delphi - 如何在delphi 10中的任务线程内使用for循环

Question

如何在 TTask 中正确使用 for 循环？我总是只获取备忘录中 ListBox1 的最后一项，例如，如果 ListBox1 中有 5 个项目，我会在 memo1 中获取 ListBox1 最后一项 5 次!代码有什么问题吗？

var
  i: Integer;
  lPath: string;
begin
  for i := 0 to ListBox1.Items.Count - 1 do
  begin
    lPath := ListBox1.Items.Strings[i];
    TTask.Create(
      procedure
      var
        lHTTP: TIdHTTP;
        IdSSL: TIdSSLIOHandlerSocketOpenSSL;
      begin
        lHTTP := TIdHTTP.Create(nil);

        TThread.Synchronize(nil,
          procedure
          begin
            Form1.Caption := 'Task Running...';
          end
        );

        try
          lHTTP.ReadTimeout := 30000;
          lHTTP.HandleRedirects := True;
          IdSSL := TIdSSLIOHandlerSocketOpenSSL.Create(lHTTP);
          IdSSL.SSLOptions.Method := sslvTLSv1;
          IdSSL.SSLOptions.Mode := sslmClient;
          lHTTP.IOHandler := IdSSL;
        Finally
          try
            lHTTP.Get('http://website.com/'+lPath, TStream(nil));
          Finally
            lHTTP.Free;
          end;
        end;

        TThread.Synchronize(nil,
          procedure
          begin
            Memo1.Lines.Add(lPath);
           end
        );

      end
    ).Start;

  end;
end;

Answer 1

这是改编后的代码:

// Current method:
procedure TMyForm.XYZ
var
  i: Integer;
  lPath: string;
begin
  for i := 0 to ListBox1.Items.Count - 1 do
  begin
    lPath := ListBox1.Items.Strings[i];
    StartDownloadTask(lPath);
  end;
end;

// Put task creation in separate method:
procedure TMyForm.StartDownloadTask(lPath: string)
begin
    TTask.Create(
      procedure
      var
        lHTTP: TIdHTTP;
        IdSSL: TIdSSLIOHandlerSocketOpenSSL;
      begin
        lHTTP := TIdHTTP.Create(nil);

        TThread.Synchronize(nil,
          procedure
          begin
            Form1.Caption := 'Task Running...';
          end
        );

        try
          lHTTP.ReadTimeout := 30000;
          lHTTP.HandleRedirects := True;
          IdSSL := TIdSSLIOHandlerSocketOpenSSL.Create(lHTTP);
          IdSSL.SSLOptions.Method := sslvTLSv1;
          IdSSL.SSLOptions.Mode := sslmClient;
          lHTTP.IOHandler := IdSSL;
        Finally
          try
            lHTTP.Get('http://website.com/'+lPath, TStream(nil));
          Finally
            lHTTP.Free;
          end;
        end;

        TThread.Synchronize(nil,
          procedure
          begin
            Memo1.Lines.Add(lPath);
           end
        );

      end
    ).Start;
end;

有关背景的说明请参阅 https://stackoverflow.com/a/13349520/101087 中的说明

其他答案中最重要的部分:

Note that variable capture captures variables—not values. If a variable's value changes after being captured by constructing an anonymous method, the value of the variable the anonymous method captured changes too, because they are the same variable with the same storage.

To capture the value of the loop variable, wrap creation of the task in a separate function: