ARC（iOS）下的Delphi TThread未发布 [英] Delphi TThread under ARC (iOS) not being released
问题描述
采取这个简单的例子：
TMyThread = class（TThread）
protected
procedure Execute;覆盖
public
析构函数Destroy;覆盖
结束
TForm2 = class（TForm）
Button1：TButton;
Button2：TButton;
Button3：TButton;
procedure Button1Click（Sender：TObject）;
程序Button2Click（发件人：TObject）;
程序Button3Click（发件人：TObject）;
private
FThread：TMyThread;
public
end;
{TMyThread}
析构函数TMyThread.Destroy;
begin
ShowMessage（'Destroy'）;
继承了Destroy;
end;
程序TMyThread.Execute;
开始
睡眠（5000）;
end;
{TForm2}
程序TForm2.Button1Click（Sender：TObject）;
begin
FThread：= TMyThread.Create（TRUE）;
FThread.FreeOnTerminate：= TRUE;
FThread.Start;
结束
程序TForm2.Button2Click（Sender：TObject）;
begin
ShowMessage（FThread.RefCount.ToString）;
结束
程序TForm2.Button3Click（发件人：TObject）;
begin
FThread：= nil;
结束
好的，按下Button1会产生一个线程。线程启动后，如果您单击Button2，它将显示一个RefCount值3！那么，1是对我的FThread变量的引用，还有另外两个TThread在内部创建的引用...我已经挖掘了源代码，发现RefCount在这里增加了：
构造函数TThread.Create（CreateSuspended：Boolean）;
ErrCode：= BeginThread（nil，@ThreadProc，指针（自身），FThreadID）;
如果ErrCode<> 0 then
raise EThread.CreateResFmt（@SThreadCreateError，[SysErrorMessage（ErrCode）]）;
{$ ENDIF POSIX}
而这里：
function ThreadProc（Thread：TThread）：Integer;
var
FreeThread：Boolean;
begin
TThread.FCurrentThread：= Thread;
嗯...线程完成后（在我的情况下，5秒钟后），RefCount将减少到2（因为我已经将FreeOnTerminate设置为TRUE，但是如果我不将FreeOnTerminate设置为TRUE，则RefCount仍将是3）。
看到问题？线程从未完成，析构函数从未被调用，如果我调用 FThread：= nil ，那么RefCount应该从2减为1（或者从3到2， code> FreeOnTerminate = FALSE ），线程永远不会在ARC下发布...
也许我没有任何东西，因为我习惯于使用没有ARC的线程...所以，我在这里缺少什么？还是在ARC下的TThread实现中有错误？
也许这个TThread的定义
code> private class threadvar
FCurrentThread：TThread;
应该像
private class threadvar
[Weak] FCurrentThread：TThread;
在qc中挖掘以下问题和解决方法
线程参数应以 const
function ThreadProc（Thread：TThread）：Integer; < - pass_by_reference推送
var up ref_count。
FreeThread：Boolean;
begin
TThread.FCurrentThread：= Thread;
如果您将其作为 const 将不会升到3.
通常这不是问题，因为ref_count在函数退出时减少，但这里：
功能epilog从来没有发生过，因为pthread_exit（）跳出了代码。
这只是解决方案的一部分，但还需要做更多的工作...
我已经想到了这个潜在的解决方法：
将这些MOD设置为Classes单位：
更改：
function ThreadProc（Thread：TThread）：Integer;
to：
code> function ThreadProc（const Thread：TThread）：Integer;
并添加：
TThread.FCurrentThread：= nil;
此行后：
如果FreeThread然后Thread.Free; $ t $ b 在TThread后代覆盖 DoTerminate ：
程序TMyThread.DoTerminate;
begin
try
inherited;
finally
__ObjRelease;
结束
结束
因此调用线程：
FMyThread.Free; //这首先不会做任何事情，因为引用将为nil
FMyThread：= TMyThread.Create（True）;
//不要设置FreeOnTerminate
FMyThread.OnTerminate：= ThreadTerminate;
FMyThread.Resume;
这（至少对我来说，在设备上）导致线程在后续调用中被销毁。
注意：在ARC条件下，永远不会在本地声明对线程的引用，因为当它脱离范围，线程被破坏，并且Execute方法从不被调用，更不用说其导致的其他问题。
What is a proper way to terminate a thread using Delphi for iOS under ARC management?
Take this simple example:
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;
Ok, pressing Button1 will spawn a thread. After thread is started, if you click Button2 it will display a RefCount value of 3!! Well, 1 is the reference to my FThread variable, and there are 2 additional references that TThread creates internally... I have digged into source code and found that RefCount is increased here:
constructor TThread.Create(CreateSuspended: Boolean);
ErrCode := BeginThread(nil, @ThreadProc, Pointer(Self), FThreadID);
if ErrCode <> 0 then
raise EThread.CreateResFmt(@SThreadCreateError, [SysErrorMessage(ErrCode)]);
{$ENDIF POSIX}
And here:
function ThreadProc(Thread: TThread): Integer;
var
FreeThread: Boolean;
begin
TThread.FCurrentThread := Thread;
Well... After thread is finished (In my case, after 5 seconds), the RefCount will decrease to 2 (Because I have set FreeOnTerminate to TRUE, but if I don´t set FreeOnTerminate to TRUE, the RefCount will still be 3).
See the problem? Thread is never finished and destructor is never called, if I call FThread := nil, then, the RefCount should decrease from 2 to 1 (Or from 3 to 2 in case FreeOnTerminate = FALSE), and thread is never released under ARC...
Maybe I´m missing something because I´m used to working with threads with no ARC... So, what am I missing here? Or is there a bug in TThread implementation under ARC?
Maybe this definition of TThread
private class threadvar
FCurrentThread: TThread;
should be something like
private class threadvar
[Weak] FCurrentThread: TThread;
After some digging in qc the following issues and workaround show up:
Thread parameters should be passed as const
function ThreadProc(Thread: TThread): Integer; <<-- pass_by_reference pushes
var up the ref_count.
FreeThread: Boolean;
begin
TThread.FCurrentThread := Thread;
Had you passed it as const the ref_count would have not gone up to 3.
Normally this is not a problem, because the ref_count gets decreased on exit of the function, but here:
the function epilog is never exectued because pthread_exit() jumps out of the code.
This is only part of the solution though, quite a bit more needs to be done...
Full workaround by Dave Nottage
After much fiddling around, I've come up with this potential workaround:
Make these mods to the Classes unit: Change:
function ThreadProc(Thread: TThread): Integer;
to:
function ThreadProc(const Thread: TThread): Integer;
and add:
TThread.FCurrentThread := nil;
after this line:
if FreeThread then Thread.Free;
Override DoTerminate in the TThread descendant, thus:
procedure TMyThread.DoTerminate;
begin
try
inherited;
finally
__ObjRelease;
end;
end;
Call the thread thus:
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.
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