Occasion-driven generic hooks for Swift

Dependencies, protocols and kinds

Once 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 possible SwiftUI, however there are a lot of different dependencies that we are able to use. Even after we do not import something we normally create separate constructions or courses to construct smaller elements as an alternative of 1 gigantic spaghetti-like file, operate or no matter. Think about 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 exhibits us the fundamentals of the separation of issues precept. The App struct the illustration of our major software, which is an easy “Hiya World!” app, with a twist. The identify just isn’t 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 applying will at all times require the NameProvider sort at compilation time. This isn’t unhealthy, however typically it isn’t what we actually need.

How can we clear up 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 whole codebase a bit more durable to know, additionally did not actually solved something, as a result of we nonetheless cannot compile our software with out the MyNameProvider dependency. That class have to be a part of the bundle regardless of what number of protocols we create. After all 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 software, however hey is not this getting a bit of bit sophisticated? 🤔

What did we acquire right here? To begin with we overcomplicated a very easy factor. Alternatively, we eradicated an precise dependency from the App struct itself. That is an important factor, as a result of now we may create a mock identify supplier and check our software occasion with that, we are able to inject any type of Swift object into the app that conforms to the NameProvider protocol.

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

We will not transfer out the supplier reference from the applying itself. 😳

Occasion-driven structure

The EDA design sample permits us to create loosely coupled software program elements and providers with out forming an precise dependency between the members. Think about the next different:

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 clarify this snippet step-by-step. The MyNameProvider struct getName operate signature modified a bit, as a result of in an event-driven world we’d like a unified operate signature to deal with all type of eventualities. Fortuitously we do not have to erease the return sort to Any because of the superb generic help in Swift. This HookArguments sort shall be simply an alias for a dictionary that has String keys and it could actually 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 operate with a generic return sort, it truly returns an elective generic worth, therefore the guard assertion with the express String sort. Lengthy story quick, we name one thing that may return us a String worth, in different phrases we hearth 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 all the things.

I am not saying that this method is easier than the protocol oriented model, however it’s very completely different for positive. Sadly we nonetheless need to 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] sort, this fashion we’re going to have the ability to go round any type of values below given keys for the hook capabilities. Subsequent we outline a protocol for invoking these capabilities, and eventually we construct up a operate signature for our hooks, that is going for use throughout 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 go round blocks as an alternative of object pointers after we register a brand new hook operate. It may be fairly useful typically to put in writing an occasion handler with out creating extra courses or structs. We’re going to additionally must affiliate these hook operate pointers with an occasion identify and an precise a return sort…

public last class HookFunctionPointer {

    public var identify: String
    public var pointer: HookFunction
    public var returnType: Any.Sort
    
    public init(identify: String, operate: HookFunction, returnType: Any.Sort) {
        self.identify = identify
        self.pointer = operate
        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 stay and you may name these occasions by way of this storage pointer when you could 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 operate = AnonymousHookFunction { args -> Any in
            block(args)
        }
        let pointer = HookFunctionPointer(identify: identify, operate: operate, 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 sophisticated at first sight, however if you begin enjoying round with these strategies it’s going to all make sense. I am nonetheless unsure in regards to the naming conventions, for instance the HookStorage can be a worldwide occasion storage so possibly it might be higher to name it one thing associated to the occasion time period. When you have a greater thought, be at liberty to tweet me.

Oh, I nearly forgot that I needed to point out you the way to register an nameless hook operate. 😅

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

That is it you do not occasion have to put in writing the return sort, the Swift compiler this time is wise sufficient to determine the ultimate operate 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 methods. The code implementation thought comes from the Vapor’s routing abstraction, however it’s barely modified to match my wants.

The event-driven design method 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 let you know extra about it… 🪶