SwiftUI emulating React custom hooks (DynamicProperty composition) https://twitter.com/inamiy/status/1313343537132433409

SwiftUI-emulating-React-Hooks.swift

import SwiftUI

struct ContentView: View {
    @UseCounter
    var counter: Int = 0

    @UseCounterEffect(initialCount: 1, onUpdate: { count in
        print("===> update: \(count)")
    })
    var counterEffect: Void

    var body: some View {
        return VStack {
            Text("\(self.counter)")
                .padding()

            Button("Incr") {
                self._counter.increment()
                self._counterEffect.increment()
            }
        }
    }
}

@propertyWrapper
struct UseCounter<T: SignedNumeric>: DynamicProperty /*Required*/ {
    @State
    private var state: T

    init(wrappedValue: T) {
        self._state = State(wrappedValue: wrappedValue)
    }

    var wrappedValue: T {
        get { state }
    }

    func increment() {
        state += 1
    }
}

@propertyWrapper
struct UseCounterEffect<T: SignedNumeric>: DynamicProperty /*Required*/ {
    @State
    private var state: T

    private let onUpdate: (T) -> ()

    init(initialCount: T, onUpdate: @escaping (T) -> ()) {
        self._state = State(wrappedValue: initialCount)
        self.onUpdate = onUpdate
    }

    // Hide internal values.
    var wrappedValue: Void { () }

    func update() {
        onUpdate(state)
    }

    func increment() {
        state += 1
    }
}