c# - 公开 ViewModel 事件以绑定(bind)到自定义 DependencyProperty

Question

是否有可能从我的 ViewModel 公开一个公共(public)事件，以便允许它绑定(bind)到我的 View 中的自定义 DependencyProperty？

我的应用程序是使用 .NET 4.5 框架用 C# 编写的。它具有 MVVM 架构，在 View 中没有代码隐藏和自定义 DependencyProperty 类，用于将 View 的 WPF 特定行为绑定(bind)到 ViewModel 公开的属性。

我希望 ViewModel 能够公开一组属性，这些属性表示 View 需要响应的事件。例如，当顶级 ViewModel 对象即将被处置时，我希望 WPF View 实现通过关闭相应的 Window 来响应。当配置过程显示对话窗口、用户输入并确认信息并且 ViewModel 已将其传递给模型并且不再需要时，可能会发生这种情况。

我知道有很多问题专门针对解决“ViewModel 的显示对话框”问题；这不是其中之一，我有解决方案。

我已经通读了 DependencyProperties 的 MSDN 文档，但找不到任何特定于绑定(bind)到事件属性的内容。

我想实现的是类似于下面的代码。此代码生成，但在显示 MainWindow 时导致典型的 System.Windows.Data Error: 40 : BindingExpression path error: 'RequestCloseEvent' property not found 错误。

我知道有很多问题与“请帮助我调试我的 System.Windows.Data 错误:40 问题”有关；这(可能)也不是其中之一。(但如果真的就是这样，我会很高兴。)

WindowBindableProperties.cs 中自定义 DependencyProperty 的来源:

using System;
using System.Threading;
using System.Windows;

namespace WpfEventBinding
{
    public static class WindowBindableProperties
    {
        #region ViewModelTerminatingEventProperty

        /// <summary>
        /// Register the ViewModelTerminatingEvent custom DependencyProperty.
        /// </summary>
        private static DependencyProperty _viewModelTerminatingEventProperty = 
            DependencyProperty.RegisterAttached
        (
            "ViewModelTerminatingEvent",
            typeof(ViewModelTerminatingEventHandler),
            typeof(WindowBindableProperties),
            new PropertyMetadata(null, ViewModelTerminatingEventPropertyChanged)
        );

        /// <summary>
        /// Identifies the ViewModelTerminatingEvent dependency property.
        /// </summary>
        public static DependencyProperty ViewModelTerminatingEventProperty
        { get { return _viewModelTerminatingEventProperty; } }

        /// <summary>
        /// Gets the attached ViewModelTerminatingEvent dependecy property.
        /// </summary>
        /// <param name="dependencyObject">The window attached to the WindowViewModel.</param>
        /// <returns>The ViewModelTerminatingEventHandler bound to this property</returns>
        public static ViewModelTerminatingEventHandler GetViewModelTerminatingEvent
        (DependencyObject dependencyObject)
        {
            return (dependencyObject.GetValue(ViewModelTerminatingEventProperty)
                as ViewModelTerminatingEventHandler);
        }

        /// <summary>
        /// Sets the ViewModelTerminatingEvent dependency property.
        /// </summary>
        public static void SetViewModelTerminatingEvent(
            DependencyObject dependencyObject,
            ViewModelTerminatingEventHandler value)
        {
            dependencyObject.SetValue(ViewModelTerminatingEventProperty, value);
        }

        /// <summary>
        /// Gets the ViewModelTerminatingEvent dependency property.
        /// </summary>
        private static void ViewModelTerminatingEventPropertyChanged(
            DependencyObject d, 
            DependencyPropertyChangedEventArgs e)
        {
            Window instance = d as Window;
            if (null != instance)
            {
                if (null != e.OldValue)
                {
                    throw new System.InvalidOperationException(
                    "ViewModelTerminatingEvent dependency property cannot be changed.");
                }

                if (null != e.NewValue)
                {
                    // Attach the Window.Close() method to the ViewModel's event
                    var newEvent = (e.NewValue as ViewModelTerminatingEventHandler);
                    newEvent += new ViewModelTerminatingEventHandler(() => instance.Close());
                }
            }
        }

        #endregion
    }
}

MainWindow.xaml 的来源:(此示例包含用于简化停止按钮实现的代码隐藏。)

<Window x:Class="WpfEventBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:v="clr-namespace:WpfEventBinding"
        v:WindowBindableProperties.ViewModelTerminatingEvent="{Binding Path=RequestCloseEvent}"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="{Binding Path=CloseCommandName}" Click="StopButton_Click" ></Button>
    </Grid>
</Window>

