Merge commit '7621e2f8dec938cf48181c8b10afc9b01f444e68' into beta

submodules/ComponentFlow/Source/Base/ChildComponentTransitions.swift

@@ -0,0 +1,110 @@
+import Foundation
+import UIKit
+
+public extension ComponentTransition.Appear {
+    static func `default`(scale: Bool = false, alpha: Bool = false) -> ComponentTransition.Appear {
+        return ComponentTransition.Appear { component, view, transition in
+            if scale {
+                transition.animateScale(view: view, from: 0.01, to: 1.0)
+            }
+            if alpha {
+                transition.animateAlpha(view: view, from: 0.0, to: 1.0)
+            }
+        }
+    }
+
+    static func scaleIn() -> ComponentTransition.Appear {
+        return ComponentTransition.Appear { component, view, transition in
+            transition.animateScale(view: view, from: 0.01, to: 1.0)
+        }
+    }
+}
+
+public extension ComponentTransition.AppearWithGuide {
+    static func `default`(scale: Bool = false, alpha: Bool = false) -> ComponentTransition.AppearWithGuide {
+        return ComponentTransition.AppearWithGuide { component, view, guide, transition in
+            if scale {
+                transition.animateScale(view: view, from: 0.01, to: 1.0)
+            }
+            if alpha {
+                transition.animateAlpha(view: view, from: 0.0, to: 1.0)
+            }
+            transition.animatePosition(view: view, from: CGPoint(x: guide.x - view.center.x, y: guide.y - view.center.y), to: CGPoint(), additive: true)
+        }
+    }
+}
+
+public extension ComponentTransition.Disappear {
+    static func `default`(scale: Bool = false, alpha: Bool = true) -> ComponentTransition.Disappear {
+        return ComponentTransition.Disappear { view, transition, completion in
+            if scale {
+                transition.setScale(view: view, scale: 0.01, completion: { _ in
+                    if !alpha {
+                        completion()
+                    }
+                })
+            }
+            if alpha {
+                transition.setAlpha(view: view, alpha: 0.0, completion: { _ in
+                    completion()
+                })
+            }
+            if !alpha && !scale {
+                completion()
+            }
+        }
+    }
+}
+
+public extension ComponentTransition.DisappearWithGuide {
+    static func `default`(alpha: Bool = true) -> ComponentTransition.DisappearWithGuide {
+        return ComponentTransition.DisappearWithGuide { stage, view, guide, transition, completion in
+            switch stage {
+            case .begin:
+                if alpha {
+                    transition.setAlpha(view: view, alpha: 0.0, completion: { _ in
+                        completion()
+                    })
+                }
+                transition.setFrame(view: view, frame: CGRect(origin: CGPoint(x: guide.x - view.bounds.width / 2.0, y: guide.y - view.bounds.height / 2.0), size: view.bounds.size), completion: { _ in
+                    if !alpha {
+                        completion()
+                    }
+                })
+            case .update:
+                transition.setFrame(view: view, frame: CGRect(origin: CGPoint(x: guide.x - view.bounds.width / 2.0, y: guide.y - view.bounds.height / 2.0), size: view.bounds.size))
+            }
+        }
+    }
+}
+
+public extension ComponentTransition.Update {
+    static let `default` = ComponentTransition.Update { component, view, transition in
+        let position = component._position ?? CGPoint()
+        let size = component.size
+        view.layer.anchorPoint = component._anchorPoint ?? CGPoint(x: 0.5, y: 0.5)
+        if let scale = component._scale {
+            transition.setBounds(view: view, bounds: CGRect(origin: CGPoint(), size: size))
+            transition.setPosition(view: view, position: position)
+            transition.setScale(view: view, scale: scale)
+        } else {
+            if view is UIScrollView {
+                let frame = component.size.centered(around: component._position ?? CGPoint())
+                if view.frame != frame {
+                    transition.setFrame(view: view, frame: frame)
+                }
+            } else {
+                if component._anchorPoint != nil {
+                    view.bounds = CGRect(origin: CGPoint(), size: size)
+                } else {
+                    transition.setBounds(view: view, bounds: CGRect(origin: CGPoint(), size: size))
+                }
+                transition.setPosition(view: view, position: position)
+            }
+        }
+        let opacity = component._opacity ?? 1.0
+        if view.alpha != opacity {
+            transition.setAlpha(view: view, alpha: opacity)
+        }
+    }
+}

submodules/ComponentFlow/Source/Base/CombinedComponent.swift

@@ -0,0 +1,898 @@
+import Foundation
+import UIKit
+
+private func updateChildAnyComponent<EnvironmentType>(
+    id: _AnyChildComponent.Id,
+    component: AnyComponent<EnvironmentType>,
+    view: UIView,
+    availableSize: CGSize,
+    transition: ComponentTransition
+) -> _UpdatedChildComponent {
+    let parentContext = _AnyCombinedComponentContext.current
+
+    if !parentContext.updateContext.updatedViews.insert(id).inserted {
+        preconditionFailure("Child component can only be processed once")
+    }
+
+    let context = view.context(component: component)
+    var isEnvironmentUpdated = false
+    var isStateUpdated = false
+    var isComponentUpdated = false
+    var availableSizeUpdated = false
+
+    if context.environment.calculateIsUpdated() {
+        context.environment._isUpdated = false
+        isEnvironmentUpdated = true
+    }
+
+    if context.erasedState.isUpdated {
+        context.erasedState.isUpdated = false
