Multi-Threading

- January 09, 2017

What mechanisms does iOS provide to support multi-threading?

NSThread creates a new low-level thread which can be started by calling the start method.

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

NSOperationQueue allows a pool of threads to be created and used to execute NSOperations in parallel. NSOperations can also be run on the main thread by asking NSOperationQueue for the mainQueue.
```
NSOperationQueue* myQueue = [[NSOperationQueue alloc] init];
[myQueue addOperation:anOperation]; 
[myQueue addOperationWithBlock:^{
 /* Do something. */
}];
```
GCD or Grand Central Dispatch is a modern feature of Objective-C that provides a rich set of methods and API's to use in order to support common multi-threading tasks. GCD provides a way to queue tasks for dispatch on either the main thread, a concurrent queue (tasks are run in parallel) or a serial queue (tasks are run in FIFO order).
```
dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(myQueue, ^{
  printf("Do some work here.\n");
});
```