How does implement of Store and ViewStore in TCA

ParseTCA.md

Redux and TCA Learning Record 1

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TCA

We can use Store and compose a new ViewStore to binding a smaller UI Block in subView or subViewController. How does TCA implements in binding each other ?

I crate some code to reproduce it.

Implement

Test

Store.swift

import Foundation
import Combine

class Store<State, Action> {
    var publisher: CurrentValueSubject<State, Never>
    var void:  Reducer<State, Action>
    var cancelable: AnyCancellable?
    
    init(state: State, reducer: @escaping Reducer<State, Action>) {
        publisher = CurrentValueSubject(state)
        self.void = reducer
    }
    
    
    deinit {
        print("\(Self.self) is deinit")
    }
    
    func send(action: Action) {
        void(&publisher.value, action)
    }
    
    func viewStore()-> ViewStore<State, Action> {
        let child = ViewStore(value: publisher.value, root: {
            [weak self] action in
            self?.send(action: action)
        })
        
        child.cancelable = publisher
            .dropFirst()
            .sink(receiveValue: { [weak child] in
                child?.value = $0
            })
        
        return child
    }
}

class ViewStore<State, Action> {
    var cancelable: AnyCancellable?
    var publisher: CurrentValueSubject<State, Never>
    var root: ((Action)-> Void)?
    
    init(value: State, root: @escaping (Action)-> Void) {
        self.publisher = CurrentValueSubject(value)
        self.root = root
    }
    
    func send(action: Action) {
        root?(action)
    }
    
    deinit {
        print("\(Self.self) is deinit")
    }
}

extension ViewStore {
    var value: State {
        get {
            self.publisher.value
        }
        
        set {
            self.publisher.send(newValue)
        }
    }
}

zTest.swift

import Foundation
import Combine

enum CounterAction {
    case decrement
    case increment
}

struct CounterState {
    var count: Int = 0
}

typealias Reducer<State, Action> = (inout State, Action)-> Void

let reducer: Reducer<CounterState, CounterAction> = { state , action in
    switch action {
    case .decrement:
        state.count -= 1
    case .increment:
        state.count += 1
    }
}


var bag: Set<AnyCancellable> = []
var parent: Store! = Store(state: CounterState(), reducer: reducer)

var child: ViewStore! = parent.viewStore()

child.publisher.print().sink(receiveValue: {_ in }).store(in: &bag)

child.send(action: .decrement) // -1
child.send(action: .increment) // 0
child.send(action: .decrement) // -1
child.send(action: .decrement) // -2
child.send(action: .decrement) // -3


print(parent.publisher.value.count == -3)
print(child.value.count == -3)