Question

I’m working on a small app written in objective-c with the help of the cocoa framework and I am having a multithreading issue. I would really appreciate it if somebody could help me with some guidance on how terminate a secondary(worker) thread from the main thread?

- (IBAction)startWorking:(id)sender {      [NSThread detachNewThreadSelector:@selector(threadMain:) toTarget:self withObject:nil]; }  - (void)threadMain {   // do a lot of boring, time consuming I/O here.. }  - (IBAction)stop:(id)sender {   // what now? } 

I’ve found something on apple’s docs but what is missing from this example is the part where the runloop input source changes the exitNow value.

Also, I won’t be using many threads in my app so I would prefer a simple solution (with less overhead) rather than a more complex one that is able to manage many threads easily, but with more overhead generated (eg. using locks maybe(?) instead of runloops)

Thanks in advance

Answer 1

I think the easiest way is to use NSThread’s -(void)cancel method. You’ll need a reference to the thread you’ve created, as well. Your example code would look something like this, if you can do the worker thread as a loop:

- (IBAction)startWorking:(id)sender {      myThread = [[NSThread alloc] initWithTarget:self selector:@selector(threadMain:) object:nil];      [myThread start]; }  - (void)threadMain {     while(1)     {         // do IO here         if([[NSThread currentThread] isCancelled])             break;     } }  - (IBAction)stop:(id)sender {    [myThread cancel];    [myThread release];    myThread = nil;  } 

Of course, this will only cancel the thread between loop iterations. So, if you’re doing some long blocking computation, you’ll have to find a way to break it up into pieces so you can check isCancelled periodically.