On 11 Jul 2016, at 06:35, Gerriet M. Denkmann <[email protected]> wrote:
>
> I have a subclass of NSThread (called MyThread), which runs a RunLoop in main.
> When it gets cancelled, it leaves the RunLoop and main will exit.
>
> Some other thread does:
>
> if ( myThread.isCancelled )
> {
> // probably the RunLoop of myThread is no longer running
> // adding stuff to a non-running RunLoop would result in [myThread
> dealloc] never called (some retain circle)
> }
> else
> {
> [ myThread performSelector: @selector(someMethod)
> onThread: myThread
> withObject: nil
> waitUntilDone: NO
> ];
> }
>
> But what if:
> “Some other thread” tests for isCancelled and sees: “not cancelled”
> Now yet another thread takes over and cancels myThread.
> Then “Some other thread” runs again and does performSelector… (bad)
>
> How to avoid this scenario?
You need to use locks, I’m afraid; essentially your MyThread needs a
cancellation lock, which it takes before cancelling the thread and also around
the code above, such that it can’t be cancelled between the -isCancelled test
and the -performSelector:onThread:withObject:waitUntilDone: call.
I’d be inclined to override the -cancel method, e.g.
- (void)cancel {
[cancellationLock lock];
[super cancel];
[cancellationLock unlock];
}
and then add a -(BOOL)performSelector:withObject:waitUntilDone: method that does
- (BOOL)performSelector:(SEL)sel withObject:(id)obj waitUntilDone:(BOOL)wait
{
[cancellationLock lock];
if ([self isCancelled]) {
[cancellationLock unlock];
return NO;
}
// This is your own method, right? You might just want to paste the code
in here...
[self performSelector:sel onThread:self withObject:obj waitUntilDone:wait];
[cancellationLock unlock];
}
then you can write
if (![thread performSelector:@selector(someMethod) withObject:nil
waitUntilDone:NO]) {
// Thread was cancelled
}
To be extra-safe, you might want to put some @try...@finally wrappers in there
to handle the -unlock.
Kind regards,
Alastair.
--
http://alastairs-place.net
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