如何在没有代码隐藏的情况下处理ViewModel中的WPF路由命令？

Question

根据我对MVVM的理解,直接在ViewModel中处理路由命令是一种很好的做法.

当路由命令在ViewModel中定义为RelayCommand(或DelegateCommand)时,很容易直接绑定到命令,如下所示:Command = {Binding MyViewModelDefinedCommand}.

实际上,对于在我的ViewModel之外定义的路由命令,我在View后面的代码中处理这些命令并将调用转发到ViewModel.但我发现我必须这样做很尴尬.它违背了推荐的MVVM良好做法.我认为应该有一种更优雅的方式来实现这项工作.

如何直接在ViewModel中处理"System.Windows.Input.ApplicationCommands"或在Viewmodel外部定义的任何路由命令.换句话说,对于在ViewModel之外定义的命令,我如何直接处理ViewBodel的CommandBinding回调"CommandExecute"和/或"CommandCanExecute"？这有可能吗？如果有,怎么样？如果不是,为什么？

Answer 1

我将这个问题改为:

如何在没有代码隐藏的情况下处理ViewModel中的WPF路由命令？

对此,我会回答:好问题!

WPF没有提供内置的方法来执行此操作,当您第一次启动WPF并且每个人都告诉您"Code-Behind is evil"(它确实是)时,这尤其令人讨厌.所以你必须自己构建它.

自己建造它

那么,如何自己创建这样的功能呢？好吧,首先我们需要一个等价的CommandBinding:

/// <summary>
///  Allows associated a routed command with a non-routed command.  Used by
///  <see cref="RoutedCommandHandlers"/>.
/// </summary>
public class RoutedCommandHandler : Freezable
{
  public static readonly DependencyProperty CommandProperty = DependencyProperty.Register(
    "Command",
    typeof(ICommand),
    typeof(RoutedCommandHandler),
    new PropertyMetadata(default(ICommand)));

  /// <summary> The command that should be executed when the RoutedCommand fires. </summary>
  public ICommand Command
  {
    get { return (ICommand)GetValue(CommandProperty); }
    set { SetValue(CommandProperty, value); }
  }

  /// <summary> The command that triggers <see cref="ICommand"/>. </summary>
  public ICommand RoutedCommand { get; set; }

  /// <inheritdoc />
  protected override Freezable CreateInstanceCore()
  {
    return new RoutedCommandHandler();
  }

  /// <summary>
  ///  Register this handler to respond to the registered RoutedCommand for the
  ///  given element.
  /// </summary>
  /// <param name="owner"> The element for which we should register the command
  ///  binding for the current routed command. </param>
  internal void Register(FrameworkElement owner)
  {
    var binding = new CommandBinding(RoutedCommand, HandleExecuted, HandleCanExecute);
    owner.CommandBindings.Add(binding);
  }

  /// <summary> Proxy to the current Command.CanExecute(object). </summary>
  private void HandleCanExecute(object sender, CanExecuteRoutedEventArgs e)
  {
    e.CanExecute = Command?.CanExecute(e.Parameter) != null;
    e.Handled = true;
  }

  /// <summary> Proxy to the current Command.Execute(object). </summary>
  private void HandleExecuted(object sender, ExecutedRoutedEventArgs e)
  {
    Command?.Execute(e.Parameter);
    e.Handled = true;
  }
}

然后我们需要一个实际将RoutedCommandHandler与特定元素相关联的类.为此,我们将RoutedCommandHandlers作为附加属性的集合,如下所示:

/// <summary>
///  Holds a collection of <see cref="RoutedCommandHandler"/> that should be
///  turned into CommandBindings.
/// </summary>
public class RoutedCommandHandlers : FreezableCollection<RoutedCommandHandler>
{
  /// <summary>
  ///  Hide this from WPF so that it's forced to go through
  ///  <see cref="GetCommands"/> and we can auto-create the collection
  ///  if it doesn't already exist.  This isn't strictly necessary but it makes
  ///  the XAML much nicer.
  /// </summary>
  private static readonly DependencyProperty CommandsProperty = DependencyProperty.RegisterAttached(
    "CommandsPrivate",
    typeof(RoutedCommandHandlers),
    typeof(RoutedCommandHandlers),
    new PropertyMetadata(default(RoutedCommandHandlers)));

