Bindable.swift

/// Similar to a `Binding`, but this is also observable/dynamic.
@propertyDelegate
@dynamicMemberLookup
final class Derived<A>: BindableObject {
    let didChange = PassthroughSubject<A, Never>()
    fileprivate var cancellables: [AnyCancellable] = []
    private let get: () -> (A)
    private let mutate: ((inout A) -> ()) -> ()
    init(get: @escaping () -> A, mutate: @escaping ((inout A) -> ()) -> ()) {
        self.get = get
        self.mutate = mutate
    }
    var value: A {
        get { get() }
        set { mutate { $0 = newValue } }
    }
    subscript<U: Equatable>(dynamicMember keyPath: WritableKeyPath<A, U>) -> Derived<U> {
        let result = Derived<U>(get: {
            let value = self.get()[keyPath: keyPath]
            return value
        }, mutate: { f in
            self.mutate { (a: inout A) in
                f(&a[keyPath: keyPath])
            }
        })
        let c = didChange.map { $0[keyPath: keyPath] }.removeDuplicates().sink { value in
            result.didChange.send(value)
        }
        cancellables.append(AnyCancellable(c))
        return result
    }
    
    var binding: Binding<A> {
        return Binding<A>(getValue: { self.value }, setValue: { self.value = $0 })
    }
    
    deinit {
        for c in cancellables {
            c.cancel()
        }
    }
}

final class SimpleStore<A>: BindableObject {
    let didChange = 
        PassthroughSubject<A, Never>()
    init(_ value: A) { self.value = value }
    var value: A {
        didSet {
            didChange.send(value)
        }
    }
    
    var bindable: Derived<A> {
        let result = Derived<A>(get: {
            self.value
        }, mutate: { f in
            f(&self.value)
        })
        let c = self.didChange.sink { [weak result] value in
            result?.didChange.send(value)
        }
        result.cancellables.append(AnyCancellable(c))
        return result
    }
}