.Net并行编程系列之三：创建带时间限制（Timeout）的异步任务并取得异步任务的结果

浏览数：30 / 时间：2015年06月09日

尝试创建基于MVVM三层架构的异步任务：

场景：View层触发ViewModel层的动作请求，ViewModel层异步的从Model层查询数据，当数据返回或者请求超时时正确更新ViewModel层数据并触发View层的UI更新。

要求：View层保持UI响应，ViewModel层实现有超时控制的异步调用并返回结果

---------------------------

实现三个Worker，Worker1演示调用超时返回，Worker2演示无超时成功返回，Worker3演示同样演示无超时返回

设计WPF窗口如下：

<Window x:Class="TimeOutTask.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:TimeOutTask"
        Title="TaskTimeOutDemo" Height="480" Width="650">
    <Window.DataContext>
        <local:ViewModel></local:ViewModel>
    </Window.DataContext>
    <Window.Resources>
        <Style TargetType="{x:Type Button}">
            <Setter Property="Height" Value="30"></Setter>
            <Setter Property="Margin" Value="5"></Setter>
            <Setter Property="Width" Value="80"></Setter>
        </Style>
        <Style TargetType="{x:Type ListBox}">
            <Setter Property="Height" Value="120"></Setter>
            <Setter Property="Margin" Value="5"></Setter>
            <Setter Property="Width" Value="500"></Setter>
        </Style>
    </Window.Resources>
    <StackPanel>
        <StackPanel Orientation="Horizontal">
            <Button Command="{Binding Worker1Command}" Content="Start Worker1"  />
            <ListBox  Background="LightGray" ItemsSource="{Binding Worker1StatusCollection}"  />
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <Button  Command="{Binding Worker2Command}" Content="Start Worker2"  />
            <ListBox  Background="LightGray" ItemsSource="{Binding Worker2StatusCollection}"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <Button  Command="{Binding Worker3Command}" Content="Start Worker3"  />
            <ListBox Background="LightGray" ItemsSource="{Binding Worker3StatusCollection}"/>
        </StackPanel>
    </StackPanel>
</Window>

ViewModel类设计如下：

其中 synchronizationContext 为UI线程的同步上下文，作为Task返回结果是的回调更新ViewModel层数据之用，注意:View层和ViewModel是同时运行在主线程之上的。

注意我们创建的实际工作task的方法是Task(Action action, TaskCreationOptions creationOptions);

其中TaskCreationOptions 参数选择的是 TaskCreationOptions.LongRunning，此参数保证创建的task始终运行在同一个线程之上，减少线程间切换上下文的损失。

另外，传入timeout的方式是Task的方法public bool Wait(int millisecondsTimeout);返回True时表示在规定时间内返回了结果，放回false表示未在指定时间内返回结果。

    public class ViewModel : INotifyPropertyChanged
    {
        Model engineSimulator;
        private SynchronizationContext synchronizationContext;
        public ViewModel()
        {
            engineSimulator = new Model();
            synchronizationContext = System.Threading.SynchronizationContext.Current;
            worker1StatusCollection = new ObservableCollection<string>();
            Worker1Command = new DelegateCommand(Worker1, () => !IsWorker1Busy);

            Worker2StatusCollection = new ObservableCollection<string>();
            Worker2Command = new DelegateCommand(Worker2, () => !IsWorker2Busy);

            Worker3StatusCollection = new ObservableCollection<string>();
            Worker3Command = new DelegateCommand(Worker3, () => !IsWorker3Busy);
        }

