Thread pools in CocoaTouch?

Question

I have a set of tasks that are done repeatedly and instead of creating a new thread each time this user-invoked task needs to be performed, I'd like to use a thread-pool.

In the typical flow to create a new thread, you have to setup an auto-release pool each time the thread entry point is invoked. It seems that the performance for this operation may be trivial according to http://www.mikeash.com/?page=pyblog/autorelease-is-fast.html but what I want to do is avoid the costs associated with setting up and tearing down a lots of these threads during the execution of my app. In other frameworks/languages, I've just retrieved an idle thread and have it do the work. When it's done with the work, the thread gets returned back to the pool.

I don't see any threadpool objects in the iPhone SDK just NSThread. What's a good way to do this?

How I'm setting up my thread:

// create thread using supplied entry point
[NSThread detachNewThreadSelector:@selector(myMethod) 
     toTarget:self 
     withObject:nil];

// thread entry-point
- (void)myMethod {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    *** code that should be run in the new thread goes here ***

    [pool release];
}

Answer 1

The way you want to go about this is to use an NSOperationQueue, which will hold a queue of tasks to be done and executes as many in parallel as you wish (configurable), each on a background thread. I assume that the implementation re-uses threads after a given operation is complete.

You use NSOperationQueue in conjunction with NSOperations - you can have either synchronous calls in there (since the work is all done on a background thread) or you can still do asynchronous calls with a little more work (have to define a few more methods).

Here's a tutorial you may find of use.

Answer 2

I would recommend using NSOperationQueue for what you want. You can set the number of threads the queue has.

Answer 3

I think the other answers are spot on with NSOperationQueue, but for the sake of completeness you should be aware of these projects:

• PLBlocks (compiler + runtime support for blocks on iPhone)
• WiganWallgate (implementation of grand central dispatch on iPhone)