iOS Dev Course: Dispatch
How to execute code concurrently? How to manage dispatch work units?
Let’s make a simple project with Dispatch framework in quick guide!
Overview
Dispatch comprises language features, runtime libraries, and system enhancements that provide systemic, comprehensive improvements to the support for concurrent code execution on multicore hardware in macOS, iOS, watchOS, and tvOS.
The BSD subsystem, Core Foundation, and Cocoa APIs have all been extended to use these enhancements to help both the system and your application to run faster, more efficiently, and with improved responsiveness. Consider how difficult it is for a single application to use multiple cores effectively, let alone doing it on different computers with different numbers of computing cores or in an environment with multiple applications competing for those cores. GCD, operating at the system level, can better accommodate the needs of all running applications, matching them to the available system resources in a balanced fashion.
Read more on Apple Developer
Synchronous and Asynchronous Execution
Each work item can be executed either synchronously or asynchronously. When a work item is executed synchronously with the sync method, the program waits until execution finishes before the method call returns. When a work item is executed asynchronously with the async method, the method call returns immediately.
Run following code in playground with sync / async execution
Serial and Concurrent Queues
A dispatch queue can be either serial, so that work items are executed one at a time, or it can be concurrent, so that work items are dequeued in order, but run all at once and can finish in any order. Both serial and concurrent queues process work items in first in, first-out (FIFO) order.
Modify code in playground and run again.
Now we have one concurrent queue.
*Default is serial
System-Provided Queues
When an app launches, the system automatically creates a special queue called the main queue. Work items enqueued to the main queue execute serially on your app’s main thread. You can access the main queue using the main type property.
Important
Attempting to synchronously execute a work item on the main queue results in dead-lock.
Run code on playground.
That demonstrates how to exec code on main queue from background.
In addition to the serial main queue, the system also creates a number of global concurrent dispatch queues. You can access the global concurrent queue that best matches a specified quality of service (QoS) using the global(attributes:) type method.
That code demonstrates queue init with different qos modes.
Managing Units of Work
Work items allow you to configure properties of individual units of work directly. They also allow you to address individual work units for the purposes of waiting for their completion, getting notified about their completion, and/or canceling them.
DispatchWorkItem
DispatchWorkItem encapsulates work that can be performed. A work item can be dispatched onto a DispatchQueue and within a DispatchGroup. A DispatchWorkItem can also be set as a DispatchSource event, registration, or cancel handler.
Run code in playground with WorkItem manage demonstation.
Using Dispatch Groups
Grouping blocks allows for aggregate synchronization. Your application can submit multiple blocks and track when they all complete, even though they might run on different queues. This behavior can be helpful when progress can’t be made until all of the specified tasks are complete.
DispatchGroup
DispatchGroup allows for aggregate synchronization of work. You can use them to submit multiple different work items and track when they all complete, even though they might run on different queues. This behavior can be helpful when progress can’t be made until all of the specified tasks are complete.
You’ve done it! Clap! Clap!
What next? Try to retrive image data from Network using separated queue and display it in UIImageView using .main queue.
Remember, the receipt to success is continuous learning and communication with colleagues including, self-education.
This was a small guide about Dispatch framework. Read more on Apple Developer site. Don’t stop self-education.
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If you still have questions — write about it in the comments. I’ll answer them in future articles.