+        isStateUpdated = true
+    }
+
+    if context.erasedComponent != component {
+        isComponentUpdated = true
+    }
+    context.erasedComponent = component
+
+    if context.layoutResult.availableSize != availableSize {
+        context.layoutResult.availableSize = availableSize
+        availableSizeUpdated = true
+    }
+
+    let isUpdated = isEnvironmentUpdated || isStateUpdated || isComponentUpdated || availableSizeUpdated
+
+    if !isUpdated, let size = context.layoutResult.size {
+        return _UpdatedChildComponent(
+            id: id,
+            component: component,
+            view: view,
+            context: context,
+            size: size
+        )
+    } else {
+        let size = component._update(
+            view: view,
+            availableSize: availableSize,
+            environment: context.environment,
+            transition: transition
+        )
+        context.layoutResult.size = size
+
+        return _UpdatedChildComponent(
+            id: id,
+            component: component,
+            view: view,
+            context: context,
+            size: size
+        )
+    }
+}
+
+public class _AnyChildComponent {
+    fileprivate enum Id: Hashable {
+        case direct(Int)
+        case mapped(Int, AnyHashable)
+    }
+
+    fileprivate var directId: Int {
+        return Int(bitPattern: Unmanaged.passUnretained(self).toOpaque())
+    }
+}
+
+public final class _ConcreteChildComponent<ComponentType: Component>: _AnyChildComponent {
+    fileprivate var id: Id {
+        return .direct(self.directId)
+    }
+
+    public func update(component: ComponentType, @EnvironmentBuilder environment: () -> Environment<ComponentType.EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        let parentContext = _AnyCombinedComponentContext.current
+        if !parentContext.updateContext.configuredViews.insert(self.id).inserted {
+            preconditionFailure("Child component can only be configured once")
+        }
+
+        var transition = transition
+
+        let view: ComponentType.View
+        if let current = parentContext.childViews[self.id] {
+            // TODO: Check if the type is the same
+            view = current.view as! ComponentType.View
+        } else {
+            view = component.makeView()
+            transition = transition.withAnimation(.none)
+        }
+
+        let context = view.context(component: component)
+        EnvironmentBuilder._environment = context.erasedEnvironment
+        let environmentResult = environment()
+        EnvironmentBuilder._environment = nil
+        context.erasedEnvironment = environmentResult
+
+        return updateChildAnyComponent(
+            id: self.id,
+            component: AnyComponent(component),
+            view: view,
+            availableSize: availableSize,
+            transition: transition
+        )
+    }
+}
+
+public extension _ConcreteChildComponent where ComponentType.EnvironmentType == Empty {
+    func update(component: ComponentType, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        return self.update(component: component, environment: {}, availableSize: availableSize, transition: transition)
+    }
+}
+
+public final class _UpdatedChildComponentGuide {
+    fileprivate let instance: _ChildComponentGuide
+
+    fileprivate init(instance: _ChildComponentGuide) {
+        self.instance = instance
+    }
+}
+
+public final class _ChildComponentGuide {
+    fileprivate var directId: Int {
+        return Int(bitPattern: Unmanaged.passUnretained(self).toOpaque())
+    }
+
+    fileprivate var id: _AnyChildComponent.Id {
+        return .direct(self.directId)
+    }
+
+    public func update(position: CGPoint, transition: ComponentTransition) -> _UpdatedChildComponentGuide {
+        let parentContext = _AnyCombinedComponentContext.current
+
+        let previousPosition = parentContext.guides[self.id]
+
+        if parentContext.updateContext.configuredGuides.updateValue(_AnyCombinedComponentContext.UpdateContext.ConfiguredGuide(previousPosition: previousPosition ?? position, position: position), forKey: self.id) != nil {
+            preconditionFailure("Child guide can only be configured once")
+        }
+
+        for disappearingView in parentContext.disappearingChildViews {
+            if disappearingView.guideId == self.id {
+                disappearingView.transitionWithGuide?(
+                    stage: .update,
+                    view: disappearingView.view,
+                    guide: position,
+                    transition: transition,
+                    completion: disappearingView.completion
+                )
+            }
+        }
+
+        return _UpdatedChildComponentGuide(instance: self)
+    }
+}
+
+public final class _UpdatedChildComponent {
+    fileprivate let id: _AnyChildComponent.Id
+    fileprivate let component: _TypeErasedComponent
+    fileprivate let view: UIView
+    fileprivate let context: _TypeErasedComponentContext
+
+    public let size: CGSize
+
+    var _removed: Bool = false
+    var _anchorPoint: CGPoint?
+    var _position: CGPoint?
+    var _scale: CGFloat?
+    var _opacity: CGFloat?
+    var _cornerRadius: CGFloat?
+    var _clipsToBounds: Bool?
+    var _allowsGroupOpacity: Bool?
+    var _shadow: Shadow?
+    
+    fileprivate var transitionAppear: ComponentTransition.Appear?
+    fileprivate var transitionAppearWithGuide: (ComponentTransition.AppearWithGuide, _AnyChildComponent.Id)?
+    fileprivate var transitionDisappear: ComponentTransition.Disappear?
+    fileprivate var transitionDisappearWithGuide: (ComponentTransition.DisappearWithGuide, _AnyChildComponent.Id)?
+    fileprivate var transitionUpdate: ComponentTransition.Update?
