ios - Swift 3.1 deprecates initialize(). How can I achieve the same thing?

Question

Objective-C declares a class function, initialize(), that is run once for each class, before it is used. It is often used as an entry point for exchanging method implementations (swizzling), among other things.

Swift 3.1 deprecates this function with a warning:

Method 'initialize()' defines Objective-C class method 'initialize', which is not guaranteed to be invoked by Swift and will be disallowed in future versions

How can this be resolved, while still maintaining the same behaviour and features that I currently implement using the initialize() entry point?

Answer 1

Easy/Simple Solution

A common app entry point is an application delegate's applicationDidFinishLaunching. We could simply add a static function to each class that we want to notify on initialization, and call it from here.

This first solution is simple and easy to understand. For most cases, this is what I'd recommend. Although the next solution provides results that are more similar to the original initialize() function, it also results in slightly longer app start up times. I no longer think it is worth the effort, performance degradation, or code complexity in most cases. Simple code is good code.

Read on for another option. You may have reason to need it (or perhaps parts of it).

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Not So Simple Solution

The first solution doesn't necessarily scale so well. And what if you are building a framework, where you'd like your code to run without anyone needing to call it from the application delegate?

Step One

Define the following Swift code. The purpose is to provide a simple entry point for any class that you would like to imbue with behavior akin to initialize() - this can now be done simply by conforming to SelfAware. It also provides a single function to run this behavior for every conforming class.

protocol SelfAware: class {
    static func awake()
}

class NothingToSeeHere {

    static func harmlessFunction() {

        let typeCount = Int(objc_getClassList(nil, 0))
        let types = UnsafeMutablePointer<AnyClass>.allocate(capacity: typeCount)
        let autoreleasingTypes = AutoreleasingUnsafeMutablePointer<AnyClass>(types)
        objc_getClassList(autoreleasingTypes, Int32(typeCount))
        for index in 0 ..< typeCount { (types[index] as? SelfAware.Type)?.awake() }
        types.deallocate(capacity: typeCount)

    }

}

Step Two

That's all good and well, but we still need a way to actually run the function we defined, i.e. NothingToSeeHere.harmlessFunction(), on application startup. Previously, this answer suggested using the Objective-C code to do this. However, it seems that we can do what we need using only Swift. For macOS or other platforms where UIApplication is not available, a variation of the following will be needed.

extension UIApplication {

    private static let runOnce: Void = {
        NothingToSeeHere.harmlessFunction()
    }()

    override open var next: UIResponder? {
        // Called before applicationDidFinishLaunching
        UIApplication.runOnce
        return super.next
    }

}

Step Three

We now have an entry point at application startup, and a way to hook into this from classes of your choice. All that is left to do: instead of implementing initialize(), conform to SelfAware and implement the defined method, awake().