        #region Worker1
        private ObservableCollection<string> worker1StatusCollection;
        public ObservableCollection<string> Worker1StatusCollection
        {
            get { return worker1StatusCollection; }
            set { PropertyChanged.ChangeAndNotify(ref worker1StatusCollection, value, () => Worker1StatusCollection); }
        }
        public void Worker1()
        {
            Worker1StatusCollection.Add(string.Format("Start Worker1 at:{0}", DateTime.Now.ToLongTimeString()));
            IsWorker1Busy = true;
            ((DelegateCommand)Worker1Command).RaiseCanExecuteChanged();
            Task.Factory.StartNew((Action)(() =>
            {
                var longRunningTaskWithTimeout = new Task<string>(() =>
                {
                    return engineSimulator.GetDataWithLongTime();
                }, TaskCreationOptions.LongRunning);

                longRunningTaskWithTimeout.Start();
                int miliSec = 5000;
                if (longRunningTaskWithTimeout.Wait(miliSec))
                {
                    synchronizationContext.Post(s =>
                    {
                        Worker1StatusCollection.Add(string.Format("Task completed with allotted time:{0}s" + miliSec / 1000));
                        Worker1StatusCollection.Add("Task Status:" + longRunningTaskWithTimeout.Status.ToString());
                        Worker1StatusCollection.Add("ResultFromEngine:"+longRunningTaskWithTimeout.Result);
                        Worker1StatusCollection.Add(string.Format("End Worker1 at:{0}", DateTime.Now.ToLongTimeString()));
                        IsWorker1Busy = false;
                        ((DelegateCommand)Worker1Command).RaiseCanExecuteChanged();
                    }, null);
                }
                else
                {
                    synchronizationContext.Post(s =>
                    {
                        Worker1StatusCollection.Add(string.Format("Task Not completed with allotted time:{0}s", miliSec / 1000));

                        Worker1StatusCollection.Add("Status:" + longRunningTaskWithTimeout.Status.ToString());
                        Worker1StatusCollection.Add(string.Format("End Worker1 at:{0}", DateTime.Now.ToLongTimeString()));
                        IsWorker1Busy = false;
                        ((DelegateCommand)Worker1Command).RaiseCanExecuteChanged();
                    }, null);
                }
            }));
        }
        private bool isWorker1Busy;
        public bool IsWorker1Busy
        {
            get { return isWorker1Busy; }
            set { PropertyChanged.ChangeAndNotify(ref isWorker1Busy, value, () => IsWorker1Busy); }
        }
        public ICommand Worker1Command { get; private set; }
        #endregion

        #region Worker2
        private ObservableCollection<string> worker2StatusCollection;
        public ObservableCollection<string> Worker2StatusCollection
        {
            get { return worker2StatusCollection; }
            set { PropertyChanged.ChangeAndNotify(ref worker2StatusCollection, value, () => Worker2StatusCollection); }
        }
        public void Worker2()
        {
            Worker2StatusCollection.Add(string.Format("Start Worker2 at:{0}", DateTime.Now.ToLongTimeString()));
            IsWorker2Busy = true;
            ((DelegateCommand)Worker2Command).RaiseCanExecuteChanged();
            Task.Factory.StartNew((Action)(() =>
            {
                var longRunningTaskWithTimeout = new Task<string>(() =>
                {
                    return engineSimulator.GetDataWithLongTime();
                }, TaskCreationOptions.LongRunning);

                longRunningTaskWithTimeout.Start();
                int miliSec = 11000;
                if (longRunningTaskWithTimeout.Wait(miliSec))
                {
                    synchronizationContext.Post(s =>
                    {
                        Worker2StatusCollection.Add(string.Format("Task completed with allotted time:{0}s", miliSec / 1000));
                        Worker2StatusCollection.Add("Tast Status:" + longRunningTaskWithTimeout.Status.ToString());
                        Worker2StatusCollection.Add("ResultFromEngine:" + longRunningTaskWithTimeout.Result);
                        Worker2StatusCollection.Add(string.Format("End Worker2 at:{0}", DateTime.Now.ToLongTimeString()));
                        IsWorker2Busy = false;
                        ((DelegateCommand)Worker2Command).RaiseCanExecuteChanged();
                    }, null);
                }
                else
                {
                    synchronizationContext.Post(s =>
                    {
                        Worker2StatusCollection.Add(string.Format("Task Not completed with allotted time:{0}s", miliSec / 1000));
                        Worker2StatusCollection.Add("Task Status:" + longRunningTaskWithTimeout.Status.ToString());
                        Worker2StatusCollection.Add(string.Format("End Worker2 at:{0}", DateTime.Now.ToLongTimeString()));
                        IsWorker2Busy = false;
                        ((DelegateCommand)Worker2Command).RaiseCanExecuteChanged();
                    }, null);
                }
            }));
        }
        private bool isWorker2Busy;
        public bool IsWorker2Busy
        {
            get { return isWorker2Busy; }
            set { PropertyChanged.ChangeAndNotify(ref isWorker2Busy, value, () => IsWorker2Busy); }
        }
        public ICommand Worker2Command { get; private set; }
        #endregion