+    fileprivate var gestures: [Gesture] = []
+
+    fileprivate init(
+        id: _AnyChildComponent.Id,
+        component: _TypeErasedComponent,
+        view: UIView,
+        context: _TypeErasedComponentContext,
+        size: CGSize
+    ) {
+        self.id = id
+        self.component = component
+        self.view = view
+        self.context = context
+        self.size = size
+    }
+
+    @discardableResult public func appear(_ transition: ComponentTransition.Appear) -> _UpdatedChildComponent {
+        self.transitionAppear = transition
+        self.transitionAppearWithGuide = nil
+        return self
+    }
+
+    @discardableResult public func appear(_ transition: ComponentTransition.AppearWithGuide, guide: _UpdatedChildComponentGuide) -> _UpdatedChildComponent {
+        self.transitionAppear = nil
+        self.transitionAppearWithGuide = (transition, guide.instance.id)
+        return self
+    }
+
+    @discardableResult public func disappear(_ transition: ComponentTransition.Disappear) -> _UpdatedChildComponent {
+        self.transitionDisappear = transition
+        self.transitionDisappearWithGuide = nil
+        return self
+    }
+
+    @discardableResult public func disappear(_ transition: ComponentTransition.DisappearWithGuide, guide: _UpdatedChildComponentGuide) -> _UpdatedChildComponent {
+        self.transitionDisappear = nil
+        self.transitionDisappearWithGuide = (transition, guide.instance.id)
+        return self
+    }
+
+    @discardableResult public func update(_ transition: ComponentTransition.Update) -> _UpdatedChildComponent {
+        self.transitionUpdate = transition
+        return self
+    }
+
+    @discardableResult public func removed(_ removed: Bool) -> _UpdatedChildComponent {
+        self._removed = removed
+        return self
+    }
+    
+    @discardableResult public func anchorPoint(_ anchorPoint: CGPoint) -> _UpdatedChildComponent {
+        self._anchorPoint = anchorPoint
+        return self
+    }
+
+    @discardableResult public func position(_ position: CGPoint) -> _UpdatedChildComponent {
+        self._position = position
+        return self
+    }
+    
+    @discardableResult public func scale(_ scale: CGFloat) -> _UpdatedChildComponent {
+        self._scale = scale
+        return self
+    }
+    
+    @discardableResult public func opacity(_ opacity: CGFloat) -> _UpdatedChildComponent {
+        self._opacity = opacity
+        return self
+    }
+
+    @discardableResult public func cornerRadius(_ cornerRadius: CGFloat) -> _UpdatedChildComponent {
+        self._cornerRadius = cornerRadius
+        return self
+    }
+
+    @discardableResult public func clipsToBounds(_ clipsToBounds: Bool) -> _UpdatedChildComponent {
+        self._clipsToBounds = clipsToBounds
+        return self
+    }
+    
+    @discardableResult public func allowsGroupOpacity(_ allowsGroupOpacity: Bool) -> _UpdatedChildComponent {
+        self._allowsGroupOpacity = allowsGroupOpacity
+        return self
+    }
+    
+    @discardableResult public func shadow(_ shadow: Shadow?) -> _UpdatedChildComponent {
+        self._shadow = shadow
+        return self
+    }
+
+    @discardableResult public func gesture(_ gesture: Gesture) -> _UpdatedChildComponent {
+        self.gestures.append(gesture)
+        return self
+    }
+}
+
+public final class _EnvironmentChildComponent<EnvironmentType>: _AnyChildComponent {
+    fileprivate var id: Id {
+        return .direct(self.directId)
+    }
+
+    func update(component: AnyComponent<EnvironmentType>, @EnvironmentBuilder environment: () -> Environment<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        let parentContext = _AnyCombinedComponentContext.current
+        if !parentContext.updateContext.configuredViews.insert(self.id).inserted {
+            preconditionFailure("Child component can only be configured once")
+        }
+
+        var transition = transition
+
+        let view: UIView
+        if let current = parentContext.childViews[self.id] {
+            // Check if the type is the same
+            view = current.view
+        } else {
+            view = component._makeView()
+            transition = .immediate
+        }
+
+        let viewContext = view.context(component: component)
+        EnvironmentBuilder._environment = viewContext.erasedEnvironment
+        let environmentResult = environment()
+        EnvironmentBuilder._environment = nil
+        viewContext.erasedEnvironment = environmentResult
+
+        return updateChildAnyComponent(
+            id: self.id,
+            component: component,
+            view: view,
+            availableSize: availableSize,
+            transition: transition
+        )
+    }
+}
+
+public extension _EnvironmentChildComponent where EnvironmentType == Empty {
+    func update(component: AnyComponent<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        return self.update(component: component, environment: {}, availableSize: availableSize, transition: transition)
+    }
+}
+
+public extension _EnvironmentChildComponent {
+    func update<ComponentType: Component>(_ component: ComponentType, @EnvironmentBuilder environment: () -> Environment<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent where ComponentType.EnvironmentType == EnvironmentType {
+        return self.update(component: AnyComponent(component), environment: environment, availableSize: availableSize, transition: transition)
+    }
+    
+    func update(_ component: AnyComponent<EnvironmentType>, @EnvironmentBuilder environment: () -> Environment<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        return self.update(component: component, environment: environment, availableSize: availableSize, transition: transition)
+    }
+
+    func update<ComponentType: Component>(_ component: ComponentType, @EnvironmentBuilder environment: () -> Environment<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent where ComponentType.EnvironmentType == EnvironmentType, EnvironmentType == Empty {
+        return self.update(component: AnyComponent(component), environment: {}, availableSize: availableSize, transition: transition)
+    }
+}
+
+public final class _EnvironmentChildComponentFromMap<EnvironmentType>: _AnyChildComponent {
