Occasion-driven generic hooks for Swift

Dependencies, protocols and kinds

After we write Swift, we are able to import frameworks and different third social gathering libraries. It is fairly pure, simply take into consideration Basis, UIKit or these days it is extra doubtless SwiftUI, however there are numerous different dependencies that we are able to use. Even after we do not import something we normally create separate buildings or courses to construct smaller parts as a substitute of 1 gigantic spaghetti-like file, perform or no matter. Take into account the next instance:

struct NameProvider {
    func getName() -> String { "John Doe" }
}


struct App {
    let supplier = NameProvider()
    
    func run() {
        let identify = supplier.getName()
        print("Hiya (identify)!")
    }
}

let app = App()
app.run()

It reveals us the fundamentals of the separation of considerations precept. The App struct the illustration of our most important utility, which is a straightforward “Hiya World!” app, with a twist. The identify will not be hardcoded into the App object, however it’s coming from a NameProvider struct.

The factor that it’s best to discover is that we have created a static dependency between the App and the NameProvider object right here. We do not have to import a framework to create a dependency, these objects are in the identical namespace, however nonetheless the appliance will at all times require the NameProvider kind at compilation time. This isn’t unhealthy, however generally it is not what we actually need.

How can we resolve this? Wait I’ve an thought, let’s create a protocol! 😃

import Basis

struct MyNameProvider: NameProvider {
    func getName() -> String { "John Doe" }
}


protocol NameProvider {
    func getName() -> String
}

struct App {
    let supplier: NameProvider
    
    func run() {
        let identify = supplier.getName()
        print("Hiya (identify)!")
    }
}

let supplier = MyNameProvider()
let app = App(supplier: supplier)
app.run()

Oh no, this simply made our complete codebase a bit tougher to grasp, additionally did not actually solved something, as a result of we nonetheless cannot compile our utility with out the MyNameProvider dependency. That class should be a part of the bundle irrespective of what number of protocols we create. In fact we may transfer the NameProvider protocol right into a standalone Swift bundle, then we may create one other bundle for the protocol implementation that depends on that one, then use each as a dependency after we construct our utility, however hey is not this getting a bit bit difficult? 🤔

What did we acquire right here? Initially we overcomplicated a extremely easy factor. However, we eradicated an precise dependency from the App struct itself. That is an awesome factor, as a result of now we may create a mock identify supplier and take a look at our utility occasion with that, we are able to inject any sort of Swift object into the app that conforms to the NameProvider protocol.

Can we alter the supplier at runtime? Effectively, sure, that is additionally potential we may outline the supplier as a variable and alter its worth in a while, however there’s one factor that we will not resolve with this strategy.

We won’t transfer out the supplier reference from the appliance itself. 😳

Occasion-driven structure

The EDA design sample permits us to create loosely coupled software program parts and providers with out forming an precise dependency between the individuals. Take into account the next various:

struct MyNameProvider {
    func getName(_: HookArguments) -> String { "John Doe" }
}

struct App {

    func run() {
        guard let identify: String = hooks.invoke("name-event") else {
            fatalError("Somebody should present a name-event handler.")
        }
        print("Hiya (identify)!")
    }
}

let hooks = HookStorage()

let supplier = MyNameProvider()
hooks.register("name-event", use: supplier.getName)

let app = App()
app.run()

Do not attempt to compile this but, there are some extra issues that we’ll must implement, however first I’m going to elucidate this snippet step-by-step. The MyNameProvider struct getName perform signature modified a bit, as a result of in an event-driven world we’d like a unified perform signature to deal with all sort of eventualities. Fortuitously we do not have to erease the return kind to Any because of the wonderful generic help in Swift. This HookArguments kind shall be simply an alias for a dictionary that has String keys and it might have Any worth.

Now contained in the App struct we call-out for the hook system and invoke an occasion with the “name-event” identify. The invoke methodology is a perform with a generic return kind, it truly returns an non-obligatory generic worth, therefore the guard assertion with the specific String kind. Lengthy story brief, we name one thing that may return us a String worth, in different phrases we fireplace the identify occasion. 🔥

The final half is the setup, first we have to initialize our hook system that may retailer all of the references for the occasion handlers. Subsequent we create a supplier and register our handler for the given occasion, lastly we make the app and run every part.

