AsyncCallback 到 BackgroundWorker

Question

我想使用 .NET-FTP 库 (http://netftp.codeplex.com)。该库提供 BeginOpenRead(string,AsyncCallback,object) 以使用异步编程模型下载内容。我对Callback的实现和例子基本一样：

static void BeginOpenReadCallback(IAsyncResult ar) {
        FtpClient conn = ar.AsyncState as FtpClient;

        try {
            if (conn == null)
                throw new InvalidOperationException("The FtpControlConnection object is null!");

            using (Stream istream = conn.EndOpenRead(ar)) {
                byte[] buf = new byte[8192];

                try {
                    DateTime start = DateTime.Now;

                    while (istream.Read(buf, 0, buf.Length) > 0) {
                        double perc = 0;

                        if (istream.Length > 0)
                            perc = (double)istream.Position / (double)istream.Length;

                        Console.Write("\rTransferring: {0}/{1} {2}/s {3:p}         ",
                                      istream.Position.FormatBytes(),
                                      istream.Length.FormatBytes(),
                                      (istream.Position / DateTime.Now.Subtract(start).TotalSeconds).FormatBytes(),
                                      perc);
                    }
                }
                finally {
                    Console.WriteLine();
                    istream.Close();
                }
            }
        }
        catch (Exception ex) {
            Console.WriteLine(ex.ToString());
        }
        finally {
            m_reset.Set();
        }
    }

在异步方法的工作完成后，如果触发一个 Completed 事件（由启动异步方法的线程以使 UI 没有问题）将结果传递给 Main- 那就太好了-线。就像 BackgroundWorker 一样（使用 RunWorkerCompleted）。

我怎样才能意识到这一点？

Answer 1

尝试将APM 模式转换为TAP 模式（more info）：

static public Task<Stream> OpenReadAsync(FtpClient ftpClient, string url)
{
    return Task.Factory.FromAsync(
         (asyncCallback, state) =>
             ftpClient.BeginOpenRead(url, asyncCallback, state),
         (asyncResult) =>
             ftpClient.EndOpenRead((asyncResult));
}

那你就可以使用async/await，不用担心同步上下文了：

Stream istream = await OpenReadAsync(ftpClient, url); 

另外，你可以使用Stream.ReadAsync:

while (await istream.ReadAsync(buf, 0, buf.Length) > 0) 
{
    // ...
}

BackgroundWorker 已被基于任务的 API 取代，因此可能是双赢的局面（更多信息：Task.Run vs BackgroundWorker 和 here）。

[更新]如果您在 VS2012+ 中工作，您可以使用 Microsoft.Bcl.Async 定位 .NET 4.0，并且仍然使用现代语言和 TPL 功能，例如 async/await。我已经经历过，我强烈推荐它，因为它让未来移植到 .NET 4.5 变得轻而易举。

否则，您可以使用Task.ContinueWith(callback, TaskScheduler.FromCurrentSynchronizationContext()) 继续处理 UI 线程。这是related example。

Answer 2

最简单的方法是将SynchronizationContext 传递给BeginOpenRead 并在回调中使用它。

private class StateHolder
{
    public StateHolder(FtpClient client, SynchronizationContext context)
    {
        Client = client;
        Context = context;

        //SynchronizationContext.Current can return null, this creates a new context that posts to the Thread Pool if called.
        if(Context == null)
            Context = new SynchronizationContext();
    }

    public FtpClient Client {get; private set;}
    public SynchronizationContext Context {get; private set;}
}

//...

ftpClient.BeginOpenRead(someString,BeginOpenReadCallback, new StateHolder(ftpClient, SynchronizationContext.Current));

然后在你的回调中使用你传入的那个状态对象。

void BeginOpenReadCallback(IAsyncResult ar) 
{
    StateHolder state = ar.AsyncState as StateHolder;
    FtpClient conn = state.client;

    //... Everything else the same in the function.

    //state.Context can't be null because we set it in the constructor.
    state.Context.Post(OnCompleted, conn);

}

protected virtual void OnCompleted(object state) //I use object instead of FtpClient to make the "state.Context.Post(OnCompleted, conn);" call simpler.
{
    var conn = state as FtpClient;
    var tmp = Completed; //This is the event people subscribed to.
    (tmp != null)
    {
        tmp(this, new CompletedArgs(conn)); //Assumes you followed the standard Event pattern and created the necessary classes.
    }
}