+    private let id: Id
+
+    fileprivate init(id: Id) {
+        self.id = id
+    }
+
+    public func update(component: AnyComponent<EnvironmentType>, @EnvironmentBuilder environment: () -> Environment<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        let parentContext = _AnyCombinedComponentContext.current
+        if !parentContext.updateContext.configuredViews.insert(self.id).inserted {
+            preconditionFailure("Child component can only be configured once")
+        }
+
+        var transition = transition
+
+        let view: UIView
+        if let current = parentContext.childViews[self.id] {
+            // Check if the type is the same
+            view = current.view
+        } else {
+            view = component._makeView()
+            transition = transition.withAnimation(.none)
+        }
+
+        let viewContext = view.context(component: component)
+        EnvironmentBuilder._environment = viewContext.erasedEnvironment
+        let environmentResult = environment()
+        EnvironmentBuilder._environment = nil
+        viewContext.erasedEnvironment = environmentResult
+
+        return updateChildAnyComponent(
+            id: self.id,
+            component: component,
+            view: view,
+            availableSize: availableSize,
+            transition: transition
+        )
+    }
+}
+
+public extension _EnvironmentChildComponentFromMap where EnvironmentType == Empty {
+    func update(component: AnyComponent<EnvironmentType>, availableSize: CGSize, transition: ComponentTransition) -> _UpdatedChildComponent {
+        return self.update(component: component, environment: {}, availableSize: availableSize, transition: transition)
+    }
+}
+
+public final class _EnvironmentChildComponentMap<EnvironmentType, Key: Hashable> {
+    private var directId: Int {
+        return Int(bitPattern: Unmanaged.passUnretained(self).toOpaque())
+    }
+
+    public subscript(_ key: Key) -> _EnvironmentChildComponentFromMap<EnvironmentType> {
+        get {
+            return _EnvironmentChildComponentFromMap<EnvironmentType>(id: .mapped(self.directId, key))
+        }
+    }
+}
+
+public final class CombinedComponentContext<ComponentType: Component> {
+    fileprivate let escapeGuard = EscapeGuard()
+
+    private let context: ComponentContext<ComponentType>
+    public let view: UIView
+
+    public let component: ComponentType
+    public let availableSize: CGSize
+    public let transition: ComponentTransition
+    private let addImpl: (_ updatedComponent: _UpdatedChildComponent) -> Void
+
+    public var environment: Environment<ComponentType.EnvironmentType> {
+        return self.context.environment
+    }
+    public var state: ComponentType.State {
+        return self.context.state
+    }
+
+    fileprivate init(
+        context: ComponentContext<ComponentType>,
+        view: UIView,
+        component: ComponentType,
+        availableSize: CGSize,
+        transition: ComponentTransition,
+        add: @escaping (_ updatedComponent: _UpdatedChildComponent) -> Void
+    ) {
+        self.context = context
+        self.view = view
+        self.component = component
+        self.availableSize = availableSize
+        self.transition = transition
+        self.addImpl = add
+    }
+
+    public func add(_ updatedComponent: _UpdatedChildComponent) {
+        self.addImpl(updatedComponent)
+    }
+}
+
+public protocol CombinedComponent: Component {
+    typealias Body = (CombinedComponentContext<Self>) -> CGSize
+
+    static var body: Body { get }
+}
+
+private class _AnyCombinedComponentContext {
+    class UpdateContext {
+        struct ConfiguredGuide {
+            var previousPosition: CGPoint
+            var position: CGPoint
+        }
+
+        var configuredViews: Set<_AnyChildComponent.Id> = Set()
+        var updatedViews: Set<_AnyChildComponent.Id> = Set()
+        var configuredGuides: [_AnyChildComponent.Id: ConfiguredGuide] = [:]
+    }
+
+    private static var _current: _AnyCombinedComponentContext?
+    static var current: _AnyCombinedComponentContext {
+        return self._current!
+    }
+
+    static func push(_ context: _AnyCombinedComponentContext) -> _AnyCombinedComponentContext? {
+        let previous = self._current
+
+        precondition(context._updateContext == nil)
+        context._updateContext = UpdateContext()
+        self._current = context
+
+        return previous
+    }
+
+    static func pop(_ context: _AnyCombinedComponentContext, stack: _AnyCombinedComponentContext?) {
+        precondition(context._updateContext != nil)
+        context._updateContext = nil
+
+        self._current = stack
+    }
+
+    class ChildView {
+        let view: UIView
+        var index: Int
+        var transition: ComponentTransition.Disappear?
+        var transitionWithGuide: (ComponentTransition.DisappearWithGuide, _AnyChildComponent.Id)?
+
+        var gestures: [UInt: UIGestureRecognizer] = [:]
+
+        init(view: UIView, index: Int) {
+            self.view = view
+            self.index = index
+        }
+
+        func updateGestures(_ gestures: [Gesture]) {
+            var validIds: [UInt] = []
+            for gesture in gestures {
+                validIds.append(gesture.id.id)
+                if let current = self.gestures[gesture.id.id] {
+                    gesture.update(gesture: current)
+                } else {
+                    let gestureInstance = gesture.create()
+                    self.gestures[gesture.id.id] = gestureInstance
+                    self.view.isUserInteractionEnabled = true
+                    self.view.addGestureRecognizer(gestureInstance)
+                }
+            }
+            var removeIds: [UInt] = []
+            for id in self.gestures.keys {
+                if !validIds.contains(id) {
+                    removeIds.append(id)
+                }
+            }
+            for id in removeIds {
+                if let gestureInstance = self.gestures.removeValue(forKey: id) {
+                    self.view.removeGestureRecognizer(gestureInstance)
+                }
+            }
+        }
+    }
+
+    class DisappearingChildView {
+        let view: UIView
+        let guideId: _AnyChildComponent.Id?