I am not saying that this strategy is easier than the protocol oriented model, however it’s very completely different for certain. Sadly we nonetheless must construct our occasion handler system, so let’s get began.

public typealias HookArguments = [String: Any]


public protocol HookFunction {
    func invoke(_: HookArguments) -> Any
}


public typealias HookFunctionSignature<T> = (HookArguments) -> T

As I discussed this earlier than, the HookArguments is only a typealias for the [String:Any] kind, this fashion we’re going to have the ability to move round any sort of values underneath given keys for the hook capabilities. Subsequent we outline a protocol for invoking these capabilities, and eventually we construct up a perform signature for our hooks, that is going for use through the registration course of. 🤓

public struct AnonymousHookFunction: HookFunction {

    non-public let functionBlock: HookFunctionSignature<Any>

    
    public init(_ functionBlock: @escaping HookFunctionSignature<Any>) {
        self.functionBlock = functionBlock
    }

    
    public func invoke(_ args: HookArguments) -> Any {
        functionBlock(args)
    }
}

The AnonymousHookFunction is a helper that we are able to use to move round blocks as a substitute of object pointers after we register a brand new hook perform. It may be fairly helpful generally to write down an occasion handler with out creating extra courses or structs. We’re going to additionally must affiliate these hook perform pointers with an occasion identify and an precise a return kind…

public last class HookFunctionPointer {

    public var identify: String
    public var pointer: HookFunction
    public var returnType: Any.Kind
    
    public init(identify: String, perform: HookFunction, returnType: Any.Kind) {
        self.identify = identify
        self.pointer = perform
        self.returnType = returnType
    }
}

The HookFunctionPointer is used contained in the hook storage, that is the core constructing block for this whole system. The hook storage is the place the place all of your occasion handlers reside and you’ll name these occasions by way of this storage pointer when you might want to set off an occasion. 🔫

public last class HookStorage {
    
    non-public var pointers: [HookFunctionPointer]

    public init() {
        self.pointers = []
    }

    public func register<ReturnType>(_ identify: String, use block: @escaping HookFunctionSignature<ReturnType>) {
        let perform = AnonymousHookFunction { args -> Any in
            block(args)
        }
        let pointer = HookFunctionPointer(identify: identify, perform: perform, returnType: ReturnType.self)
        pointers.append(pointer)
    }

    
    public func invoke<ReturnType>(_ identify: String, args: HookArguments = [:]) -> ReturnType? {
        pointers.first { $0.identify == identify && $0.returnType == ReturnType.self }?.pointer.invoke(args) as? ReturnType
    }

    
    public func invokeAll<ReturnType>(_ identify: String, args: HookArguments = [:]) -> [ReturnType] {
        pointers.filter { $0.identify == identify && $0.returnType == ReturnType.self }.compactMap { $0.pointer.invoke(args) as? ReturnType }
    }
}

I do know, this looks like fairly difficult at first sight, however once you begin taking part in round with these strategies it will all make sense. I am nonetheless undecided concerning the naming conventions, for instance the HookStorage can be a world occasion storage so possibly it would be higher to name it one thing associated to the occasion time period. You probably have a greater thought, be at liberty to tweet me.

Oh, I nearly forgot that I wished to point out you the right way to register an nameless hook perform. 😅

hooks.register("name-event") { _ in "John Doe" }

That is it you do not occasion have to write down the return kind, the Swift compiler this time is sensible sufficient to determine the ultimate perform signature. This magic solely works with one-liners I suppose… ✨

This text was a follow-up on the modules and hooks in Swift, additionally closely impressed by the my previous Entropy framework, Drupal and the WordPress hook programs. The code implementation thought comes from the Vapor’s routing abstraction, however it’s barely modified to match my wants.

The event-driven design strategy is a really good structure and I actually hope that we’ll see the long run advantage of utilizing this sample inside Feather. I am unable to wait to inform you extra about it… 🪶