使用asyc await从事件更新UI

Question

我试图了解如何在使用async/await模式时从事件更新UI.下面是我在WinForm应用程序上使用的测试代码.我甚至不确定这是正确的方法.允许pwe_StatusUpdate方法更新UI的必要条件是什么？抛出跨线程操作错误.

谢谢阅读.

 // calling code
    ProcessWithEvents pwe = new ProcessWithEvents();
    pwe.StatusUpdate += pwe_StatusUpdate;
    await pwe.Run();



 void pwe_StatusUpdate(string updateMsg)
    {
      // Error Here: Cross-thread operation not valid: Control '_listBox_Output' accessed from a thread other than the thread it was created on.
      _listBox_Output.Items.Add(updateMsg);
    }

-

// Class with long running process and event    
public delegate void StatusUpdateHandler(string updateMsg);

 public class ProcessWithEvents
  {
    public event StatusUpdateHandler StatusUpdate;

    public async Task Run()
    {
        await Task.Run(() =>
        {
            for (int i = 0; i < 10; i++)
                {

                    RaiseUpdateEvent(String.Format("Update {0}", i));

                    Thread.Sleep(500);
                }
            });

        }

        private void RaiseUpdateEvent(string msg)
        {
        if (StatusUpdate != null)
            StatusUpdate(msg);
        }
   }

-

Answer 1

该async模式支持进度更新.

简而言之,您的async方法可以采用IProgress<T>,并且您的调用代码会传递该接口的实现(通常Progress<T>).

public class ProcessWithUpdates
{
  public async Task Run(IProgress<string> progress)
  {
    await Task.Run(() =>
    {
      for (int i = 0; i < 10; i++)
      {
        if (progress != null)
          progress.Report(String.Format("Update {0}", i));
        Thread.Sleep(500);
      }
    });
  }
}

// calling code
ProcessWithUpdates pwp = new ProcessWithUpdates();
await pwp.Run(new Progress<string>(pwp_StatusUpdate));

Answer 2

你应该使用Invoke的方法Control。它在 Control 的线程中执行一些代码。您还可以检查InvokeRequired属性以检查是否需要调用Invoke方法（它检查调用者是否位于与创建控件的线程不同的线程上）。

简单的例子：

void SomeAsyncMethod()
{
    // Do some work             

    if (this.InvokeRequired)
    {
        this.Invoke((MethodInvoker)(() =>
            {
                DoUpdateUI();

            }
        ));
    }
    else
    {
        DoUpdateUI();
    }
}

void DoUpdateUI()
{
    // Your UI update code here
}

在某些情况下，您应该在调用方法之前检查IsHandleCreated属性。如果返回 false 那么你需要等待 Control 的句柄将被创建ControlInvokeIsHandleCreated