WPF:无法使用命令绑定(bind)和线程控制按钮的启用/禁用状态

Question

我有一个 WPF 应用程序，它只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，会启动一个禁用按钮的线程，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，它总是无法正确地重新启用按钮!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(我使用的是 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 知道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 内部:

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

编辑(现在我有时间了):实际问题很可能是您在 CanExecuteThread 时没有通知属性变化。它应该提高 PropertyChanged为了让 WPF 检测到更改:

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

以上假设您的 ViewModel基类有 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更好，但您明白了。