  /// <summary>
  ///  Gets the collection of RoutedCommandHandler for a given element, creating
  ///  it if it doesn't already exist.
  /// </summary>
  public static RoutedCommandHandlers GetCommands(FrameworkElement element)
  {
    RoutedCommandHandlers handlers = (RoutedCommandHandlers)element.GetValue(CommandsProperty);
    if (handlers == null)
    {
      handlers = new RoutedCommandHandlers(element);
      element.SetValue(CommandsProperty, handlers);
    }

    return handlers;
  }

  private readonly FrameworkElement _owner;

  /// <summary> Each collection is tied to a specific element. </summary>
  /// <param name="owner"> The element for which this collection is created. </param>
  public RoutedCommandHandlers(FrameworkElement owner)
  {
    _owner = owner;

    // because we auto-create the collection, we don't know when items will be
    // added.  So, we observe ourself for changes manually. 
    var self = (INotifyCollectionChanged)this;
    self.CollectionChanged += (sender, args) =>
                              {
                                // note this does not handle deletions, that's left as an exercise for the
                                // reader, but most of the time, that's not needed! 
                                ((RoutedCommandHandlers)sender).HandleAdditions(args.NewItems);
                              };
  }

  /// <summary> Invoked when new items are added to the collection. </summary>
  /// <param name="newItems"> The new items that were added. </param>
  private void HandleAdditions(IList newItems)
  {
    if (newItems == null)
      return;

    foreach (RoutedCommandHandler routedHandler in newItems)
    {
      routedHandler.Register(_owner);
    }
  }

  /// <inheritdoc />
  protected override Freezable CreateInstanceCore()
  {
    return new RoutedCommandHandlers(_owner);
  }
}

然后,就像在元素上使用类一样简单:

<local:RoutedCommandHandlers.Commands>
  <local:RoutedCommandHandler RoutedCommand="Help" Command="{Binding TheCommand}" />
</local:RoutedCommandHandlers.Commands>

Interaction.Behavior实现

知道了上述内容,您可能会问:

哇,那很好,但那是很多代码.我已经在使用Expression Behaviors,所以有没有办法简化这一点？

对此,我会回答:好问题!

如果您已经在使用Interaction.Behaviors,那么您可以使用以下实现:

/// <summary>
///  Allows associated a routed command with a non-ordinary command. 
/// </summary>
public class RoutedCommandBinding : Behavior<FrameworkElement>
{
  public static readonly DependencyProperty CommandProperty = DependencyProperty.Register(
    "Command",
    typeof(ICommand),
    typeof(RoutedCommandBinding),
    new PropertyMetadata(default(ICommand)));

  /// <summary> The command that should be executed when the RoutedCommand fires. </summary>
  public ICommand Command
  {
    get { return (ICommand)GetValue(CommandProperty); }
    set { SetValue(CommandProperty, value); }
  }

  /// <summary> The command that triggers <see cref="ICommand"/>. </summary>
  public ICommand RoutedCommand { get; set; }

  protected override void OnAttached()
  {
    base.OnAttached();

    var binding = new CommandBinding(RoutedCommand, HandleExecuted, HandleCanExecute);
    AssociatedObject.CommandBindings.Add(binding);
  }

  /// <summary> Proxy to the current Command.CanExecute(object). </summary>
  private void HandleCanExecute(object sender, CanExecuteRoutedEventArgs e)
  {
    e.CanExecute = Command?.CanExecute(e.Parameter) != null;
    e.Handled = true;
  }

  /// <summary> Proxy to the current Command.Execute(object). </summary>
  private void HandleExecuted(object sender, ExecutedRoutedEventArgs e)
  {
    Command?.Execute(e.Parameter);
    e.Handled = true;
  }
}

使用相应的XAML:

<i:Interaction.Behaviors>
  <local:RoutedCommandBinding RoutedCommand="Help" Command="{Binding TheCommand}" />
</i:Interaction.Behaviors>