iOS concurrency, the run loop, and GCD BN.University 

Concurrency in iOS Mostly managed out of your control 

main.m Only area where you’re starting things off Everything* else is going to be event driven Otherwise, responding to an event, or working through a queue 

The Run Loop You’re almost never going to touch a run loop yourself Important to know about conceptually 

The run loop Polls event sources and calls handlers 

UI Threading There is a very defined “main” thread in iOS (and OS X) All UI mutation must happen here Luckily, it’s easy to get back to 

Queues Queues are the base concept behind threading in modern ObjC They handle the thread they’re running on under the hood Accessed by enqueuing an executable chunk of code or block 

Blocks Blocks are closures/lambdas in Objective C Used all over, they’re just a block of runnable code Can be called directly or enqueued with libdispatch 

Libdispatch This is a really magical library provided by Apple It handles how many threads to use for your machine Transparently lets you take advantage of multiple cores/hyperthreading/etc. 

Tools for Concurrency This is a really magical library provided by Apple It handles how many threads to use for your machine Transparently lets you take advantage of multiple cores/hyperthreading/etc. 

Tools for Concurrency NSOperation{,Queue} Grand Central Dispatch NSThread POSIX threads 

POSIX threads Everything is built on top of these Can run objc and thread-communication, requires a run loop manually Mostly for performance critical long running tasks 

Tools for Concurrency NSOperation{,Queue} Grand Central Dispatch NSThread POSIX threads 

NSThread Abstraction layer on top of posix threads Easy to spin off a background thread for a particular selector Easy to bring in a run loop if communication is needed 

NSThread Server Connections? File System Watching? Database Connection? Regular (cron-like) event? Really only useful for true long running tasks or background general purpose threads (the system will make those background threads) All are event based in Cocoa, last use a timer 

Tools for Concurrency NSOperation{,Queue} Grand Central Dispatch NSThread POSIX threads 

Grand Central Dispatch Really interesting stuff Built on top of libdispatch block based interface FIFO queues each with their own priority Synchronization primitives 

Grand Central Dispatch Parallel Array Iteration dispatch_apply can replace for loops almost without change runs in parallel as long as the queue is parallel 

Grand Central Dispatch Dispatching a background task That code will run on a background queue of low priority This is a very common pattern for long running tasks in response to UI interaction 

Grand Central Dispatch Calling back for UI Updates This lets you run some long running task, such as loading an image, processing it (edge detection maybe?) and displaying it Keeps the main queue open for other UI updates 

Tools for Concurrency NSOperation{,Queue} Grand Central Dispatch NSThread POSIX threads 

NSOperation{,Queue} This takes a reusable class-based task approach Built on top of GCD 

NSOperation{,Queue} Each NSOperation is a class and can have individual priorities NSOperationQueue processes with FIFO + priority 

NSOperation{,Queue} Image Metrics Colorize Resize UI JPEG Upload NSOperationQueue also handles dependencies You build a web of dependent operations Just adding operations you depend on to a property NSOperationQueue figures out how to run through it as fast as possible 

Demo! We’re going to build NSOperations to - download an image - resize it - then put it into an image view. In order to get back to the main queue to work with the UI, we’re going to use GCD