异步等待创建文件

Question

外部应用程序创建文件await的最简洁方法是什么？

    async Task doSomethingWithFile(string filepath)
    {
        // 1. await for path exists
        // 2. Do something with file
    }

Answer 1

因此，第一个关键点是，当文件系统事件在特定路径发生更改时，您可以使用FileSystemWatcher来通知。例如，如果您希望在特定位置创建文件时收到通知，则可以找到。

接下来，我们可以创建一个方法，当文件系统观察程序触发相关事件时，使用TaskCompletionSource来触发任务的完成。

public static Task WhenFileCreated(string path)
{
    if (File.Exists(path))
        return Task.FromResult(true);

    var tcs = new TaskCompletionSource<bool>();
    FileSystemWatcher watcher = new FileSystemWatcher(Path.GetDirectoryName(path));

    FileSystemEventHandler createdHandler = null;
    RenamedEventHandler renamedHandler = null;
    createdHandler = (s, e) =>
    {
        if (e.Name == Path.GetFileName(path))
        {
            tcs.TrySetResult(true);
            watcher.Created -= createdHandler;
            watcher.Dispose();
        }
    };

    renamedHandler = (s, e) =>
    {
        if (e.Name == Path.GetFileName(path))
        {
            tcs.TrySetResult(true);
            watcher.Renamed -= renamedHandler;
            watcher.Dispose();
        }
    };

    watcher.Created += createdHandler;
    watcher.Renamed += renamedHandler;

    watcher.EnableRaisingEvents = true;

    return tcs.Task;
}

请注意，首先检查文件是否存在，以便在适用时立即退出。它还使用创建的和重命名的处理程序，因为任一选项都可以允许文件在将来的某个时刻存在。 FileSystemWatcher也只监视目录，因此获取指定路径的目录然后检查事件处理程序中每个受影响文件的文件名很重要。

另请注意，代码在完成后会删除事件处理程序。

这允许我们写：

public static async Task Foo()
{
    await WhenFileCreated(@"C:\Temp\test.txt");
    Console.WriteLine("It's aliiiiiive!!!");
}

Answer 2

使用自定义ReactiveExtension运算符的完整解决方案：WaitIf。 这要求Genesis.RetryWithBackoff通过NuGet可用

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reactive.Concurrency;
using System.Reactive.Linq;
using System.Reactive.Threading.Tasks;


public class TestWatcher
{

    public static void Test()
    {

        FileSystemWatcher Watcher = new FileSystemWatcher("C:\\test")
        {
            EnableRaisingEvents = true,
        };

        var Created = Observable
            .FromEventPattern<FileSystemEventHandler, FileSystemEventArgs>(h => Watcher.Created += h, h => Watcher.Created -= h)
            .Select(e => e.EventArgs.FullPath);
        var CreatedAndNotLocked = Created.WaitIf(IsFileLocked,100, attempt =>TimeSpan.FromMilliseconds(100), Scheduler.Default);
        var FirstCreatedAndNotLocked = CreatedAndNotLocked.Take(1)
            .Finally(Watcher.Dispose);
        var task = FirstCreatedAndNotLocked.GetAwaiter().ToTask();
        task.Wait();
        Console.WriteLine(task.Result);

    }

    public bool IsFileLocked(string filePath)
    {
        var ret = false;
        try
        {
            using (File.Open(filePath, FileMode.Open)) { }
        }
        catch (IOException e)
        {
            var errorCode = Marshal.GetHRForException(e) & ((1 << 16) - 1);
            ret = errorCode == 32 || errorCode == 33;
        }
        return ret;
    }
}



public static class ObservableExtensions
{


    public class NotReadyException : Exception
    {
        public NotReadyException (string message) : base(message)
        {
        }
    }

    public static IObservable<T> WaitIf<T>(
      this IObservable<T> @this,
      Func<T, bool> predicate,
      int? retryCount = null,
      Func<int, TimeSpan> strategy = null,
      Func<Exception, bool> retryOnError = null,
      IScheduler scheduler = null)
    {
        scheduler = scheduler ?? DefaultScheduler.Instance;
        return @this.SelectMany(f =>
        Observable.Defer(() =>
           Observable.FromAsync<bool>(() => Task.Run<bool>(() => predicate.Invoke(f)),scheduler)
           .SelectMany(b => b ? Observable.Throw<T>(new NotReadyException(f + " not ready")) :
                           Observable.Return(f)
        ).RetryWithBackoff(retryCount, strategy, retryOnError, scheduler)));
    }
}

