异步等待继续未按预期工作

Question

我正在开发一个WPF Prism 应用程序。我有一个DelgateCommand，它负责填充一个ObservableCollection，它由UI线程异步拥有using async and await。该集合又被绑定到一个图表。

为了使multiple threads能够访问集合，我启用了synchronization，如下：

BindingOperations.EnableCollectionSynchronization(ChartBoundCollection, _lock);

命令处理逻辑如下：

    private async void ShowPatientVisitsVsDays()
        {
          IsChartBeingPopulated = true;
          this.ChartSubTitle = "Requests Vs Days";
          this.SeriesTitle = "Days";
          ChartBoundCollection.Clear();
          await ShowRequestsVsDaysAsync();
          IsChartBeingPopulated = false;
        }

填充可观察集合的逻辑如下：

private async Task ShowRequestsVsDaysAsync()
    {
        await Task.Run(() =>
            {
                if (PatientVisits.Count() > 0)
                {
                    var days = PatientVisits.Select(p => p.VisitDate.Value.Date).Distinct();
                    foreach (var i in days)
                    {

                        var dayVisitCount = PatientVisits.Where(p => p.VisitDate.Value.Date == i).Count();
                        ChartBoundCollection.Add(new PatientVisitVsXAxis() { XAxis = i.ToShortDateString(), NumberOfPatientVisits = dayVisitCount });
                    }
                }
            });
    }

我面临的问题是，在设置了 await 的异步方法完成后，我设置 IsChartBeingPopulated = false 的延续没有得到执行。

await ShowRequestsVsDaysAsync();

因此 IsChartBeingPopulated 甚至在异步方法之前设置 完成了。

命令处理程序 ShowPatientVisitsVsDays() 由点击调用 视图上的按钮。该按钮绑定到以下 命令：

ShowPatientVisitsVsDaysCommand = new DelegateCommand(ShowPatientVisitsVsDays);

IsChartBeingPopulated 用于设置属于“扩展 WPF 工具包”的BusyIndiator 控件的IsBusy DependencyProperty。 这个想法是在绑定集合中填充图表数据时显示BusyIndicator。

<xctk:BusyIndicator IsBusy="{Binding Path=IsChartBeingPopulated}" >
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="*"></RowDefinition>
            </Grid.RowDefinitions>
            <chart:ClusteredColumnChart Grid.Row="0" ChartTitle="Patient Visits History" ChartSubTitle="{Binding Path=ChartSubTitle}">
                <chart:ClusteredColumnChart.Series>
                    <chart:ChartSeries SeriesTitle="{Binding Path=SeriesTitle}" DisplayMember="XAxis" ValueMember="NumberOfPatientVisits" ItemsSource="{Binding Path=ChartBoundCollection}" />
                </chart:ClusteredColumnChart.Series>
            </chart:ClusteredColumnChart>
        </Grid>
    </xctk:BusyIndicator>

不确定是什么问题。有人知道是什么原因造成的吗？

Answer 1

您没有同步对集合的访问。 BindingOperations.EnableCollectionSynchronization 不会神奇地使收集线程安全。它只确保数据绑定引擎不会在不获取锁的情况下枚举集合。

在添加和清除集合时，您仍然需要锁定_lock 对象。

有关EnableCollectionSynchronization 的更多信息，请参阅here。

Answer 2

你可以这样做；

    private async void ShowPatientVisitsVsDays()
    {
        IsChartBeingPopulated = true;
        this.ChartSubTitle = "Requests Vs Days";
        this.SeriesTitle = "Days";
        new ChartBoundCollection().Clear();
        IsChartBeingPopulated = await ShowRequestsVsDaysAsync();//here we are waiting till the async method is finished.
    }


    private async Task<bool> ShowRequestsVsDaysAsync()
    {
       return await Task.Run(() =>
        {
            if (PatientVisits.Any())//replace Count with Any to avoid unwanted enumerations.
            {
                var days = PatientVisits.Select(p => p.VisitDate.Value.Date).Distinct();
                foreach (var i in days)
                {

                    var dayVisitCount = PatientVisits.Count(p => p.VisitDate.Value.Date == i);
                    chartBoundCollection.Add(new PatientVisitVsXAxis() { XAxis = i.ToShortDateString(), NumberOfPatientVisits = dayVisitCount });
                }
            }
            Thread.Sleep(5000);//this is for testing. Sleep the thread for 5secs. Now your busyIndicator must be visible for 5secs minimum.
            return false;//return false, so that we can use it to populate IsChartBeingPopulated 
        });
    }

更新：我认为您对异步等待有一些疑问。下面的代码将帮助您清除它们。

创建一个新的控制台应用程序并将此代码放在Program 类中。

根据您在下面的评论，ShowPatientVisitsVsDays 中的文本应该在从“ShowRequestsVsDaysAsync”方法打印任何内容之前打印出来。它是这样工作的吗？对此进行测试并告诉我们。

   static void Main(string[] args)
    {
        Console.WriteLine("Main started");
        ShowPatientVisitsVsDays();
        Console.ReadLine();
    }

   private static async void ShowPatientVisitsVsDays()
    {
        await ShowRequestsVsDaysAsync();            
        Console.WriteLine("ShowPatientVisitsVsDays() method is going to SLEEP");
        Thread.Sleep(2000);
        Console.WriteLine("ShowPatientVisitsVsDays() method finished");
    }

   private static async Task ShowRequestsVsDaysAsync()
    {
        await Task.Run(() =>
        {
            Console.WriteLine("ASYNC ShowRequestsVsDaysAsync() is going to SLEEP.");
            Thread.Sleep(5000);
            Console.WriteLine("ASYNC ShowRequestsVsDaysAsync finished.");
        });
    }
 }