+        let transition: ComponentTransition.Disappear?
+        let transitionWithGuide: ComponentTransition.DisappearWithGuide?
+        let completion: () -> Void
+
+        init(
+            view: UIView,
+            guideId: _AnyChildComponent.Id?,
+            transition: ComponentTransition.Disappear?,
+            transitionWithGuide: ComponentTransition.DisappearWithGuide?,
+            completion: @escaping () -> Void
+        ) {
+            self.view = view
+            self.guideId = guideId
+            self.transition = transition
+            self.transitionWithGuide = transitionWithGuide
+            self.completion = completion
+        }
+    }
+
+    var childViews: [_AnyChildComponent.Id: ChildView] = [:]
+    var childViewIndices: [_AnyChildComponent.Id] = []
+    var guides: [_AnyChildComponent.Id: CGPoint] = [:]
+    var disappearingChildViews: [DisappearingChildView] = []
+
+    private var _updateContext: UpdateContext?
+    var updateContext: UpdateContext {
+        return self._updateContext!
+    }
+}
+
+private final class _CombinedComponentContext<ComponentType: CombinedComponent>: _AnyCombinedComponentContext {
+    var body: ComponentType.Body?
+}
+
+private var UIView_CombinedComponentContextKey: Int?
+
+private extension UIView {
+    func getCombinedComponentContext<ComponentType: CombinedComponent>(_ type: ComponentType.Type) -> _CombinedComponentContext<ComponentType> {
+        if let context = objc_getAssociatedObject(self, &UIView_CombinedComponentContextKey) as? _CombinedComponentContext<ComponentType> {
+            return context
+        } else {
+            let context = _CombinedComponentContext<ComponentType>()
+            objc_setAssociatedObject(self, &UIView_CombinedComponentContextKey, context, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
+            return context
+        }
+    }
+}
+
+public extension ComponentTransition {
+    final class Appear {
+        private let f: (_UpdatedChildComponent, UIView, ComponentTransition) -> Void
+
+        public init(_ f: @escaping (_UpdatedChildComponent, UIView, ComponentTransition) -> Void) {
+            self.f = f
+        }
+
+        public func callAsFunction(component: _UpdatedChildComponent, view: UIView, transition: ComponentTransition) {
+            self.f(component, view, transition)
+        }
+    }
+
+    final class AppearWithGuide {
+        private let f: (_UpdatedChildComponent, UIView, CGPoint, ComponentTransition) -> Void
+
+        public init(_ f: @escaping (_UpdatedChildComponent, UIView, CGPoint, ComponentTransition) -> Void) {
+            self.f = f
+        }
+
+        public func callAsFunction(component: _UpdatedChildComponent, view: UIView, guide: CGPoint, transition: ComponentTransition) {
+            self.f(component, view, guide, transition)
+        }
+    }
+
+    final class Disappear {
+        private let f: (UIView, ComponentTransition, @escaping () -> Void) -> Void
+
+        public init(_ f: @escaping (UIView, ComponentTransition, @escaping () -> Void) -> Void) {
+            self.f = f
+        }
+
+        public func callAsFunction(view: UIView, transition: ComponentTransition, completion: @escaping () -> Void) {
+            self.f(view, transition, completion)
+        }
+    }
+
+    final class DisappearWithGuide {
+        public enum Stage {
+            case begin
+            case update
+        }
+
+        private let f: (Stage, UIView, CGPoint, ComponentTransition, @escaping () -> Void) -> Void
+
+        public init(_ f: @escaping (Stage, UIView, CGPoint, ComponentTransition, @escaping () -> Void) -> Void
+        ) {
+            self.f = f
+        }
+
+        public func callAsFunction(stage: Stage, view: UIView, guide: CGPoint, transition: ComponentTransition, completion: @escaping () -> Void) {
+            self.f(stage, view, guide, transition, completion)
+        }
+    }
+
+    final class Update {
+        private let f: (_UpdatedChildComponent, UIView, ComponentTransition) -> Void
+
+        public init(_ f: @escaping (_UpdatedChildComponent, UIView, ComponentTransition) -> Void) {
+            self.f = f
+        }
+
+        public func callAsFunction(component: _UpdatedChildComponent, view: UIView, transition: ComponentTransition) {
+            self.f(component, view, transition)
+        }
+    }
+}
+
+public extension CombinedComponent {
+    func makeView() -> UIView {
+        return UIView()
+    }
+
+    func update(view: View, availableSize: CGSize, state: State, environment: Environment<EnvironmentType>, transition: ComponentTransition) -> CGSize {
+        let context = view.getCombinedComponentContext(Self.self)
+        
+        let storedBody: Body
+        if let current = context.body {
+            storedBody = current
+        } else {
+            storedBody = Self.body
+            context.body = storedBody
+        }
+
+        let viewContext = view.context(component: self)
+
+        var nextChildIndex = 0
+        var addedChildIds = Set<_AnyChildComponent.Id>()
+
+        let contextStack = _AnyCombinedComponentContext.push(context)
+
+        let escapeStatus: EscapeGuard.Status
+        let size: CGSize
+        do {
+            let bodyContext = CombinedComponentContext<Self>(
+                context: viewContext,
+                view: view,
+                component: self,
+                availableSize: availableSize,
+                transition: transition,
+                add: { updatedChild in
+                    if !addedChildIds.insert(updatedChild.id).inserted {
+                        preconditionFailure("Child component can only be added once")
+                    }
+
+                    let index = nextChildIndex
+                    nextChildIndex += 1
+
+                    if let previousView = context.childViews[updatedChild.id] {
+                        precondition(updatedChild.view === previousView.view)
+
+                        if index != previousView.index {
+                            assert(index < previousView.index)
+                            for i in index ..< previousView.index {
+                                if let moveView = context.childViews[context.childViewIndices[i]] {
+                                    moveView.index += 1
+                                }
+                            }
