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Universal Self

Available from Swift 5.1

Paul Hudson      @twostraws

SE-0068 expands Swift’s use of Self so that it refers to the containing type when used inside classes, structs, and enums. This is particularly useful for dynamic types, where the exact type of something needs to be determined at runtime.

As an example, consider this code:

class NetworkManager {
    class var maximumActiveRequests: Int {
        return 4
    }

    func printDebugData() {
        print("Maximum network requests: \(NetworkManager.maximumActiveRequests).")
    }
}

That declares a static maximumActiveRequests property for a network manager, and adds a printDebugData() method to print the static property. That works fine right now, but when NetworkManager is subclassed things become more complicated:

class ThrottledNetworkManager: NetworkManager {
    override class var maximumActiveRequests: Int {
        return 1
    }
}

That subclass changes maximumActiveRequests so that it allows only one request at a time, but if we call printDebugData() it will print out the value from its parent class:

let manager = ThrottledNetworkManager()
manager.printDebugData()

That should print out 1 rather than 4, and that’s where SE-0068 comes in: we can now write Self (with a capital S) to refer to the current type. So, we can rewrite printDebugData() to this:

class ImprovedNetworkManager {
    class var maximumActiveRequests: Int {
        return 4
    }

    func printDebugData() {
        print("Maximum network requests: \(Self.maximumActiveRequests).")
    }
}

This means Self works the same way as it did for protocols in earlier Swift versions.

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Other changes in Swift 5.1…

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