WWDC24 SALE: Save 50% on all my Swift books and bundles! >>

Effectful read-only properties

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0310 upgrades Swift’s read-only properties to support the async and throws keywords, either individually or together, making them significantly more flexible.

To demonstrate this, we could create a BundleFile struct that attempts to load the contents of a file in our app’s resource bundle. Because the file might not be there, might be there but can’t be read for some reason, or might be readable but so big it takes time to read, we could mark the contents property as async throws like this:

import Foundation

enum FileError: Error {
    case missing, unreadable
}

struct BundleFile {
    let filename: String

    var contents: String {
        get async throws {
            guard let url = Bundle.main.url(forResource: filename, withExtension: nil) else {
                throw FileError.missing
            }

            do {
                return try String(contentsOf: url)
            } catch {
                throw FileError.unreadable
            }
        }
    }
}

Because contents is both async and throwing, we must use try await when trying to read it:

func printHighScores() async throws {
    let file = BundleFile(filename: "highscores")
    try await print(file.contents)
}
Save 50% in my WWDC sale.

SAVE 50% To celebrate WWDC24, all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!

Other changes in Swift 5.5…

Download all Swift 5.5 changes as a playground Link to Swift 5.5 changes

Browse changes in all Swift versions

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.