+                            context.childViewIndices.remove(at: previousView.index)
+                            context.childViewIndices.insert(updatedChild.id, at: index)
+                            previousView.index = index
+                            view.insertSubview(previousView.view, at: index)
+                        }
+
+                        previousView.updateGestures(updatedChild.gestures)
+                        previousView.transition = updatedChild.transitionDisappear
+                        previousView.transitionWithGuide = updatedChild.transitionDisappearWithGuide
+
+                        (updatedChild.transitionUpdate ?? ComponentTransition.Update.default)(component: updatedChild, view: updatedChild.view, transition: transition)
+                    } else {
+                        for i in index ..< context.childViewIndices.count {
+                            if let moveView = context.childViews[context.childViewIndices[i]] {
+                                moveView.index += 1
+                            }
+                        }
+
+                        context.childViewIndices.insert(updatedChild.id, at: index)
+                        let childView = _AnyCombinedComponentContext.ChildView(view: updatedChild.view, index: index)
+                        context.childViews[updatedChild.id] = childView
+
+                        childView.updateGestures(updatedChild.gestures)
+                        childView.transition = updatedChild.transitionDisappear
+                        childView.transitionWithGuide = updatedChild.transitionDisappearWithGuide
+
+                        view.insertSubview(updatedChild.view, at: index)
+
+                        updatedChild.view.layer.anchorPoint = updatedChild._anchorPoint ?? CGPoint(x: 0.5, y: 0.5)
+                        
+                        if let scale = updatedChild._scale {
+                            updatedChild.view.bounds = CGRect(origin: CGPoint(), size: updatedChild.size)
+                            updatedChild.view.center = updatedChild._position ?? CGPoint()
+                            updatedChild.view.transform = CGAffineTransform(scaleX: scale, y: scale)
+                        } else {
+                            if updatedChild.view is UIScrollView {
+                                updatedChild.view.frame = updatedChild.size.centered(around: updatedChild._position ?? CGPoint())
+                            } else {
+                                updatedChild.view.bounds = CGRect(origin: CGPoint(), size: updatedChild.size)
+                                if updatedChild.view.layer.anchorPoint != CGPoint(x: 0.5, y: 0.5) {
+                                    updatedChild.view.layer.position = updatedChild._position ?? CGPoint()
+                                } else {
+                                    updatedChild.view.center = updatedChild._position ?? CGPoint()
+                                }
+                            }
+                        }
+                        
+                        updatedChild.view.alpha = updatedChild._opacity ?? 1.0
+                        updatedChild.view.clipsToBounds = updatedChild._clipsToBounds ?? false
+                        updatedChild.view.layer.cornerRadius = updatedChild._cornerRadius ?? 0.0
+                        if let allowsGroupOpacity = updatedChild._allowsGroupOpacity {
+                            updatedChild.view.layer.allowsGroupOpacity = allowsGroupOpacity
+                        }
+                        if let shadow = updatedChild._shadow {
+                            updatedChild.view.layer.shadowColor = shadow.color.withAlphaComponent(1.0).cgColor
+                            updatedChild.view.layer.shadowRadius = shadow.radius
+                            updatedChild.view.layer.shadowOpacity = Float(shadow.color.alpha)
+                            updatedChild.view.layer.shadowOffset = shadow.offset
+                        } else {
+                            updatedChild.view.layer.shadowColor = nil
+                            updatedChild.view.layer.shadowRadius = 0.0
+                            updatedChild.view.layer.shadowOpacity = 0.0
+                        }
+                        updatedChild.view.context(typeErasedComponent: updatedChild.component).erasedState._updated = { [weak viewContext] transition, isLocal in
+                            guard let viewContext = viewContext else {
+                                return
+                            }
+                            viewContext.state.updated(transition: transition, isLocal: isLocal)
+                        }
+
+                        if let transitionAppearWithGuide = updatedChild.transitionAppearWithGuide {
+                            guard let guide = context.updateContext.configuredGuides[transitionAppearWithGuide.1] else {
+                                preconditionFailure("Guide should be configured before using")
+                            }
+                            transitionAppearWithGuide.0(
+                                component: updatedChild,
+                                view: updatedChild.view,
+                                guide: guide.previousPosition,
+                                transition: transition
+                            )
+                        } else if let transitionAppear = updatedChild.transitionAppear {
+                            transitionAppear(
+                                component: updatedChild,
+                                view: updatedChild.view,
+                                transition: transition
+                            )
+                        }
+                    }
+                }
+            )
+
+            escapeStatus = bodyContext.escapeGuard.status
+            size = storedBody(bodyContext)
+        }
+
+        assert(escapeStatus.isDeallocated, "Body context should not be stored for later use")
+
+        if nextChildIndex < context.childViewIndices.count {
+            for i in nextChildIndex ..< context.childViewIndices.count {
+                let id = context.childViewIndices[i]
+                if let childView = context.childViews.removeValue(forKey: id) {
+                    let view = childView.view
+                    let completion: () -> Void = { [weak context, weak view] in