Answer 3

这是Servy的solution的功能更丰富的版本。它允许监视特定的文件系统状态和事件，以涵盖不同的场景。超时和CancellationToken都可以取消。

[Flags]
public enum WatchFileType
{
    Created = 1,
    Deleted = 2,
    Changed = 4,
    Renamed = 8,
    Exists = 16,
    ExistsNotEmpty = 32,
    NotExists = 64,
}

public static Task<WatchFileType> WatchFile(string filePath,
    WatchFileType watchTypes,
    int timeout = Timeout.Infinite,
    CancellationToken cancellationToken = default)
{
    var tcs = new TaskCompletionSource<WatchFileType>();
    var fileName = Path.GetFileName(filePath);
    var folderPath = Path.GetDirectoryName(filePath);
    var fsw = new FileSystemWatcher(folderPath);
    fsw.Filter = fileName;

    if (watchTypes.HasFlag(WatchFileType.Created)) fsw.Created += Handler;
    if (watchTypes.HasFlag(WatchFileType.Deleted)) fsw.Deleted += Handler;
    if (watchTypes.HasFlag(WatchFileType.Changed)) fsw.Changed += Handler;
    if (watchTypes.HasFlag(WatchFileType.Renamed)) fsw.Renamed += Handler;

    void Handler(object sender, FileSystemEventArgs e)
    {
        WatchFileType result;
        switch (e.ChangeType)
        {
            case WatcherChangeTypes.Created: result = WatchFileType.Created; break;
            case WatcherChangeTypes.Deleted: result = WatchFileType.Deleted; break;
            case WatcherChangeTypes.Changed: result = WatchFileType.Changed; break;
            case WatcherChangeTypes.Renamed: result = WatchFileType.Renamed; break;
            default: throw new NotImplementedException(e.ChangeType.ToString());
        }
        fsw.Dispose();
        tcs.TrySetResult(result);
    }

    fsw.Error += (object sender, ErrorEventArgs e) =>
    {
        fsw.Dispose();
        tcs.TrySetException(e.GetException());
    };

    CancellationTokenRegistration cancellationTokenReg = default;

    fsw.Disposed += (object sender, EventArgs e) =>
    {
        cancellationTokenReg.Dispose();
    };

    fsw.EnableRaisingEvents = true;

    var fileInfo = new FileInfo(filePath);
    if (watchTypes.HasFlag(WatchFileType.Exists) && fileInfo.Exists)
    {
        fsw.Dispose();
        tcs.TrySetResult(WatchFileType.Exists);
    }
    if (watchTypes.HasFlag(WatchFileType.ExistsNotEmpty)
        && fileInfo.Exists && fileInfo.Length > 0)
    {
        fsw.Dispose();
        tcs.TrySetResult(WatchFileType.ExistsNotEmpty);
    }
    if (watchTypes.HasFlag(WatchFileType.NotExists) && !fileInfo.Exists)
    {
        fsw.Dispose();
        tcs.TrySetResult(WatchFileType.NotExists);
    }

    if (cancellationToken.CanBeCanceled)
    {
        cancellationTokenReg = cancellationToken.Register(() =>
        {
            fsw.Dispose();
            tcs.TrySetCanceled(cancellationToken);
        });
    }

    if (tcs.Task.IsCompleted || timeout == Timeout.Infinite)
    {
        return tcs.Task;
    }

    // Handle timeout
    var cts = new CancellationTokenSource();
    var delayTask = Task.Delay(timeout, cts.Token);
    return Task.WhenAny(tcs.Task, delayTask).ContinueWith(_ =>
    {
        cts.Cancel();
        if (tcs.Task.IsCompleted) return tcs.Task;
        fsw.Dispose();
        return Task.FromCanceled<WatchFileType>(cts.Token);
    }, TaskContinuationOptions.ExecuteSynchronously).Unwrap();
}

用法示例：

var result = await WatchFile(@"..\..\_Test.txt",
    WatchFileType.Exists | WatchFileType.Created, 5000);

在此示例中，结果通常为WatchFileType.Exists或WatchFileType.Created。在特殊情况下，如果文件不存在并且没有创建5000毫秒，则会抛出TaskCanceledException。

场景
•WatchFileType.Exists | WatchFileType.Created：用于一次性创建的文件。
•WatchFileType.ExistsNotEmpty | WatchFileType.Changed：用于首先创建为空然后再填充数据的文件。
•WatchFileType.NotExists | WatchFileType.Deleted：用于将要删除的文件。

Answer 4

我就是这样做的：

await Task.Run(() => {while(!File.Exists(@"yourpath.extension")){} return;});
//do all the processing

您也可以将其打包成方法：

public static Task WaitForFileAsync(string path)
{
    if (File.Exists(path)) return Task.FromResult<object>(null);
    var tcs = new TaskCompletionSource<object>();
    FileSystemWatcher watcher = new FileSystemWatcher(Path.GetDirectoryName(path));
    watcher.Created += (s, e) => 
    { 
        if (e.FullPath.Equals(path))
        { 
            tcs.TrySetResult(null);
            if (watcher != null)
            {
                watcher.EnableRaisingEvents = false;
                watcher.Dispose();
            }
        } 
    };
    watcher.Renamed += (s, e) =>
    {
        if (e.FullPath.Equals(path))
        {
            tcs.TrySetResult(null);
            if (watcher != null)
            {
                watcher.EnableRaisingEvents = false;
                watcher.Dispose();
            }
        }
    };
    watcher.EnableRaisingEvents = true;
    return tcs.Task;
}

然后只需将其用作：

await WaitForFileAsync("yourpath.extension");
//do all the processing