了解 Silverlight 调度程序

Question

我遇到了无效的跨线程访问问题，但经过一些研究，我设法使用调度程序修复了它。

现在在我的应用程序中我有延迟加载的对象。我将使用 WCF 进行异步调用，并像往常一样使用 Dispatcher 来更新我的对象 DataContext，但它不适用于此场景。不过，我确实在此处找到了解决方案。这是我不明白的地方。

在我的 UserControl 中，我有代码来调用对象上的 Toggle 方法。对该方法的调用是在 Dispatcher 中进行的，如下所示。

Dispatcher.BeginInvoke( () => _CurrentPin.ToggleInfoPanel() );

正如我之前提到的，这还不足以让 Silverlight 满意。我必须在我的对象内进行另一个调度程序调用。我的对象不是 UIElement，而是一个处理所有自己的加载/保存的简单类。

打电话解决了问题

Deployment.Current.Dispatcher.BeginInvoke( () => dataContext.Detail = detail );

因此，通过在班级内

。为什么我必须调用调度程序两次才能实现此目的？高层的呼吁还不够吗？ UIElement 中的 Deployment.Current.Dispatcher 和 Dispatcher 之间有区别吗？

Answer 1

理想情况下，存储一个 Dispatcher 实例，您可以在其他地方使用该实例，而无需对其进行线程检查。

调用任何单例 .Current 实例实际上可能会导致调用跨线程访问检查。通过首先存储它，您可以避免这种情况以实际获取共享实例。

我使用“SmartDispatcher”，它在线程外调用时使用调度程序，否则仅调用。它解决了此类问题。

帖子：http://www.jeff.wilcox.name/2010/04/ propertychangedbase-crossthread/

代码：

// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Public License (Ms-PL).
// Please see http://go.microsoft.com/fwlink/?LinkID=131993 for details.
// All other rights reserved.

using System.ComponentModel;

namespace System.Windows.Threading
{
    /// <summary>
    /// A smart dispatcher system for routing actions to the user interface
    /// thread.
    /// </summary>
    public static class SmartDispatcher
    {
        /// <summary>
        /// A single Dispatcher instance to marshall actions to the user
        /// interface thread.
        /// </summary>
        private static Dispatcher _instance;

        /// <summary>
        /// Backing field for a value indicating whether this is a design-time
        /// environment.
        /// </summary>
        private static bool? _designer;

        /// <summary>
        /// Requires an instance and attempts to find a Dispatcher if one has
        /// not yet been set.
        /// </summary>
        private static void RequireInstance()
        {
            if (_designer == null)
            {
                _designer = DesignerProperties.IsInDesignTool;
            }

            // Design-time is more of a no-op, won't be able to resolve the
            // dispatcher if it isn't already set in these situations.
            if (_designer == true)
            {
                return;
            }

            // Attempt to use the RootVisual of the plugin to retrieve a
            // dispatcher instance. This call will only succeed if the current
            // thread is the UI thread.
            try
            {
                _instance = Application.Current.RootVisual.Dispatcher;
            }
            catch (Exception e)
            {
                throw new InvalidOperationException("The first time SmartDispatcher is used must be from a user interface thread. Consider having the application call Initialize, with or without an instance.", e);
            }

            if (_instance == null)
            {
                throw new InvalidOperationException("Unable to find a suitable Dispatcher instance.");
            }
        }

        /// <summary>
        /// Initializes the SmartDispatcher system, attempting to use the
        /// RootVisual of the plugin to retrieve a Dispatcher instance.
        /// </summary>
        public static void Initialize()
        {
            if (_instance == null)
            {
                RequireInstance();
            }
        }

        /// <summary>
        /// Initializes the SmartDispatcher system with the dispatcher
        /// instance.
        /// </summary>
        /// <param name="dispatcher">The dispatcher instance.</param>
        public static void Initialize(Dispatcher dispatcher)
        {
            if (dispatcher == null)
            {
                throw new ArgumentNullException("dispatcher");
            }

            _instance = dispatcher;

            if (_designer == null)
            {
                _designer = DesignerProperties.IsInDesignTool;
            }
        }

        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        public static bool CheckAccess()
        {
            if (_instance == null)
            {
                RequireInstance();
            }

            return _instance.CheckAccess();
        }

        /// <summary>
        /// Executes the specified delegate asynchronously on the user interface
        /// thread. If the current thread is the user interface thread, the
        /// dispatcher if not used and the operation happens immediately.
        /// </summary>
        /// <param name="a">A delegate to a method that takes no arguments and 
        /// does not return a value, which is either pushed onto the Dispatcher 
        /// event queue or immediately run, depending on the current thread.</param>
        public static void BeginInvoke(Action a)
        {
            if (_instance == null)
            {
                RequireInstance();
            }

            // If the current thread is the user interface thread, skip the
            // dispatcher and directly invoke the Action.
            if (_instance.CheckAccess() || _designer == true)
            {
                a();
            }
            else
            {
                _instance.BeginInvoke(a);
            }
        }
    }
}

Answer 2

如果您使用 MVVM light 工具包，您可以使用 Extras dll 中 Galasoft.MvvmLight.Threading 命名空间中的 DispatcherHelper 类。它检查访问并使用调度程序的静态属性，类似于 SmartDispatcher。

在您的 App.xaml.cs 启动事件调用中：

DispatcherHelper.Initialize();

然后在您需要使用调度程序的任何地方使用：

   DispatcherHelper.CheckBeginInvokeOnUI(() => // do stuff; );