异步等待死锁

Question

我在 Windows Phone 8.1 上使用 Accelerometer 传感器。我必须从传感器的 ReadingChanged 回调中访问 UI。我还有一个 DispatcherTimer，每两秒更新一次传感器的 ReportInterval。当计时器触发并尝试设置加速度计的 ReportInterval 时程序会阻塞。下面的示例是重现错误的最小可执行示例。

namespace TryAccelerometer
{        
    public sealed partial class MainPage : Page
    {
        private Accelerometer acc;
        private DispatcherTimer timer;                
        private int numberAcc = 0;
        private int numberTimer = 0;

        public MainPage()
        {
            this.InitializeComponent();
            this.NavigationCacheMode = NavigationCacheMode.Required;

            acc = Accelerometer.GetDefault();                                    
            acc.ReadingChanged += acc_ReadingChanged;

            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromSeconds(2);
            timer.Tick += timer_Tick;
            timer.Start();            
        }

        async void acc_ReadingChanged(Accelerometer sender, AccelerometerReadingChangedEventArgs args)
        {            
            await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
            {
                //HERE I WILL HAVE TO ACCESS THE UI, BUT FOR SAKE OF SIMPLICITY I WROTE AN INCREMENT
                numberAcc++;
            });
        }

        void timer_Tick(object sender, object e)
        {
            numberTimer++;            
            //PUT A BREAKPOINT HERE BELOW AND SEE THAT THE PROGRAM BLOCKS
            acc.ReportInterval = acc.ReportInterval++;
        }
        /// <summary>
        /// Invoked when this page is about to be displayed in a Frame.
        /// </summary>
        /// <param name="e">Event data that describes how this page was reached.
        /// This parameter is typically used to configure the page.</param>
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            // TODO: Prepare page for display here.

            // TODO: If your application contains multiple pages, ensure that you are
            // handling the hardware Back button by registering for the
            // Windows.Phone.UI.Input.HardwareButtons.BackPressed event.
            // If you are using the NavigationHelper provided by some templates,
            // this event is handled for you.
        }
    }
}

我不明白为什么会出现死锁。提前谢谢你。

Answer 1

好吧，我被难住了。

Dispatcher.RunAsync 不应该导致死锁。因此，为了找出问题的确切位置，我在多行中重写了您的代码：

async void acc_ReadingChanged(Accelerometer sender, AccelerometerReadingChangedEventArgs args)
{
    var view = Windows.ApplicationModel.Core.CoreApplication.MainView;

    var window = view.CoreWindow;

    var dispatcher = window.Dispatcher;

    await dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () => { numberAcc++; });
}

真正的罪魁祸首是var window = view.CoreWindow;。很难解释为什么没有看到 WinRT 源代码，我猜 WinRT 需要切换到 UI 线程以检索对 window 的引用和 [=15= 之间存在一些奇怪的交互] 属性 Accelerometer 同步执行 ReadingChanged 事件。

从那里，我可以想到一些解决方案：

以另一种方式检索调度程序：

async void acc_ReadingChanged(Accelerometer sender, AccelerometerReadingChangedEventArgs args)
{
    await this.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () => { numberAcc++; });
}

当然，能不能，要看你实际的代码。

重写您的代码以使用 Timer 而不是 DispatcherTimer。我知道您需要使用 UI 线程来检索文本框（或类似的东西）的值，但是如果您使用数据绑定（有或没有 MVVM 模式），那么您应该能够访问从任何线程

在另一个线程中更改 ReportInterval。不过感觉真的很黑。

void timer_Tick(object sender, object e)
{
    numberTimer++;
    Task.Run(() => { acc.ReportInterval = acc.ReportInterval++; });
}

Answer 2

根据@KooKiz 的解释和@StephenCleary 的评论，我找到了另一种可能的解决方案。由于我们已经了解问题出在这里：

var window = view.CoreWindow;

我们可以缓存调度程序并将其保存为实例变量。这样做，我们避免在计时器的同时访问它：

namespace TryAccelerometer
{        
    public sealed partial class MainPage : Page
    {
        private Accelerometer acc;
        private DispatcherTimer timer;                
        private int numberAcc = 0;
        private int numberTimer = 0;
        private CoreDispatcher dispatcher;

        public MainPage()
        {
            this.InitializeComponent();
            this.NavigationCacheMode = NavigationCacheMode.Required;

            dispatcher = Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher;

            acc = Accelerometer.GetDefault();                                    
            acc.ReadingChanged += acc_ReadingChanged;

            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromSeconds(2);
            timer.Tick += timer_Tick;
            timer.Start();            
        }

        async void acc_ReadingChanged(Accelerometer sender, AccelerometerReadingChangedEventArgs args)
        {            
            await dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
            {
                numberAcc++;
            });
        }

        void timer_Tick(object sender, object e)
        {
            numberTimer++;            
            acc.ReportInterval = acc.ReportInterval++;
            //acc.ReadingChanged -= acc_ReadingChanged;
        }
        /// <summary>
        /// Invoked when this page is about to be displayed in a Frame.
        /// </summary>
        /// <param name="e">Event data that describes how this page was reached.
        /// This parameter is typically used to configure the page.</param>
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            // TODO: Prepare page for display here.

            // TODO: If your application contains multiple pages, ensure that you are
            // handling the hardware Back button by registering for the
            // Windows.Phone.UI.Input.HardwareButtons.BackPressed event.
            // If you are using the NavigationHelper provided by some templates,
            // this event is handled for you.
        }
    }
}

这样就不会发生死锁。

Answer 3

我在 WinRT 上遇到死锁问题后创建了这个扩展，它解决了我的问题（到目前为止）：

using global::Windows.ApplicationModel.Core;
using global::Windows.UI.Core;

public static class UIThread
{
    private static readonly CoreDispatcher Dispatcher;

    static DispatcherExt()
    {
        Dispatcher = CoreApplication.MainView.CoreWindow.Dispatcher;
    }

    public static async Task Run(DispatchedHandler handler)
    {
        await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, handler);
    }
}

用法

public async Task Foo()
{
    await UIThread.Run(() => { var test = 0; });
}

异步等待死锁

Async Await deadlock

c#

asynchronous

windows-phone

async-await

windows-phone-8.1