Objective C: Concurrent and Serial Dispatch Queue

The main reason you use the default queue over the main queue is to run tasks in the background.

For instance, if I am downloading a file from the internet and I want to update the user on the progress of the download, I will run the download in the priority default queue and update the UI in the main queue asynchronously.

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
    //Background Thread
    dispatch_async(dispatch_get_main_queue(), ^(void){
        //Run UI Updates
    });
});

Serial queues are useful when you want your tasks to execute in a specific order. A serial queue executes only one task at a time and always pulls tasks from the head of the queue. You might use a serial queue instead of a lock to protect a shared resource or mutable data structure. Unlike a lock, a serial queue ensures that tasks are executed in a predictable order. And as long as you submit your tasks to a serial queue asynchronously, the queue can never deadlock.

Unlike concurrent queues, which are created for you, you must explicitly create and manage any serial queues you want to use. You can create any number of serial queues for your application but should avoid creating large numbers of serial queues solely as a means to execute as many tasks simultaneously as you can. If you want to execute large numbers of tasks concurrently, submit them to one of the global concurrent queues. When creating serial queues, try to identify a purpose for each queue, such as protecting a resource or synchronizing some key behavior of your application.

Listing 3-2 shows the steps required to create a custom serial queue. The dispatch_queue_create function takes two parameters: the queue name and a set of queue attributes. The debugger and performance tools display the queue name to help you track how your tasks are being executed. The queue attributes are reserved for future use and should be NULL.

Listing 3-2  Creating a new serial queue

dispatch_queue_t queue;

queue = dispatch_queue_create("com.example.MyQueue", NULL);

In addition to any custom queues you create, the system automatically creates a serial queue and binds it to your application’s main thread. For more information about getting the queue for the main thread, see Getting Common Queues at Runtime.

GCD queue can be create to run as a serial queue. Just create your own queue: dispatch_queue_t queue = dispatch_queue_create("com.my.download", NULL); You can do this .m part where you functie are declared something like: dispatch_queue_t getDownloadQueue() { static dispatch_once_t onceToken; static dispatch_queue_t ruleQueue; dispatch_once(&onceToken, ^{ ruleQueue = dispatch_queue_create("com.my.download", NULL); }); return ruleQueue; }

All of the DISPATCH_QUEUE_PRIORITY_X queues are concurrent queues (meaning they can execute multiple tasks at once), and are FIFO in the sense that tasks within a given queue will begin executing using "first in, first out" order. This is in comparison to the main queue (from dispatch_get_main_queue()), which is a serial queue (tasks will begin executing and finish executing in the order in which they are received).