ios - ARC (iOS) 下的 Delphi TThread 未发布

Question

在 ARC 管理下使用 Delphi for iOS 终止线程的正确方法是什么？

举个简单的例子:

  TMyThread = class(TThread)
  protected
    procedure Execute; override;
  public
    destructor Destroy; override;
  end;

  TForm2 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
  private
    FThread: TMyThread;
  public
  end;

{ TMyThread }
destructor TMyThread.Destroy;
begin

  ShowMessage('Destroy');

  inherited Destroy;

end;

procedure TMyThread.Execute;
begin

  Sleep(5000);

end;

{ TForm2 }
procedure TForm2.Button1Click(Sender: TObject);
begin
  FThread := TMyThread.Create(TRUE);
  FThread.FreeOnTerminate := TRUE;
  FThread.Start;
end;

procedure TForm2.Button2Click(Sender: TObject);
begin
  ShowMessage(FThread.RefCount.ToString);
end;

procedure TForm2.Button3Click(Sender: TObject);
begin
  FThread := nil;
end;

好的，按下 Button1 将产生一个线程。线程启动后，如果您单击 Button2，它将显示 RefCount 值为 3!好吧，1 是对我的 FThread 变量的引用，TThread 在内部创建了 2 个额外的引用......我深入研究了源代码，发现 RefCount 在这里增加了:

constructor TThread.Create(CreateSuspended: Boolean);

  ErrCode := BeginThread(nil, @ThreadProc, Pointer(Self), FThreadID);
  if ErrCode <> 0 then
    raise EThread.CreateResFmt(@SThreadCreateError, [SysErrorMessage(ErrCode)]);
  {$ENDIF POSIX}

在这里:

function ThreadProc(Thread: TThread): Integer;
var
  FreeThread: Boolean;
begin
  TThread.FCurrentThread := Thread;

嗯...线程完成后(在我的例子中，5 秒后)，RefCount 将减少到 2(因为我已将 FreeOnTerminate 设置为 TRUE，但如果我不将 FreeOnTerminate 设置为 TRUE，RefCount 将仍然是 3).

看到问题了吗？如果我调用 FThread := nil，线程永远不会完成并且永远不会调用析构函数，然后，RefCount 应该从 2 减少到 1(或者在 FreeOnTerminate = FALSE 的情况下从 3 减少到 2)，并且线程永远不会在 ARC 下释放...

也许我遗漏了一些东西，因为我习惯于使用没有 ARC 的线程...那么，我在这里遗漏了什么？还是ARC下的TThread实现有bug？

也许是TThread的这个定义

private class threadvar
  FCurrentThread: TThread;

应该是这样的

private class threadvar
  [Weak] FCurrentThread: TThread;

Answer 1

在 qc 中进行一些挖掘后，出现了以下问题和解决方法:

线程参数应该作为const传递

function ThreadProc(Thread: TThread): Integer;  <<-- pass_by_reference pushes
var                                                  up the ref_count.
  FreeThread: Boolean;
begin
  TThread.FCurrentThread := Thread;

如果您将其作为 const 传递，则 ref_count 不会上升到 3。通常这不是问题，因为 ref_count 在函数退出时会减少，但这里:

the function epilog is never exectued because pthread_exit() jumps out of the code.

这只是解决方案的一部分，还需要做更多......

Full workaround by Dave Nottage

After much fiddling around, I've come up with this potential workaround:

为类单元制作这些模组:变化:

function ThreadProc(Thread: TThread): Integer;

到:

function ThreadProc(const Thread: TThread): Integer;

并添加:

TThread.FCurrentThread := nil;

在这一行之后:

if FreeThread then Thread.Free;

覆盖 TThread 后代中的 DoTerminate，因此:

procedure TMyThread.DoTerminate;
begin
  try
    inherited;
  finally
    __ObjRelease;
  end;
end;

这样调用线程:

FMyThread.Free; // This will do nothing the first time around, since the reference will be nil
FMyThread := TMyThread.Create(True);
// DO NOT SET FreeOnTerminate
FMyThread.OnTerminate := ThreadTerminate;
FMyThread.Resume;

This (at least for me, on the device) results in the thread being destroyed on subsequent calls.

NOTE: Under ARC conditions, never declare a reference to a thread locally, because when it falls out of scope, the thread is destroyed and the Execute method is never called, not to mention other problems it causes.