Grand Central Dispatch - iOS Conf SG 2015

Grand Central DispatchBen Asher

basher@yelp.combenasher44 on Github and Twitter

Yelp’s Mission:Connecting people with great

local businesses.

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Yelp Stats:As of Q2 2015

83M 3268%83M

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A Responsive App

- Responsive UI

- Don’t make the user wait

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GCD - Getting Started

Basics of blocks in Objective-C

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- Segment of code

- Passed around as parameters

- Stored in variables

- Called like C functions ()

^{

// Some code

};

What is GCD about?

“…technology that you use to manage the execution of tasks in your app…”

- Apple Docs

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- C API (mostly)

- Open source!

- Task = ^{}

GCD - Intro

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- Schedule tasks- dispatch_queue

- Synchronize tasks- dispatch_group

- Serialize asynchronous tasks- dispatch_suspend and dispatch_resume

GCD Techniques

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Scheduling Tasks with GCD

GCD - Scheduling Tasks with Queues

Queues

Serial

Concurrent

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GCD - Scheduling Tasks with Queues

Queues

^{1}^{2}

^{1}^{2}^{3}

^{3}

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dispatch_queue_t dispatch_queue_create(const char *label,dispatch_queue_attr_t attr

);

dispatch_queue_create(“q”, DISPATCH_QUEUE_SERIAL);dispatch_queue_create(“q”, DISPATCH_QUEUE_CONCURRENT);

GCD - Scheduling Tasks with Queues

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Quality of Service Class

- Urgency of queued tasks- Relative priority

GCD - Scheduling Tasks with QueuesiOS 8+

QOS_CLASS_USER_INTERACTIVE

QOS_CLASS_USER_INITIATED

QOS_CLASS_DEFAULT

QOS_CLASS_UTILITY

QOS_CLASS_BACKGROUND

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GCD - Scheduling Tasks with QueuesiOS 8+

QOS_CLASS_USER_INTERACTIVE

QOS_CLASS_USER_INITIATED

QOS_CLASS_DEFAULT

QOS_CLASS_UTILITY

QOS_CLASS_BACKGROUND

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dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_UTILITY, 0);

QOS_CLASS_USER_INITIATED

QOS_CLASS_UTILITY

- 5 Global Queues- 1 per QOS class- Concurrent, background thread

GCD - Scheduling Tasks with Queues

dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0);

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Queues can target other queues- (a) dispatch_queue_create(“q”, DISPATCH_QUEUE_SERIAL);

GCD - Scheduling Tasks with Queues

^{2}^{3} ^{1}

(a)16

(b)

- (b) Global QOS_CLASS_DEFAULT

GCD - Scheduling Tasks with Queues

^{3} ^{2} ^{1}

dispatch_set_target_queue(someQ, targetQ);

(a) (b)

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GCD - Scheduling Tasks with Queues

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QOS_CLASS_UTILITY

QOS_CLASS_USER_INITIATED

GCD - Scheduling Tasks with Queues

Task- dispatch_block_t b = ^{};

Schedule a task in GCD- dispatch_sync(queue, ^{}) // Blocking- dispatch_async(queue, ^{}) // Non blocking

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GCD - Scheduling Tasks with Queues

- Special 6th global queue - main queue- dispatch_get_main_queue()- Executes blocks on the main thread

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GCD - Scheduling Tasks with Queues

Common Patterndispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT), ^{

// do some expensive workdispatch_async(dispatch_get_main_queue(), ^{

// show user the work i did});

});

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DemoCat Pyramid

Synchronizing Tasks with GCD

GCD - Synchronizing Tasks

Problem - User generated video- 2 tasks:

- Crop video- Generate thumbnails for

keyframe slider

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GCD - Synchronizing TasksPotential Solution - BOOL Flags

- isVideoProccessed- areSliderImagesGenerated

- (void)hideLoadingIfPossible {if (self.isVideoProcessed && self.areSliderImagesGenerated) { // Hide the loading spinner}

} 25

GCD - Synchronizing TasksPotential Solution - BOOL Flags- (void)hideLoadingIfPossible {

if (self.isVideoProcessed && self.areSliderImagesGenerated &&

self.isAudioProcessed && self.isUserReadyToSeeThis self.areYouSureUserIsReady) {

// Hide the loading spinner}

}26

Better Solution - Dispatch Groupsdispatch_group_t

- dispatch_group_create();

- Creates a new empty group

- dispatch_group_enter(group);

- Increments the number of tasks in group

- dispatch_group_leave(group);

- Decrements the number of tasks in group

GCD - Synchronizing Tasks

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Potential solution - Dispatch Groupsdispatch_group_t group = dispatch_group_create();

// For each task:// call dispatch_group_enter(group) on start// call dispatch_group_leave on tasks completion

// Call wait to block until task count is 0dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

GCD - Synchronizing Tasks

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Potential solution - Dispatch Groupsdispatch_group_t group = dispatch_group_create();

// For each task:// call dispatch_group_enter(group) on start// call dispatch_group_leave on tasks completiondispatch_async(someConcurrentQueue, ^{ // Call wait to block until task count is 0 dispatch_group_wait(group, DISPATCH_TIME_FOREVER);});

GCD - Synchronizing Tasks

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GCD - Synchronizing Tasks

Optimal Solution- dispatch_group_notify(group, queue, block);

- Read “When there are no tasks left associated with this group, notify by

enqueueing this block onto this queue”

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DemoCat Pyramid - Faster

Serialize Asynchronous Tasks

Sync Task- Easily fits into a block- Blocks until the task is done

Async Task- Task happens in another thread/process- Starting this task returns immediately

GCD - Serialize Async Tasks

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Problem - Animations

GCD - Serialize Async Tasks

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GCD - Serialize Async Tasks

Problem - Animations[UIView animateWithDuration:1.0 animations:^{

// Rotate 90 degrees} completion:^(BOOL finished) {

[UIView animateWithDuration:0.5 animations:^{ // Move down the screen 100pt } completion:NULL];}];

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DemoFun with Animation Queueing

GCD - Caveats

Cleanup is hard

- if a dispatch_group_t is deallocated with a non-zero task count, it will throw an exception

- dispatch_async“…The queue is retained by the system until the block has run to completion…” - Apple Docs

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Tools in the Toolbelt

GCD- Small synchronous tasks- Simple asynchronous tasks

NSOperation and NSOperationQueue- Complex asynchronous tasks- Cancel support

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What else can I do with GCD?

- Efficient reader/writer schemes- dispatch_barrier

- Read and write data efficiently- dispatch_io (disk) and dispatch_data (memory)

- Respond to low-level system objects- dispatch_source

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ResourcesGCD Source

- https://libdispatch.macosforge.org/trac/browser/trunk

GCD Documentation- https://developer.apple.

com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/

Demo App- https://github.com/benasher44/SGConf2015Demo

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