正确进行线程 FTP 下载（Indy/Android）

Question

我已经创建了一个 IdFTP 来从我的 FTP 服务器下载文件，但是当我尝试将其设置为 Threaded 时，它卡在了 Android 操作系统中的“正在解析主机名...”中。

没有线程（工作正常）：

uses ..., IdFTPCommon;

var
  RecordDownload: TMemoryStream;

uses System.IOUtils;

procedure TForm1.Button1Click(Sender: TObject);
begin
  IdFTP1.Host := 'motoristaajudante.ddns.net';
  IdFTP1.Port := 2121;
  IdFTP1.DataPortMin := 50100;
  IdFTP1.DataPortMax := 51100;
  IdFTP1.Username := 'anonymous';
  IdFTP1.TransferType := IdFTPCommon.TIdFTPTransferType.ftBinary;
  IdFTP1.Passive := True;
  try
    IdFTP1.Connect();
    IdFTP1.Get('00001.m4a',TPath.GetDocumentsPath + PathDelim + '00001.m4a',True,False);
  except
    IdFTP1.Disconnect;
  end;
end;

procedure TForm1.IdFTP1AfterGet(ASender: TObject; AStream: TStream);
begin
  IdFTP1.Disconnect;
end;

procedure TForm1.IdFTP1WorkEnd(ASender: TObject; AWorkMode: TWorkMode);
begin
  if FileExists(TPath.GetDocumentsPath + PathDelim + '00001.m4a') then
  begin
      ShowMessage('Downloaded!');
  end;
end;

还有我在this solution 之后编写的线程代码：

uses ..., IdFTPCommon;

type
  TLoadThread = class(TThread)
  public
    constructor Create; reintroduce;
  protected
    procedure Execute; override;
  end;

type
  TForm1 = class(TForm)
  ...
  procedure ThreadTerminated(Sender: TObject);

var
  RecordDownload: TMemoryStream;
  Loading: Boolean = False;
  zLThread: TLoadThread = nil;

uses System.IOUtils;

constructor TLoadThread.Create;
begin
  inherited Create(True);
  FreeOnTerminate := True;
end;

procedure TLoadThread.Execute;
begin
try
  Form1.IdFTP1.Connect();
  Form1.IdFTP1.Get('00001.m4a',TPath.GetDocumentsPath + PathDelim + '00001.m4a',True,False);
except
  Form1.IdFTP1.Disconnect;
end;
end;

procedure TForm1.ThreadTerminated(Sender: TObject);
begin
  zLThread := nil;
  Loading := False;
  FloatAnimation1.Enabled := False;
  FloatAnimation2.Enabled := False;
  Arc3.StartAngle := -90;
  Arc3.EndAngle := 0;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  IdFTP1.Host := 'motoristaajudante.ddns.net';
  IdFTP1.Port := 2121;
  IdFTP1.DataPortMin := 50100;
  IdFTP1.DataPortMax := 51100;
  IdFTP1.Username := 'anonymous';
  IdFTP1.TransferType := IdFTPCommon.TIdFTPTransferType.ftBinary;
  IdFTP1.Passive := True;
  zLThread := TLoadThread.Create;
  zLThread.OnTerminate := ThreadTerminated;
  zLThread.Start;
  Loading := True;
  FloatAnimation1.Enabled := True;
  FloatAnimation2.Enabled := True;
end;

procedure TForm1.IdFTP1AfterGet(ASender: TObject; AStream: TStream);
begin
  IdFTP1.Disconnect;
end;

procedure TForm1.IdFTP1WorkEnd(ASender: TObject; AWorkMode: TWorkMode);
begin
  Form1.FloatAnimation1.Enabled := False;
  Form1.FloatAnimation2.Enabled := False;
  Form1.Arc3.StartAngle := -90;
  Form1.Arc3.EndAngle := 0;
  if FileExists(TPath.GetDocumentsPath + PathDelim + '00001.m4a') then
  begin
      ShowMessage('Downloaded!');
  end;
end;

procedure TForm1.IdFTP1Status(ASender: TObject; const AStatus: TIdStatus;
  const AStatusText: string);
begin
  Memo1.Lines.Add(AStatusText);
  Application.ProcessMessages;
end;