        #region Worker3
        private ObservableCollection<string> worker3StatusCollection;
        public ObservableCollection<string> Worker3StatusCollection
        {
            get { return worker3StatusCollection; }
            set { PropertyChanged.ChangeAndNotify(ref worker3StatusCollection, value, () => Worker3StatusCollection); }
        }
        public void Worker3()
        {
            Worker3StatusCollection.Add(string.Format("Start Worker3 at:{0}", DateTime.Now.ToLongTimeString()));
            IsWorker3Busy = true;
            ((DelegateCommand)Worker3Command).RaiseCanExecuteChanged();
            Task.Factory.StartNew((Action)(() =>
            {
                var longRunningTaskWithTimeout = new Task<string>(() =>
                {
                    return engineSimulator.GetDataWithShortTime();
                }, TaskCreationOptions.LongRunning);

                longRunningTaskWithTimeout.Start();
                int miliSec = 1000;
                if (longRunningTaskWithTimeout.Wait(miliSec))
                {
                    synchronizationContext.Post(s =>
                    {
                        Worker3StatusCollection.Add(string.Format("Task completed with allotted time:{0}s", miliSec / 1000));
                        Worker3StatusCollection.Add("Task Status:" + longRunningTaskWithTimeout.Status.ToString());
                        Worker3StatusCollection.Add("ResultFromEngine:" + longRunningTaskWithTimeout.Result);
                        Worker3StatusCollection.Add(string.Format("End Worker3 at:{0}", DateTime.Now.ToLongTimeString()));
                        IsWorker3Busy = false;
                        ((DelegateCommand)Worker3Command).RaiseCanExecuteChanged();
                    }, null);
                }
                else
                {
                    synchronizationContext.Post(s =>
                    {
                        Worker3StatusCollection.Add(string.Format("Task Not completed with allotted time:{0}", miliSec / 1000));
                        Worker3StatusCollection.Add("Status:" + longRunningTaskWithTimeout.Status.ToString());
                        Worker3StatusCollection.Add(string.Format("End Worker3 at:{0}", DateTime.Now.ToLongTimeString()));
                        IsWorker3Busy = false;
                        ((DelegateCommand)Worker3Command).RaiseCanExecuteChanged();
                    }, null);
                }
            }));
        }
        private bool isWorker3Busy;
        public bool IsWorker3Busy
        {
            get { return isWorker3Busy; }
            set { PropertyChanged.ChangeAndNotify(ref isWorker3Busy, value, () => IsWorker3Busy); }
        }
        public ICommand Worker3Command { get; private set; }
        #endregion

        public event PropertyChangedEventHandler PropertyChanged;
    }

模仿Model层的类定义如下：一个方法等待10s模仿底层操作（Worker1，Worker2使用），另一个方法直接返回（供Worker3使用）

 public class Model
    {
        public string GetDataWithLongTime()
        {
            Thread.Sleep(TimeSpan.FromSeconds(10));
            return "Data from DataWithLongTime";
        }
        public string GetDataWithShortTime()
        {
            return "Data from DataWithShortTime";
        }
    }