Delphi TThread under ARC (iOS) not being released

Question

What is a proper way to terminate a thread using Delphi for iOS under ARC management?

Take this simple example:

  TMyThread = class(TThread)
  protected
         


        

Answer 1

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.