no-more-swiftui.swift

import UIKit

protocol View {
    func body() -> UIView
}


struct ModifiedView<T: View>: View {
    let content: T
    let modifier: (UIView) -> Void
    init(content: T, modifier: @escaping (UIView) -> Void) {
        self.content = content
        self.modifier = modifier
    }
    func body() -> UIView {
        let view = content.body()
        modifier(view)
        return view
    }
}

extension View {

    func background(_ background: UIColor) -> ModifiedView<Self> {
        return ModifiedView(content: self) { body in
            body.backgroundColor = background
        }
    }


    func bind<Binder: AnyObject, Target: UIView>(_ keyPath: ReferenceWritableKeyPath<Binder, Target?>, binder: Binder) -> ModifiedView<Self> {
        return ModifiedView(content: self) { body in
            binder[keyPath: keyPath] = body as? Target
        }
    }
}

@_functionBuilder
final class ViewBuilder {
    static func buildBlock<Content>(_ content: Content) -> some View where Content : View {
        return content
    }
    static func buildBlock<C0, C1>(_ c0: C0, _ c1: C1) -> some View where C0 : View, C1 : View {
        return TupleView(t1: c0, t2: c1)
    }
}

protocol TupleViewAnchor {
    func flattenViews() -> [UIView]
}

final class TupleView<T1, T2>: View, TupleViewAnchor where T1: View, T2: View {
    let t1: T1
    let t2: T2

    init(t1: T1, t2: T2) {
        self.t1 = t1
        self.t2 = t2
    }

    func flattenViews() -> [UIView] {
        var selfContents: [UIView] = []
        if let t1 = t1 as? TupleViewAnchor {
            selfContents.append(contentsOf: t1.flattenViews())
        } else {
            selfContents.append(t1.body())
        }
        if let t2 = t2 as? TupleViewAnchor {
            selfContents.append(contentsOf: t2.flattenViews())
        } else {
            selfContents.append(t2.body())
        }
        return selfContents
    }

    func body() -> UIView {
        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.distribution = .fillEqually
        for subview in flattenViews() {
            stackView.addArrangedSubview(subview)
        }
        return stackView
    }
}

final class SimpleView: View {
    let makeContent: () -> View
    init(@ViewBuilder makeContent: @escaping () -> View) {
        self.makeContent = makeContent
    }
    func body() -> UIView {
        let view = UIView()
        let content = makeContent().body()
        view.addSubview(content)
        content.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate(
            [
                view.topAnchor.constraint(equalTo: content.topAnchor),
                view.bottomAnchor.constraint(equalTo: content.bottomAnchor),
                view.leadingAnchor.constraint(equalTo: content.leadingAnchor),
                view.trailingAnchor.constraint(equalTo: content.trailingAnchor)
            ]
        )

        return view
    }
}

final class Label: View {
    let text: String
    init(text: String) {
        self.text = text
    }
    func body() -> UIView {
        let label = UILabel()
        label.text = text
        label.sizeToFit()
        label.textAlignment = .center
        return label
    }
}



final class UserView: View {

    var userNameLabel: UILabel!
    var userName: String? {
        didSet { userNameLabel.text = userName }
    }

    func body() -> UIView {
        SimpleView {
            Label(text: "Hello")
            Label(text: "John")
                .bind(\.userNameLabel, binder: self)
        }
        .background(.white)
        .body()
    }
}