避免在多线程c# MVVM应用程序中从ViewModel对象调用BeginInvoke()

1. (从你的编辑)发送更新到UI通过动作调度不仅是黑客，这是完全没有必要的。与在VM中使用Dispatcher或SynchronizationContext相比，这样做绝对没有好处。不要那样做。请。它一文不值。

2. 当绑定到实现INotifyPropertyChanged的对象时，绑定将自动处理UI线程上的调用更新*。你说这是胡扯?花点时间创建一个小型原型进行测试。去做吧。我会等. ...告诉你。

所以你的问题实际上是没有意义的——你根本不需要担心这个。

^{*这个对框架的更改是在3.5,iirc中引入的，所以如果你是基于3构建的，则不适用}

这个回答是基于Will的回答和Marcel B的评论，并被标记为社区wiki的回答。

在问题中的简单应用程序中，一个公共SynchronizationContext属性被添加到ViewModel类中。这是由视图在必要时设置的，并由ViewModel用于执行受保护的操作。在没有GUI线程的单元测试上下文中，可以模拟GUI线程，并用一个SynchronizationContext代替真实的线程。对于我的实际应用程序，其中一个视图没有任何特殊的SynchronizationContext，它只是不改变ViewModel的默认ViewContext。

App.xaml.cs

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
namespace MultiThreadingGUI
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        public App()
        {
            Startup += new StartupEventHandler(App_Startup);
        }
        void App_Startup(object sender, StartupEventArgs e)
        {
            TestViewModel vm = new TestViewModel();
            MainWindow window = new MainWindow();
            window.DataContext = vm;
            vm.Start();
            window.Show();
        }
    }
    public class TestViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public ObservableCollection<String> ListFromElsewhere { get; private set; }
        public String TextFromElsewhere { get; private set; }
        // Provides a mechanism for the ViewModel to marshal operations from
        // worker threads on the View's thread.  The GUI context will be set
        // during the MainWindow's Loaded event handler, when both the GUI
        // thread context and an instance of this class are both available.
        public SynchronizationContext ViewContext { get; set; }
        public TestViewModel()
        {
            // Provide a default context based on the current thread that
            // can be changed by the View, should it required a different one.
            // It just happens that in this simple example the Current context
            // is the GUI context, but in a complete application that may
            // not necessarily be the case.
            ViewContext = SynchronizationContext.Current;
        }
        internal void Start()
        {
            ListFromElsewhere = new ObservableCollection<string>();
            Task testTask = new Task(new Action(()=>
            {
                int count = 0;
                while (true)
                {
                    TextFromElsewhere = Convert.ToString(count++);
                    // This is Marshalled on the correct thread by the framework.
                    PropertyChangedEventHandler RaisePropertyChanged = PropertyChanged;
                    if (null != RaisePropertyChanged)
                    {
                        RaisePropertyChanged(this, 
                            new PropertyChangedEventArgs("TextFromElsewhere"));
                    }
                    // ObservableCollections (amongst other things) are thread-centric,
                    // so use the SynchronizationContext supplied by the View to
                    // perform the Add operation.
                    ViewContext.Post(
                        (param) => ListFromElsewhere.Add((String)param), TextFromElsewhere);
                    Thread.Sleep(1000);
                }
            }));
            _testTask.Start();
        }
    }
}

在这个例子中，Window的Loaded事件是在后台代码中处理的，以向ViewModel对象提供GUI SynchronizationContext。(在我的应用程序中，我没有现成的代码，并且使用了绑定依赖属性。)

MainWindow.xaml.cs

using System;
using System.Threading;
using System.Windows;
namespace MultiThreadingGUI
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            // The ViewModel object that needs to marshal some actions is
            // attached as the DataContext by the time of the loaded event.
            TestViewModel vmTest = (this.DataContext as TestViewModel);
            if (null != vmTest)
            {
                // Set the ViewModel's reference SynchronizationContext to
                // the View's current context.
                vmTest.ViewContext = (SynchronizationContext)Dispatcher.Invoke
                    (new Func<SynchronizationContext>(() => SynchronizationContext.Current));
            }
        }
    }
}

最后，在XAML.

MainWindow.xaml

<Window x:Class="MultiThreadingGUI.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow"
        SizeToContent="WidthAndHeight"
        Loaded="Window_Loaded"
        >
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Label Grid.Row="0" Grid.Column="0" Content="TextFromElsewhere:" />
        <Label Grid.Row="0" Grid.Column="1" Content="{Binding Path=TextFromElsewhere}" />
        <Label Grid.Row="1" Grid.Column="0" Content="ListFromElsewhere:" />
        <ListView x:Name="itemListView" Grid.Row="1" Grid.Column="1"
            ItemsSource="{Binding Path=ListFromElsewhere}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <Label Content="{Binding}" />
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </Grid>
</Window>

你可以在你的Base(ViewModel) class中实现一个通用的 PropertyChanged Behavior:

private void RaisePropertyChanged(string propertyName)
        {
            if (Application.Current == null || Application.Current.Dispatcher.CheckAccess())
            {
                RaisePropertyChangedUnsafe(propertyName);
            }
            else
            {
                Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.DataBind,
                    new ThreadStart(() => RaisePropertyChangedUnsafe(propertyName)));
            }
        }

和

 private void RaisePropertyChangingUnsafe(string propertyName)
        {
            PropertyChangingEventHandler handler = PropertyChanging;
            if (handler != null)
            {
                handler(this, new PropertyChangingEventArgs(propertyName));
            }
        }

此代码将检查对主GUI调度程序的访问，并将在当前或GUI线程上引发Property Changed事件。

如果使用了一个接口，那么MainWindow.xaml.cs将失去对TestViewModel的依赖。

interface ISynchronizationContext
{
    System.Threading.SynchronizationContext ViewContext { get; set; }
} 
(this.DataContext as ISynchronizationContext).ViewContext  = 
(SynchronizationContext)Dispatcher.Invoke
(new Func<SynchronizationContext>(() => SynchronizationContext.Current));