iosobjective-cgrand-central-dispatchnsoperationqueueserial-processing

Equivalent of GCD serial dispatch queue in iOS 3.x

Apple's Grand Central Dispatch (GCD) is great, but only works on iOS 4.0 or greater. Apple's documentation says, "[A] serialized operation queue does not offer quite the same behavior as a serial dispatch queue in Grand Central Dispatch does" (because the queue is not FIFO, but order is determined by dependencies and priorities).

What is the right way to achieve the same effect as GCD's serial dispatch queues while supporting OS versions before GCD was released? Or put another way, what is the recommended way to handle simple background processing (doing web service requests, etc.) in iOS apps that want to support versions less than 4.0?

Solution

  • Seems like people are going to a lot of effort to rewrite NSRunloop. Per the NSRunloop documentation:

    Your application cannot either create or explicitly manage NSRunLoop objects. Each NSThread object, including the application’s main thread, has an NSRunLoop object automatically created for it as needed.

    So surely the trivial answer would be, to create a usable queue:

    - (void)startRunLoop:(id)someObject
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    [[NSRunLoop currentRunLoop] run];

    [pool release];
}

...

NSThread *serialDispatchThread = [[NSThread alloc] 
                   initWithTarget:self 
                   selector:@selector(startRunLoop:) 
                   object:nil];
[serialDispatchThread start];

    To add a task to the queue:

    [object
    performSelector:@selector(whatever:) 
    onThread:serialDispatchThread
    withObject:someArgument
    waitUntilDone:NO];

    Per the Threading Programming Guide section on Run Loops:

    Cocoa defines a custom input source that allows you to perform a selector on any thread. ... perform selector requests are serialized on the target thread, alleviating many of the synchronization problems that might occur with multiple methods being run on one thread.

    So you've got an explicitly serial queue. Of course, mine isn't fantastically written because I've told the run loop to run forever, and you may prefer one you can terminate later, but those are easy modifications to make.