Tasks can finish in any order, and you have no knowledge of the time it will take for the next task to start, nor the number of tasks running at any given time. The queue guarantees tasks start in the order you add them. You won’t know the amount of time between one task ending and the next one beginning:Ĭoncurrent queues allow multiple tasks to run at the same time.
#1 dispatch serial
Serial queues guarantee that only one task runs at any given time. Queues can be either serial or concurrent. The key to this is to choose the right kind of dispatch queue and the right dispatching function to submit your work to the queue. GCD’s benefits are apparent when you understand how dispatch queues provide thread safety to parts of your code. You submit units of work to this queue, and GCD executes them in a FIFO order (first in, first out), guaranteeing that the first task submitted is the first one started.ĭispatch queues are thread-safe, meaning you can simultaneously access them from multiple threads. Understanding QueuesĪs mentioned before, GCD operates on dispatch queues through a class aptly named DispatchQueue. It’s important to note that parallelism requires concurrency, but concurrency doesn’t guarantee parallelism.īasically, concurrency is about structure while parallelism is about execution. Note that GCD decides how much parallelism it requires based on the system and available system resources. This allows you to use GCD to take advantage of concurrent execution.
#1 dispatch code
With GCD, you add blocks of code or work items to dispatch queues and GCD decides which thread to execute them on.Īs you structure your code, you’ll find code blocks that can run simultaneously and some that should not. Under the hood, it manages a shared thread pool. Multi-core devices, on the other hand, execute multiple threads at the same time via parallelism. They run one thread, perform a context switch, then run another thread. Single-core devices achieve concurrency through a method called time-slicing. Each thread can execute concurrently, but it’s up to the system to decide if, when and how it happens. The operating system scheduler manages the threads independently of each other. In iOS, a process or application consists of one or more threads. To understand GCD, you need to be comfortable with several concepts related to concurrency and threading. In this first part, you’ll work on a few improvements, including optimizing the googly-fying process and making PhotoManager thread-safe. You’ll fix this in the second part of the series.
#1 dispatch download
One that you may have noticed when running it is that the download complete alert is premature. PhotoManager: This manages all the Photo objects.AssetPhoto, which instantiates a photo from an instance of PHAsset.DownloadPhoto, which instantiates a photo from an instance of URL.The project includes two classes that implement the protocol: It provides an image, a thumbnail and their corresponding statuses. Photo: This protocol describes the properties of a photo.PhotoDetailViewController: Displays a selected photo from PhotoCollectionViewController and adds googly eyes to the image.It displays the selected photos as thumbnails. PhotoCollectionViewController: The initial view controller.You’ll primarily work with four classes in this tutorial: Tap the first image, and you’ll see googly eyes added to the face. Tap +, then select Le Internet to download predefined images from the internet. You should use a real device to let the CIDetector work.
#1 dispatch simulator
Note: Currently, the app doesn’t work well on the iOS Simulator - eyes will appear in random locations instead of over the people’s eyes. Your mission is to use GCD to optimize the app and ensure you can safely call code from different threads.
![1 dispatch 1 dispatch](https://i.ytimg.com/vi/WRN3xBFR8E0/maxresdefault.jpg)
You can select images on which to apply this effect from your photo library or download images from the internet. GooglyPuff is a non-optimized, “thread-unsafe” app that overlays googly eyes on detected faces using Core Image’s face detection API. You’ll build upon an existing app called GooglyPuff. In the second part, you’ll learn about some advanced functions GCD has to offer. This first part explains what GCD does and showcases several basic GCD functions. In this two-part Grand Central Dispatch tutorial, you’ll learn the ins and outs of GCD and its Swifty API. It’s an easier concurrency model to work with than locks and threads. It can help improve your app’s responsiveness by deferring computationally expensive tasks to the background. Grand Central Dispatch (GCD) is a low-level API for managing concurrent operations. Evan Dekhayser wrote a previous update, and Christine Abernathy wrote the original.
#1 dispatch update
Update note: Fabrizio Brancati updated this tutorial for iOS 15, Swift 5.5 and Xcode 13.