[原创]WPF使用Command的问题及解决方案

   今天有位网友对我说，他在WPF中使用Command时遇到了一个问题，场景如下，为一个Button实例设置了Command，并且添加了CommandBinding，然后后台开一个Timer线程不停地改变该Command的CanCommand属性，但是在界面上却并没有立即反应出来。
下面给出该重现该问题及其解决方案的代码
代码如下：

<Window x:Class="wpfCommand.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300" Loaded="Window_Loaded">
    <Grid>
        <Button Height="23" HorizontalAlignment="Left" Margin="40,0,0,41" Name="btnCanExecute" VerticalAlignment="Bottom" Width="Auto">Can Execute</Button>
        <Button Height="23" HorizontalAlignment="Right" Margin="0,0,45,41" Name="btnPaste" VerticalAlignment="Bottom" Width="75">_Paste</Button>
        <TextBox Margin="17,21,25,92" Name="txtContent" />
    </Grid>
</Window>

private bool canExecute = false;
private Timer timer = new Timer(2000);

public Window1()
{
    InitializeComponent();
}

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    btnPaste.Command = ApplicationCommands.Paste;
    btnPaste.CommandBindings.Add(new CommandBinding(ApplicationCommands.Paste,new ExecutedRoutedEventHandler(executedRoutedEventHandler),new CanExecuteRoutedEventHandler(canExecuteRoutedEventHandler)));
    
    timer.Elapsed +=new ElapsedEventHandler(timer_Elapsed);
    timer.Start();
}

void timer_Elapsed(object sender, ElapsedEventArgs e)
{
    canExecute = !canExecute;
}

private void executedRoutedEventHandler(object sender, ExecutedRoutedEventArgs e)
{
    txtContent.Text += Clipboard.GetText();
}

private void canExecuteRoutedEventHandler(object sender, CanExecuteRoutedEventArgs e)
{
    //e.CanExecute = Clipboard.ContainsText();
    e.CanExecute = canExecute;
}

的确，我试了下，只有在焦点切换时才会更新，当然，Command的内部原理我没有去了解过，相信通过了解内部机制会更好更容易的解决这个问题，我只是从表层出发，既然切换焦点可以使得Command更新，那么我们可以在CanExecute发生改变时自己来控制焦点的切换，案例代码如下：


public partial class Window1 : Window
{
    private bool canExecute = false;
    private event EventHandler CanExecuteChanged;
    private delegate void DoWork();
    private Timer timer = new Timer(2000);

    public Window1()
    {
        InitializeComponent();
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        CanExecuteChanged += new EventHandler(Window1_CanExecuteChanged);
        
        btnPaste.Command = ApplicationCommands.Paste;
        btnPaste.CommandBindings.Add(new CommandBinding(ApplicationCommands.Paste,new ExecutedRoutedEventHandler(executedRoutedEventHandler),new CanExecuteRoutedEventHandler(canExecuteRoutedEventHandler)));
        
        timer.Elapsed +=new ElapsedEventHandler(timer_Elapsed);
        timer.Start();
    }

    void Window1_CanExecuteChanged(object sender, EventArgs e)
    {
        this.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Background, (DoWork)delegate()
        {
            DependencyObject focusScope = FocusManager.GetFocusScope(this);
            UIElement focusElement = FocusManager.GetFocusedElement(focusScope) as UIElement;
            if (!btnCanExecute.Equals(focusElement))
            {
                FocusManager.SetFocusedElement(focusScope, btnCanExecute);
            }
            else
            {
                focusElement = this;
            }

            if (focusElement == null)
            {
                FocusManager.SetFocusedElement(focusScope, this);
            }
            else
            {
                FocusManager.SetFocusedElement(focusScope, focusElement);
            }
        });
    }

    void timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        canExecute = !canExecute;
        RaiseEvent();
    }

    private void executedRoutedEventHandler(object sender, ExecutedRoutedEventArgs e)
    {
        txtContent.Text += Clipboard.GetText();
    }

    private void canExecuteRoutedEventHandler(object sender, CanExecuteRoutedEventArgs e)
    {
        //e.CanExecute = Clipboard.ContainsText();
        e.CanExecute = canExecute;
    }

    private void RaiseEvent()
    {
        EventHandler temp = CanExecuteChanged;
        if (temp != null)
        {
            temp(this, new EventArgs());
        }
    }

    protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
    {
        base.OnClosing(e);
        timer.Dispose();
    }
}