Simplifying State by partially introducing unidirectional data flow in your codebase

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47. /// Model
    struct Cinnamon { let value: Int = 0 }
    enum Event { case increase }
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48. /// Model
    struct Cinnamon { let value: Int = 0 }
    enum Event { case increase }
    func reducer(state: Cinnamon, event: Event) -> Cinnamon {
    var newState = state
    if event == .increase {
    newState.value += 1
    }
    return newState
    }
    /// UI
    let builder = Form(state: Cinnamon(), reducer: reducer)
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52. struct AddressBook {
    var contacts: [Person]
    var searchTerm: String
    var scrollPosition: Int
    }
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53. struct AddressBookApp {
    struct Data {
    var contacts: [Person]
    }
    var data: Data
    struct UI {
    var searchTerm: String
    var scrollPosition: Int
    }
    var ui: UI
    }
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68. private var subscribers: [String: (State)->Void] = [:]
    public func subscribe(_ subscriber: @escaping (State)->Void)
    -> String {
    let token = UUID().uuidString
    subscribers[token] = subscriber
    subscriber(state)
    return token
    }
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70. protocol FormComponent {
    associatedtype State
    func setup(with state: State)
    func update(state: State)
    -> Void
    }
    public func subscribe
    (_ subscriber: Component) -> String
    where Component.State == State
    {...
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72. func subscribe(
    path: KeyPath,
    action: @escaping (_ oldValue: Type, _ newValue: Type) -> Void
    ) -> String
    struct Person { let name: String }
    form.subscribe(path: \Person.name) { (old, new) in
    ...
    }
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73. private var history: [State] = []
    func apply(_ change: (inout State) -> Void) {
    states.append(state)
    change(&state)
    notifySubscribers()
    }
    func undo() {
    state = history.popLast()
    notifySubscribers()
    }
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