Question

以下代码解释了这个想法

    private async void button1_Click(object sender, EventArgs e)
    {
        string result;
        CancellationToken cancellationToken = new CancellationTokenSource(3000).Token;

        try
        {
            result = await GetDataAsync(cancellationToken)
                .ContextIfSuccess();    // Should use SynchronizationContext only if Task status is RanToCompletion
        }
        catch(OperationCanceledException)
        {
            /* Context is not required */
            return;
        }
        catch (Exception ex)
        {
            /* Context is not required otherwise it can slow down UI Thread a little bit */
            Log(ex.ToString());
            return;
        }

        /* UI Thread only */
        button1.Text = result;
    }

问题是＆＃34;是否可以像ContextIfSuccess（）一样制作方法？＆＃34;

Answer 1

为了拥有您想要的方法，您需要创建自定义awaiter。它主要是样板文件，关键在于，当被要求添加延续时，使用当前同步上下文添加一个以在成功完成时运行，并且在没有运行完成时使用默认调度程序。

public struct CaptureContextOnSuccessAwaiter : INotifyCompletion
{
    private Task task;

    public CaptureContextOnSuccessAwaiter(Task task)
    {
        this.task = task;
    }

    public CaptureContextOnSuccessAwaiter GetAwaiter() { return this; }

    public void OnCompleted(Action continuation)
    {
        if (SynchronizationContext.Current != null)
        {
            task.ContinueWith(t => continuation(),
                CancellationToken.None,
                TaskContinuationOptions.OnlyOnRanToCompletion,
                TaskScheduler.FromCurrentSynchronizationContext());
            task.ContinueWith(t => continuation(),
                CancellationToken.None,
                TaskContinuationOptions.NotOnRanToCompletion,
                TaskScheduler.Default);
        }
        else
        {
            task.ContinueWith(t => continuation(),
                CancellationToken.None,
                TaskContinuationOptions.None,
                TaskScheduler.Default);
        }
    }

    public void GetResult() { task.GetAwaiter().GetResult(); }
    public bool IsCompleted { get { return task.GetAwaiter().IsCompleted; } }
}

public struct CaptureContextOnSuccessAwaiter<T> : INotifyCompletion
{
    private Task<T> task;

    public CaptureContextOnSuccessAwaiter(Task<T> task)
    {
        this.task = task;
    }

    public CaptureContextOnSuccessAwaiter<T> GetAwaiter() { return this; }

    public void OnCompleted(Action continuation)
    {
        if (SynchronizationContext.Current != null)
        {
            task.ContinueWith(t => continuation(),
                CancellationToken.None,
                TaskContinuationOptions.OnlyOnRanToCompletion,
                TaskScheduler.FromCurrentSynchronizationContext());
            task.ContinueWith(t => continuation(),
                CancellationToken.None,
                TaskContinuationOptions.NotOnRanToCompletion,
                TaskScheduler.Default);
        }
        else
        {
            task.ContinueWith(t => continuation(),
                CancellationToken.None,
                TaskContinuationOptions.None,
                TaskScheduler.Default);
        }
    }

    public T GetResult() { return task.GetAwaiter().GetResult(); }
    public bool IsCompleted { get { return task.GetAwaiter().IsCompleted; } }
}

public static CaptureContextOnSuccessAwaiter ContextIfSuccess(this Task task)
{
    return new CaptureContextOnSuccessAwaiter(task);
}

public static CaptureContextOnSuccessAwaiter<T> ContextIfSuccess<T>(this Task<T> task)
{
    return new CaptureContextOnSuccessAwaiter<T>(task);
}

Answer 2

await没有上下文（ConfigureAwait(false)）。然后，如果所需条件为真，则切换到上下文。

因此，您需要捕获SynchronizationContext.Current和Post。

这与TaskAwaiter无论如何都非常相似。它在没有上下文的情况下恢复，然后在调用者希望的情况下切换回上下文。

您应该可以将其转换为ContextIfSuccess方法。基本上，克隆TaskAwaiter源代码并决定是否在完成通知中Post。我假设这个功能已经存在。代码必须查看ConfigureAwait(...)值并有条件地应用上下文。

仅在Task RanToCompletion上使用SynchronizationContext

2 个答案: