如何让 DelegateCommand CanExecute 使用 MVVM 更新 WPF UI？

Question

在 this question 的基础上，我正在尝试制作一个带有两个按钮的 UI。按下一个按钮会禁用自身并启用另一个按钮。我正在使用 MVVM 模式和 DelegateCommand:

主窗口XAML

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:ViewModel />
    </Window.DataContext>

    <StackPanel>
        <Rectangle Height="100"></Rectangle>
        <Button Command="{Binding StartServer}" >Start Server</Button>
        <Button Command="{Binding StopServer}" >Stop Server</Button>
    </StackPanel>
</Window>

ViewModel

public class ViewModel : INotifyPropertyChanged
{
  public ViewModel()
  {
    PropertyChanged += PropertyChangedHandler;
  }

  private void PropertyChangedHandler(object sender, PropertyChangedEventArgs e)
  {
    switch (e.PropertyName)
    {
      case nameof(ServerIsRunning):
        StartServer.RaiseCanExecuteChanged();
        StopServer.RaiseCanExecuteChanged();
        break;
    }
  }

  private bool m_serverIsRunning;
  public bool ServerIsRunning
  {
    get => m_serverIsRunning;
    set
    {
      m_serverIsRunning = value;
      RaisePropertyChanged(nameof(ServerIsRunning));
    }
  }

  public DelegateCommand<object> StartServer => new DelegateCommand<object>(
    context =>
    {
      ServerIsRunning = true;
    }, e => !ServerIsRunning);

  public DelegateCommand<object> StopServer => new DelegateCommand<object>(
    context =>
    {
      ServerIsRunning = false;
    }, e => ServerIsRunning);

  public event PropertyChangedEventHandler PropertyChanged;

  public void RaisePropertyChanged(string property)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(property));
  }
}

DelegateCommand

public class DelegateCommand<T> : ICommand
{
  private readonly Action<object> ExecuteAction;
  private readonly Func<object, bool> CanExecuteFunc;

  public DelegateCommand(Action<object> executeAction, Func<object, bool> canExecuteFunc)
  {
    ExecuteAction = executeAction;
    CanExecuteFunc = canExecuteFunc;
  }

  public bool CanExecute(object parameter)
  {
    return CanExecuteFunc == null || CanExecuteFunc(parameter);
  }

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

  public event EventHandler CanExecuteChanged;

  public void RaiseCanExecuteChanged()
  {
    CanExecuteChanged?.Invoke(this, EventArgs.Empty);
  }
}

问题是，当我点击 Start Server 按钮时，按钮的 IsEnabled 状态在 UI 中没有改变。我是否使用不当的 DelegateCommand？

Answer 1

是这样的吗？

public class ViewModel : INotifyPropertyChanged
{
    bool _isServerStarted;
    public ViewModel()
    {
        StartServer = new DelegateCommand(OnStartServerExecute, OnStartServerCanExecute);
        StopServer = new DelegateCommand(OnStopServerExecute, OnStopServerCanExecute);
    }

    void OnStartServerExecute()
    {
        IsServerStarted = true;
    }
    bool OnStartServerCanExecute()
    {
        return !IsServerStarted;
    }
    void OnStopServerExecute()
    {
        IsServerStarted = false;
    }
    bool OnStopServerCanExecute()
    {
        return IsServerStarted;
    }
    void RaiseCanExecuteChanged()
    {
        StartServer.RaiseCanExecuteChanged();
        StopServer.RaiseCanExecuteChanged();
    }
    public bool IsServerStarted 
    {
        get => _isServerStarted;
        set
        {
            _isServerStarted = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(IsServerStarted)));
            RaiseCanExecuteChanged();
        }
    }

    public DelegateCommand StartServer { get; }
    public DelegateCommand StopServer { get; }

    public event PropertyChangedEventHandler PropertyChanged;
}

您只需要使用 CommandManager.InvalidateRequerySuggested() 来更新按钮状态。

Answer 2

您的代码的问题在于您的命令属性使用表达式主体，每次有人获得属性时都会执行它们，这意味着每次您请求 StartServer 命令时它都会给您一个新实例。您可以通过在 getter 上放置一个断点来自己检查这一点，您会看到它会被击中几次并始终创建委托命令的新实例。

为了使您的代码正常工作，您必须只使用每个命令的一个实例，您可以通过使用支持字段或只读属性并将其设置在构造函数中来实现：

  public MainWindowViewModel()
  {
     PropertyChanged += PropertyChangedHandler;
     StartServer = new DelegateCommand<object>(
        context =>
        {
           ServerIsRunning = true;
        }, e => !ServerIsRunning);

     StopServer = new DelegateCommand<object>(
        context =>    {
           ServerIsRunning = false;
        }, e => ServerIsRunning);
  }

  public DelegateCommand<object> StartServer
  { get; }

  public DelegateCommand<object> StopServer
  { get; }

UI 只会监听它所绑定的命令实例引发的 CanExecuteChanged 事件（通过 Command={Binding ...}.

您的代码也可以简化，因为您可以直接调用 RaiseCanExecuteChanged 而不是引发 PropertyChangedEvent 并自己倾听：

  public bool ServerIsRunning
  {
     get => m_serverIsRunning;
     set
     {
        m_serverIsRunning = value;
        StartServer.RaiseCanExecuteChanged();
        StopServer.RaiseCanExecuteChanged();
     }
  }

如何让 DelegateCommand CanExecute 使用 MVVM 更新 WPF UI？

How do I get DelegateCommand CanExecute to update WPF UI using MVVM?

c#

wpf

mvvm

delegatecommand