等待的 FileSystemWatcher 结果 [重复]答案

【问题标题】：Awaitable FileSystemWatcher Result [duplicate]等待的 FileSystemWatcher 结果 [重复]
【发布时间】：2018-10-16 22:07:15
【问题描述】：

我围绕FileSystemWatcher 创建了一个包装类。该类的目的是等待创建文本文件，然后将文件的内容读入其_result 字段。

我想向名为GetResultAsync() 的类添加一个方法，以便调用者可以await 准备好结果，但我不知道如何最好地去做。请有人指出我正确的方向吗？

internal class SingleFileWatcher : IDisposable
{
    private readonly string _targetFile;
    private readonly FileSystemWatcher _fileWatcher;
    private string _result = null;

    internal SingleFileWatcher(string theFile)
    {
        _targetFile = theFile;

        _fileWatcher = new FileSystemWatcher();
        _fileWatcher.Path = Path.GetDirectoryName(theFile);
        _fileWatcher.Filter = Path.GetFileName(theFile);
        _fileWatcher.IncludeSubdirectories = false;
        _fileWatcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite | NotifyFilters.FileName;
        _fileWatcher.Created += FileCreated;

        if (File.Exists(theFile))
        {
            ProcessFile();
        }
        else
        {
            //this must happen after other members have been set
            _fileWatcher.EnableRaisingEvents = true;
        }
    }

    internal Task<string> GetResultAsync()
    {
        //todo
    }

    private void FileCreated(Object sender, FileSystemEventArgs e)
    {
        ProcessFile();
    }

    private void ProcessFile()
    {

        FileStream stream = null;

        //filecreated is raised as soon as the file is created, but the process may still be writing to the file
        while (true)
        {
            try
            {
                stream = new FileStream(_targetFile, FileMode.Open, FileAccess.Read);

                using (var reader = new StreamReader(stream))
                {
                    stream = null;
                    _result = reader.ReadToEnd();
                }

                break;
            }
            catch (IOException)
            {
                Thread.Sleep(TimeSpan.FromMilliseconds(500));
            }
            finally
            {
                stream?.Dispose();
            }
        }
    }

    public void Dispose()
    {
        _fileWatcher.Created -= FileCreated;
        _fileWatcher.Dispose();
    }
}

【问题讨论】：

TaskCompletionSource 可能是你的朋友。
谢谢，TaskCompletionSource 为我解决了这个问题。

标签： c#

【解决方案1】：

扩展 Jeroen Mosterts 的评论。您的代码应如下所示：

internal Task<string> GetResultAsync()
{
    var tcs = new TaskCompletionSource<string>();
    ProcessFile();
    tcs.TrySetResult(_result);
    return tcs.Task;
}

这很幼稚，我建议您考虑异常处理和取消的场景。它们应该在ProcessFile 内，所以它看起来像Task<string> ProcessFile(TaskCompletionSource<string> tcs)。设置异常使用tcs.TrySetException(exception)。

【讨论】：