+                        view?.removeFromSuperview()
+
+                        if let context = context {
+                            for i in 0 ..< context.disappearingChildViews.count {
+                                if context.disappearingChildViews[i].view === view {
+                                    context.disappearingChildViews.remove(at: i)
+                                    break
+                                }
+                            }
+                        }
+                    }
+                    if let transitionWithGuide = childView.transitionWithGuide {
+                        guard let guide = context.updateContext.configuredGuides[transitionWithGuide.1] else {
+                            preconditionFailure("Guide should be configured before using")
+                        }
+                        context.disappearingChildViews.append(_AnyCombinedComponentContext.DisappearingChildView(
+                            view: view,
+                            guideId: transitionWithGuide.1,
+                            transition: nil,
+                            transitionWithGuide: transitionWithGuide.0,
+                            completion: completion
+                        ))
+                        view.isUserInteractionEnabled = false
+                        transitionWithGuide.0(
+                            stage: .begin,
+                            view: view,
+                            guide: guide.position,
+                            transition: transition,
+                            completion: completion
+                        )
+                    } else if let simpleTransition = childView.transition {
+                        context.disappearingChildViews.append(_AnyCombinedComponentContext.DisappearingChildView(
+                            view: view,
+                            guideId: nil,
+                            transition: simpleTransition,
+                            transitionWithGuide: nil,
+                            completion: completion
+                        ))
+                        view.isUserInteractionEnabled = false
+                        simpleTransition(
+                            view: view,
+                            transition: transition,
+                            completion: completion
+                        )
+                    } else {
+                        childView.view.removeFromSuperview()
+                    }
+                }
+            }
+            context.childViewIndices.removeSubrange(nextChildIndex...)
+        }
+
+        if addedChildIds != context.updateContext.updatedViews {
+            preconditionFailure("Updated and added child lists do not match")
+        }
+
+        context.guides.removeAll()
+        for (id, guide) in context.updateContext.configuredGuides {
+            context.guides[id] = guide.position
+        }
+
+        _AnyCombinedComponentContext.pop(context, stack: contextStack)
+
+        return size
+    }
+}
+
+public extension CombinedComponent {
+    static func Child<Environment>(environment: Environment.Type) -> _EnvironmentChildComponent<Environment> {
+        return _EnvironmentChildComponent<Environment>()
+    }
+
+    static func ChildMap<Environment, Key: Hashable>(environment: Environment.Type, keyedBy keyType: Key.Type) -> _EnvironmentChildComponentMap<Environment, Key> {
+        return _EnvironmentChildComponentMap<Environment, Key>()
+    }
+
+    static func Child<ComponentType: Component>(_ type: ComponentType.Type) -> _ConcreteChildComponent<ComponentType> {
+        return _ConcreteChildComponent<ComponentType>()
+    }
+
+    static func Guide() -> _ChildComponentGuide {
+        return _ChildComponentGuide()
+    }
+    
+    static func StoredActionSlot<Arguments>(_ argumentsType: Arguments.Type) -> ActionSlot<Arguments> {
+        return ActionSlot<Arguments>()
+    }
+}
+
+public struct Shadow {
+    public let color: UIColor
+    public let radius: CGFloat
+    public let offset: CGSize
+    
+    public init(
+        color: UIColor,
+        radius: CGFloat,
+        offset: CGSize
+    ) {
+        self.color = color
+        self.radius = radius
+        self.offset = offset
+    }
+}

submodules/ComponentFlow/Source/Base/Component.swift

@@ -0,0 +1,221 @@
+import Foundation
+import UIKit
+import ObjectiveC
+
+public class ComponentLayoutResult {
+    var availableSize: CGSize?
+    var size: CGSize?
+}
+
+public protocol _TypeErasedComponentContext: AnyObject {
+    var erasedEnvironment: _Environment { get }
+    var erasedState: ComponentState { get }
+
+    var layoutResult: ComponentLayoutResult { get }
+}
+
+class AnyComponentContext<EnvironmentType>: _TypeErasedComponentContext {
+    var erasedComponent: AnyComponent<EnvironmentType> {
+        get {
+            preconditionFailure()
+        } set(value) {
+            preconditionFailure()
+        }
+    }
+    var erasedState: ComponentState {
+        preconditionFailure()
+    }
+    var erasedEnvironment: _Environment {
+        get {
+            return self.environment
+        } set(value) {
+            self.environment = value as! Environment<EnvironmentType>
+        }
+    }
+
+    let layoutResult: ComponentLayoutResult
+    var environment: Environment<EnvironmentType>
+
+    init(environment: Environment<EnvironmentType>) {
+        self.layoutResult = ComponentLayoutResult()
+        self.environment = environment
+    }
+}
+
+class ComponentContext<ComponentType: Component>: AnyComponentContext<ComponentType.EnvironmentType> {
+    override var erasedComponent: AnyComponent<ComponentType.EnvironmentType> {
+        get {
+            return AnyComponent(self.component)
+        } set(value) {
+            self.component = value.wrapped as! ComponentType
+        }
+    }
+
+    var component: ComponentType
+    let state: ComponentType.State
+
+    override var erasedState: ComponentState {
+        return self.state
+    }
+
+    init(component: ComponentType, environment: Environment<ComponentType.EnvironmentType>, state: ComponentType.State) {
+        self.component = component
+        self.state = state
+        
+        super.init(environment: environment)
+    }
+}
+
+private var UIView_TypeErasedComponentContextKey: Int?
