WPF：使用命令绑定和线程控制按钮的启用/禁用状态时遇到问题

Question

我有一个 WPF 应用程序，仅包含一个按钮和一个文本框来显示一些输出。当用户单击按钮时，一个线程启动，该线程禁用按钮，将内容打印到输出文本框，然后线程停止（此时我希望再次启用按钮）。

该应用程序似乎正确禁用了按钮，并正确更新了文本框。但是，当线程完成时，它总是无法正确重新启用按钮！谁能告诉我我做错了什么？

这是我的 xaml 的片段：

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Button Grid.Row="0" HorizontalAlignment="Center" Command="{Binding ExecuteCommand}">E_xecute</Button>
    <Label Grid.Row="1" Content="Output Window:" HorizontalAlignment="Left"/>
    <TextBlock Grid.Row="2" Text="{Binding Output}"/>
</Grid>

这是我的 ViewModel 代码（我使用 Josh Smith 的 MVVM 设计）：

public class WindowViewModel : ViewModelBase
{
    private bool _threadStopped;
    private RelayCommand _executeCommand;
    private string _output;

    public WindowViewModel()
    {
        _threadStopped = true;
    }

    public string Output { get { return _output; } set { _output = value; OnPropertyChanged("Output"); } }

    public ICommand ExecuteCommand
    {
        get
        {
            if (_executeCommand == null)
            {
                _executeCommand = new RelayCommand(p => this.ExecuteThread(p), p => this.CanExecuteThread); 
            }
            return _executeCommand;
        }
    }

    public bool CanExecuteThread
    {
        get
        {
            return _threadStopped;
        }
        set
        {
            _threadStopped = value;
        }
    }

    private void ExecuteThread(object p)
    {
        ThreadStart ts = new ThreadStart(ThreadMethod);
        Thread t = new Thread(ts);
        t.Start();
    }

    private void ThreadMethod()
    {
        CanExecuteThread = false;
        Output = string.Empty;
        Output += "Thread Started:  Is the 'Execute' button disabled?\r\n";
        int countdown = 5000;

        while (countdown > 0)
        {
            Output += string.Format("Time remaining: {0}...\r\n", countdown / 1000);
            countdown -= 1000;
            Thread.Sleep(1000);
        }
        CanExecuteThread = true;
        Output += "Thread Stopped:  Is the 'Execute' button enabled?\r\n";
    }
}

Answer 1

您需要帮助 WPF 了解命令的可执行状态已更改。执行此操作的简单方法是：

CommandManager.InvalidateRequerySuggested()

在 CanExecuteThread 内：

set
{
    _threadStopped = value;
    CommandManager.InvalidateRequerySuggested()
}

编辑（现在我有时间）：实际问题可能是您在 CanExecuteThread 属性更改时没有通知。它应该引发 PropertyChanged 以便 WPF 检测到更改：

public bool CanExecuteThread
{
    get { return _threadStopped; }
    set
    {
        if (_threadStopped != value)
        {
            _threadStopped = value;
            this.OnPropertyChanged(() => this.CanExecuteThread);
        }
    }
}

上面假设您的 ViewModel 基类具有 OnPropertyChanged 方法。

也就是说，我还想指出，您可以通过简单地使用 BackgroundWorker 来简化事情：

public class WindowViewModel : ViewModel
{
    private readonly BackgroundWorker backgroundWorker;

    public WindowVieWModel()
    {
        backgroundWorker = new BackgroundWorker();
        backgroundWorker.DoWork += delegate
        {
            // do work here (what's currently in ThreadMethod)
        };
        backgroundWorker.RunWorkerCompleted += delegate
        {
            // this will all run on the UI thread after the work is done
            this.OnPropertyChanged(() => this.CanExecuteThread);
        };
    }

    ...

    public bool CanExecuteThread
    {
        get { !this.backgroundWorker.IsBusy; }
    }

    private void ExecuteThread(object p)
    {
        // this will kick off the work
        this.backgroundWorker.RunWorkerAsync();

        // this property will have changed because the worker is busy
        this.OnPropertyChanged(() => this.CanExecuteThread);
    }
}

您可以进一步重构它以使其变得更好，但您明白了。

Answer 2

这是肯特·布加特（Kent Boogaart）建议的答案，而且它有效。基本上，我必须通过将 CommandManager.InvalidateRequerySuggested 放置在 Dispatcher 调用调用中来在 UI 线程上调用 CommandManager.InvalidateRequerySuggested。另请注意，我能够删除 CanExecuteThread 属性上的 Set 访问器，因为此解决方案不再需要它。谢谢，肯特！

public class WindowViewModel : ViewModelBase
{
    private bool _threadStopped;
    private RelayCommand _executeCommand;
    private string _output;
    private Dispatcher _currentDispatcher;
    public WindowViewModel()
    {
        _threadStopped = true;
        _currentDispatcher = Dispatcher.CurrentDispatcher;
    }

    public string Output { get { return _output; } set { _output = value; OnPropertyChanged("Output"); } }

    public ICommand ExecuteCommand
    {
        get
        {
            if (_executeCommand == null)
            {
                _executeCommand = new RelayCommand(p => this.ExecuteThread(p), p => this.CanExecuteThread); 
            }
            return _executeCommand;
        }
    }

    private delegate void ZeroArgDelegate();

    public bool CanExecuteThread
    {
        get
        {
            return _threadStopped;
        }
    }

    private void ExecuteThread(object p)
    {
        ThreadStart ts = new ThreadStart(ThreadMethod);
        Thread t = new Thread(ts);
        t.Start();
    }

    private void ThreadMethod()
    {
        _threadStopped = false;
        Output = string.Empty;
        Output += "Thread Started:  Is the 'Execute' button disabled?\r\n";
        int countdown = 5000;

        while (countdown > 0)
        {
            Output += string.Format("Time remaining: {0}...\r\n", countdown / 1000);
            countdown -= 1000;
            Thread.Sleep(1000);
        }
        _threadStopped = true;
        _currentDispatcher.BeginInvoke(new ZeroArgDelegate(resetButtonState), null);
        Output += "Thread Stopped:  Is the 'Execute' button enabled?\r\n";
    }

    private void resetButtonState()
    {
        CommandManager.InvalidateRequerySuggested();
    }
}