Published by Arunprasadh C on 03 June 2022 • Last Updated on 03 June 2022
Concurrency
Swift has built-in support for writing asynchronous and parallel code in a structured way. Asynchronous code can be suspended and resumed later, although only one piece of the program executes at a time. Suspending and resuming code in your program lets it continue to make progress on short-term operations like updating its UI while continuing to work on long-running operations like fetching data over the network or parsing files. Parallel code means multiple pieces of code run simultaneously—for example, a computer with a four-core processor can run four pieces of code at the same time, with each core carrying out one of the tasks. A program that uses parallel and asynchronous code carries out multiple operations at a time; it suspends operations that are waiting for an external system, and makes it easier to write this code in a memory-safe way.
The additional scheduling flexibility from parallel or asynchronous code also comes with a cost of increased complexity. Swift lets you express your intent in a way that enables some compile-time checking—for example, you can use actors to safely access mutable state. However, adding concurrency to slow or buggy code isn’t a guarantee that it will become fast or correct. In fact, adding concurrency might even make your code harder to debug. However, using Swift’s language-level support for concurrency in code that needs to be concurrent means Swift can help you catch problems at compile time.
The rest of this topic uses the term concurrency to refer to this common combination of asynchronous and parallel code.
NOTE: If you’ve written concurrent code before, you might be used to working with threads. The concurrency model in Swift is built on top of threads, but you don’t interact with them directly. An asynchronous function in Swift can give up the thread that it’s running on, which lets another asynchronous function run on that thread while the first function is blocked.
Defining and Calling Asynchronous Functions
An asynchronous function or asynchronous method is a special kind of function or method that can be suspended while it’s partway through execution. This is in contrast to ordinary, synchronous functions and methods, which either run to completion, throw an error, or never return. An asynchronous function or method still does one of those three things, but it can also pause in the middle when it’s waiting for something. Inside the body of an asynchronous function or method, you mark each of these places where execution can be suspended.
To indicate that a function or method is asynchronous, you write the async keyword in its declaration after its parameters, similar to how you use throws to mark a throwing function. If the function or method returns a value, you write async before the return arrow (->).
Syntax:
func someLongCall() async -> String
{
//Function Body
}
For a function or method that’s both asynchronous and throwing, you write async before throws.
When calling an asynchronous method, execution suspends until that method returns. You write await in front of the call to mark the possible suspension point. This is like writing try when calling a throwing function, to mark the possible change to the program’s flow if there’s an error. Inside an asynchronous method, the flow of execution is suspended only when you call another asynchronous method—suspension is never implicit or preemptive—which means every possible suspension point is marked with await.
For example, the code below fetches the names of all the pictures in a gallery and then shows the first picture:
Example 1:
let photoNames = await listPhotos(inGallery: "Summer Vacation")
let sortedNames = photoNames.sorted()
let name = sortedNames[0]
let photo = await downloadPhoto(named: name)
show(photo)
Asynchronous functions can’t be called within main.swift of a console app since main is synchronous. Asynchronous functions can be tried out during Actual App Development.
With this, I would like to conclude this long-running Series. I hope that it was useful for learning Swift language. iOS App Development is a vast subject to be covered and can’t be covered here. You can start out by learning about the UIKit which is the Framework provided by Apple for iOS App Development.