1) Grand Central Dispatch (GCD) is a technology for managing the execution of tasks in an app. It allows scheduling blocks of code to run on concurrent queues for parallel execution and synchronization of tasks. 2) GCD uses dispatch queues, which can be serial or concurrent, to schedule tasks. Global dispatch queues corresponding to quality of service classes are available to target specific thread priorities. Dispatch groups can be used to synchronize tasks and wait for completion. 3) Common techniques with GCD include scheduling tasks asynchronously on dispatch queues, synchronizing tasks using dispatch groups, and serializing asynchronous tasks by chaining animations in completion blocks. GCD provides efficient solutions for managing threads and tasks in iOS apps.

1. Grand Central Dispatch
Ben Asher
basher@yelp.com
benasher44 on Github and Twitter

2. Yelp’s Mission:
Connecting people with great
local businesses.
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3. Yelp Stats:
As of Q2 2015
83M 3268%83M
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4. A Responsive App
- Responsive UI
- Don’t make the user wait
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5. 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
};

6. What is GCD about?
“…technology that you use to manage the
execution of tasks in your app…”
- Apple Docs
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7. - C API (mostly)
- Open source!
- Task = ^{}
GCD - Intro
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8. - Schedule tasks
- dispatch_queue
- Synchronize tasks
- dispatch_group
- Serialize asynchronous tasks
- dispatch_suspend and dispatch_resume
GCD Techniques
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9. Scheduling Tasks with GCD

10. GCD - Scheduling Tasks with Queues
Queues
Serial
Concurrent
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11. GCD - Scheduling Tasks with Queues
Queues
^{1}^{2}
^{1}^{2}^{3}
^{3}
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12. 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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13. Quality of Service Class
- Urgency of queued tasks
- Relative priority
GCD - Scheduling Tasks with Queues
iOS 8+
QOS_CLASS_USER_INTERACTIVE
QOS_CLASS_USER_INITIATED
QOS_CLASS_DEFAULT
QOS_CLASS_UTILITY
QOS_CLASS_BACKGROUND
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14. GCD - Scheduling Tasks with Queues
iOS 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

15. - 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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16. Queues can target other queues
- (a) dispatch_queue_create(“q”, DISPATCH_QUEUE_SERIAL);
GCD - Scheduling Tasks with Queues
^{2}^{3} ^{1}
(a)
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(b)
- (b) Global QOS_CLASS_DEFAULT

17. GCD - Scheduling Tasks with Queues
^{3} ^{2} ^{1}
dispatch_set_target_queue(someQ, targetQ);
(a) (b)
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18. GCD - Scheduling Tasks with Queues
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QOS_CLASS_UTILITY
QOS_CLASS_USER_INITIATED

19. 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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20. 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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21. GCD - Scheduling Tasks with Queues
Common Pattern
dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT), ^{
// do some expensive work
dispatch_async(dispatch_get_main_queue(), ^{
// show user the work i did
});
});
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22. Demo
Cat Pyramid

23. Synchronizing Tasks with GCD

24. GCD - Synchronizing Tasks
Problem - User generated video
- 2 tasks:
- Crop video
- Generate thumbnails for
keyframe slider
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25. GCD - Synchronizing Tasks
Potential Solution - BOOL Flags
- isVideoProccessed
- areSliderImagesGenerated
- (void)hideLoadingIfPossible {
if (self.isVideoProcessed &&
self.areSliderImagesGenerated) {
// Hide the loading spinner
}
} 25

26. GCD - Synchronizing Tasks
Potential Solution - BOOL Flags
- (void)hideLoadingIfPossible {
if (self.isVideoProcessed &&
self.areSliderImagesGenerated &&
self.isAudioProcessed &&
self.isUserReadyToSeeThis
self.areYouSureUserIsReady) {
// Hide the loading spinner
}
}
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27. Better Solution - Dispatch Groups
dispatch_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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28. Potential solution - Dispatch Groups
dispatch_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 0
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
GCD - Synchronizing Tasks
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29. Potential solution - Dispatch Groups
dispatch_group_t group = dispatch_group_create();
// For each task:
// call dispatch_group_enter(group) on start
// call dispatch_group_leave on tasks completion
dispatch_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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30. 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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31. Demo
Cat Pyramid - Faster

32. Serialize Asynchronous Tasks

33. 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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34. Problem - Animations
GCD - Serialize Async Tasks
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35. 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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36. Demo
Fun with Animation Queueing

37. 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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38. Tools in the Toolbelt
GCD
- Small synchronous tasks
- Simple asynchronous tasks
NSOperation and NSOperationQueue
- Complex asynchronous tasks
- Cancel support
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39. 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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40. Resources
GCD 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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