然后 FTP 状态显示它卡在“正在解析主机名 ...”。如何使其正确穿线？

Answer 1

没有理由像使用它们一样使用OnAfterGet 和OnWorkEnd 事件。 Indy 是同步的。 TIdFTP.Get() 在传输完成之前不会返回，如果发生错误，则会引发异常。

因此，摆脱这两个事件处理程序，使用try/finally 而不是try/except 调用Disconnect()，并且仅在Get() 未引发时才处理下载。

OnWorkEnd 和 OnStatus 事件在调用 Connect()、Disconnect() 和 Get() 的同一线程的上下文中触发。因此，在您的线程示例中，这将是工作线程，而不是主线程。但是您的事件处理程序不会同步对您的 UI 控件的访问。这可能会导致各种问题，包括您遇到的冻结。您必须同步（TThread.OnTerminated 事件已同步）。

话虽如此，试试这个：

非线程：

uses
  ..., IdFTPCommon;

...

procedure TForm1.Button1Click(Sender: TObject);
begin
  IdFTP1.Host := 'motoristaajudante.ddns.net';
  IdFTP1.Port := 2121;
  IdFTP1.DataPortMin := 50100;
  IdFTP1.DataPortMax := 51100;
  IdFTP1.Username := 'anonymous';
  IdFTP1.TransferType := IdFTPCommon.TIdFTPTransferType.ftBinary;
  IdFTP1.Passive := True;
  try
    IdFTP1.Connect;
    try
      IdFTP1.Get('00001.m4a', TPath.GetDocumentsPath + PathDelim + '00001.m4a', True, False);
    finally
      IdFTP1.Disconnect;
    end;
    ShowMessage('Downloaded!');
  except
    ShowMessage('Error while downloading!');
  end;
end;

线程：

uses
  ..., IdFTPCommon;

type
  TLoadThread = class(TThread)
  public
    constructor Create; reintroduce;
  protected
    procedure Execute; override;
  end;

type
  TForm1 = class(TForm)
    ...
    IdFTP1: TIdFTP;
    procedure ThreadTerminated(Sender: TObject);
    ... 
  private
    Loading: Boolean;
    zLThread: TLoadThread;
  end;

...

constructor TLoadThread.Create;
begin
  inherited Create(True);
  FreeOnTerminate := True;
end;

procedure TLoadThread.Execute;
begin
  Form1.IdFTP1.Connect;
  try
    Form1.IdFTP1.Get('00001.m4a', TPath.GetDocumentsPath + PathDelim + '00001.m4a', True, False);
  finally
    Form1.IdFTP1.Disconnect;
  end;
end;

procedure TForm1.ThreadTerminated(Sender: TObject);
begin
  zLThread := nil;
  Loading := False;
  FloatAnimation1.Enabled := False;
  FloatAnimation2.Enabled := False;
  Arc3.StartAngle := -90;
  Arc3.EndAngle := 0;
  If TThread(Sender).FatalException = nil then
    ShowMessage('Downloaded!')
  else
    ShowMessage('Error while Downloading!');
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  IdFTP1.Host := 'motoristaajudante.ddns.net';
  IdFTP1.Port := 2121;
  IdFTP1.DataPortMin := 50100;
  IdFTP1.DataPortMax := 51100;
  IdFTP1.Username := 'anonymous';
  IdFTP1.TransferType := IdFTPCommon.TIdFTPTransferType.ftBinary;
  IdFTP1.Passive := True;
  zLThread := TLoadThread.Create;
  zLThread.OnTerminate := ThreadTerminated;
  zLThread.Start;
  Loading := True;
  FloatAnimation1.Enabled := True;
  FloatAnimation2.Enabled := True;
end;

procedure TForm1.IdFTP1Status(ASender: TObject; const AStatus: TIdStatus; const AStatusText: string);
begin
  TThread.Queue(nil, 
    procedure
    begin
      Memo1.Lines.Add(AStatusText);
    end
  );
end;