.RequerySuggested fires, CanExecute在命令源消失很久之后也会触发。

为了解决这个问题我实施了IDisposable on RelayCommand，并添加了必要的代码，以便每当删除模型对象并从 UI 中删除时，都会调用 Dispose() 就其所有的RelayCommand.

这是修改后的RelayCommand（原文是here http://msdn.microsoft.com/en-us/magazine/dd419663.aspx#id0090030):

public class RelayCommand : ICommand, IDisposable
{
    #region Fields

    List<EventHandler> _canExecuteSubscribers = new List<EventHandler>();
    readonly Action<object> _execute;
    readonly Predicate<object> _canExecute;

    #endregion // Fields

    #region Constructors

    public RelayCommand(Action<object> execute)
        : this(execute, null)
    {
    }

    public RelayCommand(Action<object> execute, Predicate<object> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;
    }

    #endregion // Constructors

    #region ICommand

    [DebuggerStepThrough]
    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add
        {
            CommandManager.RequerySuggested += value;
            _canExecuteSubscribers.Add(value);
        }
        remove
        {
            CommandManager.RequerySuggested -= value;
            _canExecuteSubscribers.Remove(value);
        }
    }

    public void Execute(object parameter)
    {
        _execute(parameter);
    }

    #endregion // ICommand

    #region IDisposable

    public void Dispose()
    {
        _canExecuteSubscribers.ForEach(h => CanExecuteChanged -= h);
        _canExecuteSubscribers.Clear();
    }

    #endregion // IDisposable
}

无论我在哪里使用上述内容，我都会跟踪所有实例化的 RelayCommand，以便我可以调用Dispose()在时机成熟时：

Dictionary<string, RelayCommand> _relayCommands 
    = new Dictionary<string, RelayCommand>();

public ICommand SomeCmd
{
    get
    {
        RelayCommand command;
        string commandName = "SomeCmd";
        if (_relayCommands.TryGetValue(commandName, out command))
            return command;
        command = new RelayCommand(
            param => {},
            param => true);
        return _relayCommands[commandName] = command;
    }
}

void Dispose()
{
    foreach (string commandName in _relayCommands.Keys)
        _relayCommands[commandName].Dispose();
    _relayCommands.Clear();
}