GCD and OperationQueue.
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GCD and NSOperationQueue are concurrency APIs for executing tasks asynchronously on iOS and macOS. GCD uses queues and blocks to asynchronously dispatch tasks, while NSOperationQueue uses operations to asynchronously execute tasks. NSOperationQueue builds upon GCD by providing higher-level APIs for canceling, prioritizing and grouping operations. The document provides examples of using dispatch groups with GCD to wait for multiple tasks to finish, and using NSOperationQueue to asynchronously fetch images and create a montage image when all images have been retrieved.
GCD and OperationQueue.
- 1. Study Group GCD and NSOperationQueue Joe Hsieh 2015/04/14
- 2. Outline • Introduction of GCD (Use GCD to Take Advantage of Platform Scaling) • Introduction of NSOperationQueue and NSOperation • Relation between GCD and NSOperationQueue (When to Use GCD and When to Use Operation ?) • Example
- 3. Grand Central Dispatch (GCD) • GCD is a concurrency library implemented via FIFO queues.(In order to solve the problems caused by multi-threading) • GCD is just the marketing name for libdispatch.
- 4. Queues of GCD • Serial Queue：The order of execution and completion is FIFO. • dispatch_get_main_queue • dispatch_queue_create(“com.ninja.serial”, DISPATCH_QUEUE_SERIAL) // or NULL • Concurrent Queue：The order of execution is FIFO, but the order of completion is not necessarily FIFO. • dispatch_get_global_queue：There are many global queues Before iOS 8 ：high、default、low After iOS 8：QOS_CLASS(_USER_INTERACTIVE、_INITIATED、_UTILITY、 _BACKGROUND) dispatch_get_global_queue：There are many global queues • dispatch_queue_create(“com.ninja.concurrent”, DISPATCH_QUEUE_CONCURRENT)
- 5. Queues of GCD 3 2 1 3 2 1 Serial Queue Concurrent Queue Order of completion (each task needs 1s) 1, 2, 3。Required 3s 2, 1, 3。Required 1.1s 4 4 p.s tasks in queue are blocks. … …
- 6. Dispatching API 3 2 1 Serial Queue or Concurrent Queue 4 Caller thread:Should I wait for block 4 done? Option 1 I am waiting for block 4 dispatch_sync Option 2 fire and forget block 4 dispatch_async …
- 7. Can we know the timing of all tasks in the queue are done? • Yes, we can make it by a counter ! • Enqueue block to the queue and then counter plus 1.When block is finished, counter minus 1.When counter is zero, system will notify us.
- 8. Dispatch Groups • Dispatch groups are a GCD feature that allows you to easily group tasks. You can then wait on that set of tasks to finish or be notified through a callback when the set of tasks has finished.
- 9. Dispatch Groups dispatch_group_t group = dispatch_group_create(); dispatch_group_notify(group, dispatch_get_main_queue(), ^{ //When groupCounter is zero,system notifies main thread to execute completionBlock completionBlock(); }); // form1 dispatch_release(group); dispatch_group_enter(group); // groupCounter++ dispatch_async(group, queue, ^{ block1(); dispatch_group_leave(group); // groupCounter—- dispatch_release(group); }); // It is the same as form1 dispatch_group_async(group, queue, ^{ block2(); });
- 10. Relation between GCD and Operation Queues • Under the hood, NSOperationQueue(Objective- C) is based on GCD (C) from iOS 4 and Mac OS X 10.6. • NSOperationQueue and NSOperation provide more high-level API by low-level API of GCD.
- 11. NSOperationQueue 3 2 1 NSOperationQueue (concurrent) 4 add operation 4 into operationQueue waitUntilFinished? maxConcurrentOperationCount > 3 … The concurrent (or asynchronize) property of operation must be NO when using NSOperation with NSOperationQueue p.s tasks in queue are operations.
- 12. High-level API • Canceling operations • Operation priorities • Operation dependencies • Because NSOperation is an Objective-C object • KVO of operation properties • Reuse of Operations
- 13. Example Simple Montage Image By AFNetworking
- 14. How to do it ? • Fire four operation to fetch image • Create four operation into operationQueue. • Wait for all image fetched. • Wait for the group of image operations to complete. • Execute function to create montage image • System notifies main thread to create montage image.
- 15. Reference • http://commandshift.co.uk/blog/2014/03/19/using- dispatch-groups-to-wait-for-multiple-web-services/ • https://developer.apple.com/library/prerelease/ios/ documentation/Cocoa/Reference/ NSOperation_class/index.htm/ • http://nshipster.com/nsoperation/ • https://gist.github.com/joehsieh/ 747366e8de86a8b80983
- 16. Q&A