如何避免getfileattributes中的网络暂停?

问题不是GetFileAttributes。它通常只使用一个对底层文件系统驱动程序的调用。是那个IO在拖延。

不过,解决方案可能很简单。呼叫 CancelSynchronousIo() 一秒钟后(这显然需要第二个线程,因为第一个线程卡在getfileattributes中)。

关于代表的一个很酷的事情是你可以 BeginInvoke 和 EndInvoke 他们。只要确保被调用的方法不会抛出异常,因为[我相信]它会导致崩溃(未处理的异常)。

AttributeType attributes = default(AttributeType);

Action<string> helper =
    (path) =>
    {
        try
        {
            // GetFileAttributes
            attributes = result;
        }
        catch
        {
        }
    };
IAsyncResult asyncResult = helper.BeginInvoke();
// whatever
helper.EndInvoke();
// at this point, the attributes local variable has a valid value.

我认为您最好的解决方案是使用线程池线程来执行该工作。

• 分配工作单元以查询文件的属性
• 让 GetFileAttributes 运行到完成
• 将结果发回表单
• 当线程函数完成时,线程自动返回池(不需要终止它)

通过使用线程池,您可以节省创建新线程的成本。
你也省去了试图摆脱他们的痛苦。

然后您就有了一个方便的助手方法,它可以在线程池线程上使用 QueueUserWorkItem :

RunInThreadPoolThread(
      GetFileAttributesThreadMethod, 
      TGetFileAttributesData.Create('D:\temp\foo.xml', Self.Handle), 
      WT_EXECUTEDEFAULT);

创建对象以保存线程数据信息:

TGetFileAttributesData = class(TObject)
public
    Filename: string;
    WndParent: HWND;
    Attributes: DWORD;
    constructor Create(Filename: string; WndParent: HWND);
end;

然后创建线程回调方法:

procedure TForm1.GetFileAttributesThreadMethod(Data: Pointer);
var
    fi: TGetFileAttributesData;
begin
    fi := TObject(Data) as TGetFileAttributesData;
    if fi = nil then
        Exit;

    fi.attributes := GetFileAttributes(PWideChar(fi.Filename));

    PostMessage(fi.WndParent, WM_GetFileAttributesComplete, NativeUInt(Data), 0);
end;

然后您只需处理消息:

procedure WMGetFileAttributesComplete(var Msg: TMessage); message WM_GetFileAttributesComplete;

procedure TfrmMain.WMGetFileAttributesComplete(var Msg: TMessage);
var
    fi: TGetFileAttributesData;
begin
    fi := TObject(Pointer(Msg.WParam)) as TGetFileAttributesData;
    try
        ShowMessage(Format('Attributes of "%s": %.8x', [fi.Filename, fi.attributes]));
    finally
        fi.Free;
    end;
end;

神奇的 RunInThreadPoolThread 只是让您在线程中执行实例方法的一点错误:

它只是一个包装器,允许您对实例变量调用方法:

TThreadMethod = procedure (Data: Pointer) of object;

TThreadPoolCallbackContext = class(TObject)
public
    ThreadMethod: TThreadMethod;
    Context: Pointer;
end;

function ThreadPoolCallbackFunction(Parameter: Pointer): Integer; stdcall;
var
    tpContext: TThreadPoolCallbackContext;
begin
    try
        tpContext := TObject(Parameter) as TThreadPoolCallbackContext;
    except
        Result := -1;
        Exit;
    end;
    try
        tpContext.ThreadMethod(tpContext.Context);
    finally
        try
            tpContext.Free;
        except
        end;
    end;
    Result := 0;
end;

function RunInThreadPoolThread(const ThreadMethod: TThreadMethod; const Data: Pointer; Flags: ULONG): BOOL;
var
    tpContext: TThreadPoolCallbackContext;
begin
    {
        Unless you know differently, the flag you want to use is 0 (WT_EXECUTEDEFAULT).

        If your callback might run for a while you can pass the WT_ExecuteLongFunction flag.
                Sure, I'm supposed to pass WT_EXECUTELONGFUNCTION if my function takes a long time, but how long is long?
                http://blogs.msdn.com/b/oldnewthing/archive/2011/12/09/10245808.aspx

        WT_EXECUTEDEFAULT (0):
                By default, the callback function is queued to a non-I/O worker thread.
                The callback function is queued to a thread that uses I/O completion ports, which means they cannot perform
                an alertable wait. Therefore, if I/O completes and generates an APC, the APC might wait indefinitely because
                there is no guarantee that the thread will enter an alertable wait state after the callback completes.
        WT_EXECUTELONGFUNCTION (0x00000010):
                The callback function can perform a long wait. This flag helps the system to decide if it should create a new thread.
        WT_EXECUTEINPERSISTENTTHREAD (0x00000080)
                The callback function is queued to a thread that never terminates.
                It does not guarantee that the same thread is used each time. This flag should be used only for short tasks
                or it could affect other timer operations.
                This flag must be set if the thread calls functions that use APCs.
                For more information, see Asynchronous Procedure Calls.
                Note that currently no worker thread is truly persistent, although worker threads do not terminate if there
                are any pending I/O requests.
    }

    tpContext := TThreadPoolCallbackContext.Create;
    tpContext.ThreadMethod := ThreadMethod;
    tpContext.Context := Data;

    Result := QueueUserWorkItem(ThreadPoolCallbackFunction, tpContext, Flags);
end;

给读者的练习 创建一个 取消 旗内 GetFileAttributesData 告诉线程 它 必须释放数据对象并 不 向家长发送消息。

要说你在创造:

DWORD WINAPI GetFileAttributes(
  _In_      LPCTSTR                         lpFileName,
  _Inout_   LPOVERLAPPED                    lpOverlapped,
  _In_      LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);