MainWindow.xaml.cs 的来源(代码隐藏):

using System.Windows;

namespace WpfEventBinding
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void StopButton_Click(object sender, RoutedEventArgs e)
        {
            MainWindowViewModel vm = (DataContext as MainWindowViewModel);
            if (null != vm)
            {
                vm.Stop();
            }
        }
    }
}

MainWindowViewModel.cs 的来源:

using System;
using System.ComponentModel;

namespace WpfEventBinding
{
    public delegate void ViewModelTerminatingEventHandler();

    class MainWindowViewModel
        : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        // Raised by the ViewModel to indicate to the view that it is no longer required.
        // Causes System.Windows.Data Error: 40 : BindingExpression path error.  Is it
        // Possible to bind to an 'event' property?
        public event ViewModelTerminatingEventHandler RequestCloseEvent;

        // This has to have the public 'get' to allow binding.  Is there some way to
        // do the same thing for the 'event'?
        public String CloseCommandName { get; private set; }

        public MainWindowViewModel()
        {
            CloseCommandName = "Close";
        }

        internal void Stop()
        {
            ViewModelTerminatingEventHandler RaiseRequestCloseEvent =
                RequestCloseEvent;
            if (null != RaiseRequestCloseEvent)
            {
                RaiseRequestCloseEvent();
            }
        }

        internal void Start()
        {
            OnPropertyChanged("CloseCommandName");
            OnPropertyChanged("ViewModelTerminatingEvent");
        }

        private void OnPropertyChanged(String propertyName)
        {
            PropertyChangedEventHandler RaisePropertyChangedEvent = PropertyChanged;
            if (RaisePropertyChangedEvent != null)
            {
                var propertyChangedEventArgs = new PropertyChangedEventArgs(propertyName);
                RaisePropertyChangedEvent(this, propertyChangedEventArgs);
            }
        }
    }
}

App.xaml 的来源:

<Application x:Class="WpfEventBinding.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             >
    <Application.Resources>
        <!-- Nothing to see here.  Move along... -->
    </Application.Resources>
</Application>

App.xaml.cs 的来源

using System.Windows;

namespace WpfEventBinding
{
    public partial class App : Application
    {
        public App()
        {
            Startup += new StartupEventHandler(App_Startup);
        }

        void App_Startup(object sender, StartupEventArgs e)
        {
            MainWindowViewModel vm = new MainWindowViewModel();
            MainWindow window = new MainWindow();

            // Make sure this is set before attempting binding!
            window.DataContext = vm;
            vm.Start();
            window.Show();
        }
    }
}

似乎 public event ViewModelTerminatingEventHandler RequestCloseEvent; 语法不足以允许数据绑定(bind)发生。类似的问题是查看 public String CloseCommandName { get;私有(private)集； 被声明为 public String CloseCommandName; 而没有 { get;私有(private)集； }。但是，没有 { get;私有(private)集； } 事件，它使用 {add{} remove{}} 语法(这也不能解决问题)。

我正在尝试的是否可行？如果可行，我错过了什么？

Answer 1

View closing 表示窗口关闭事件。所以你基本上想要对 View 中的事件使用react。我最近读了这个 arcticle , 有一个很好的图像

并且还提到了EventBehavior的存在。

如果您不想隐藏任何代码，最好的办法是使用行为。行为是一个简单的附加属性，它可以执行操作，例如上升的应用程序范围的命令，然后 ViewModel 可以捕获这些操作而不会出现 MVVM 问题。

这是一个行为示例:

public static class FreezeBehavior
{
    public static bool GetIsFrozen(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsFrozenProperty);
    }
    public static void SetIsFrozen(DependencyObject obj, bool value)
    {
        obj.SetValue(IsFrozenProperty, value);
    }
    public static readonly DependencyProperty IsFrozenProperty =
        DependencyProperty.RegisterAttached("IsFrozen", typeof(bool), typeof(FreezeBehavior), new PropertyMetadata(OnIsFrozenChanged));

    private static void OnIsFrozenChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if ((bool)e.NewValue)
        {
            var freezable = d as Freezable;
            if (freezable != null && freezable.CanFreeze)
                freezable.Freeze();
        }
    }
}

是这样使用的

<DropShadowEffect ShadowDepth="2" local:FreezeBehavior.IsFrozen="True"/>

它可以附加到任何 freezable 上以卡住它。在您的情况下，您想订阅事件并调用命令或设置属性，或任何通知 ViewModel 的内容。