+
+extension UIView {
+    func context<EnvironmentType>(component: AnyComponent<EnvironmentType>) -> AnyComponentContext<EnvironmentType> {
+        return self.context(typeErasedComponent: component) as! AnyComponentContext<EnvironmentType>
+    }
+
+    func context<ComponentType: Component>(component: ComponentType) -> ComponentContext<ComponentType> {
+        return self.context(typeErasedComponent: component) as! ComponentContext<ComponentType>
+    }
+
+    func context(typeErasedComponent component: _TypeErasedComponent) -> _TypeErasedComponentContext{
+        if let context = objc_getAssociatedObject(self, &UIView_TypeErasedComponentContextKey) as? _TypeErasedComponentContext {
+            return context
+        } else {
+            let context = component._makeContext()
+            objc_setAssociatedObject(self, &UIView_TypeErasedComponentContextKey, context, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
+            return context
+        }
+    }
+}
+
+open class ComponentState {
+    open var _updated: ((ComponentTransition, Bool) -> Void)?
+    var isUpdated: Bool = false
+    
+    public init() {
+    }
+    
+    public final func updated(transition: ComponentTransition = .immediate, isLocal: Bool = false) {
+        self.isUpdated = true
+        self._updated?(transition, isLocal)
+    }
+}
+
+public final class EmptyComponentState: ComponentState {
+}
+
+public protocol _TypeErasedComponent {
+    func _makeView() -> UIView
+    func _makeContext() -> _TypeErasedComponentContext
+    func _update(view: UIView, availableSize: CGSize, environment: Any, transition: ComponentTransition) -> CGSize
+    func _isEqual(to other: _TypeErasedComponent) -> Bool
+}
+
+public protocol ComponentTaggedView: UIView {
+    func matches(tag: Any) -> Bool
+}
+
+public final class GenericComponentViewTag {
+    public init() {
+    }
+}
+
+public protocol Component: _TypeErasedComponent, Equatable {
+    associatedtype EnvironmentType = Empty
+    associatedtype View: UIView = UIView
+    associatedtype State: ComponentState = EmptyComponentState
+    
+    func makeView() -> View
+    func makeState() -> State
+    func update(view: View, availableSize: CGSize, state: State, environment: Environment<EnvironmentType>, transition: ComponentTransition) -> CGSize
+}
+
+public extension Component {
+    func _makeView() -> UIView {
+        return self.makeView()
+    }
+
+    func _makeContext() -> _TypeErasedComponentContext {
+        return ComponentContext<Self>(component: self, environment: Environment<EnvironmentType>(), state: self.makeState())
+    }
+
+    func _update(view: UIView, availableSize: CGSize, environment: Any, transition: ComponentTransition) -> CGSize {
+        let view = view as! Self.View
+        
+        return self.update(view: view, availableSize: availableSize, state: view.context(component: self).state, environment: environment as! Environment<EnvironmentType>, transition: transition)
+    }
+
+    func _isEqual(to other: _TypeErasedComponent) -> Bool {
+        if let other = other as? Self {
+            return self == other
+        } else {
+            return false
+        }
+    }
+}
+
+public extension Component where Self.View == UIView {
+    func makeView() -> UIView {
+        return UIView()
+    }
+}
+
+public extension Component where Self.State == EmptyComponentState {
+    func makeState() -> State {
+        return EmptyComponentState()
+    }
+}
+
+public class ComponentGesture {
+    public static func tap(action: @escaping() -> Void) -> ComponentGesture {
+        preconditionFailure()
+    }
+}
+
+public class AnyComponent<EnvironmentType>: _TypeErasedComponent, Equatable {
+    public let wrapped: _TypeErasedComponent
+
+    public init<ComponentType: Component>(_ component: ComponentType) where ComponentType.EnvironmentType == EnvironmentType {
+        self.wrapped = component
+    }
+
+    public static func ==(lhs: AnyComponent<EnvironmentType>, rhs: AnyComponent<EnvironmentType>) -> Bool {
+        return lhs.wrapped._isEqual(to: rhs.wrapped)
+    }
+
+    public func _makeView() -> UIView {
+        return self.wrapped._makeView()
+    }
+
+    public func _makeContext() -> _TypeErasedComponentContext {
+        return self.wrapped._makeContext()
+    }
+
+    public func _update(view: UIView, availableSize: CGSize, environment: Any, transition: ComponentTransition) -> CGSize {
+        return self.wrapped._update(view: view, availableSize: availableSize, environment: environment as! Environment<EnvironmentType>, transition: transition)
+    }
+
+    public func _isEqual(to other: _TypeErasedComponent) -> Bool {
+        return self.wrapped._isEqual(to: other)
+    }
+}
+
+public final class AnyComponentWithIdentity<Environment>: Equatable {
+    public let id: AnyHashable
+    public let component: AnyComponent<Environment>
+
+    public init<IdType: Hashable>(id: IdType, component: AnyComponent<Environment>) {
+        self.id = AnyHashable(id)
+        self.component = component
+    }
+
+    public static func == (lhs: AnyComponentWithIdentity<Environment>, rhs: AnyComponentWithIdentity<Environment>) -> Bool {
+        if lhs.id != rhs.id {
+            return false
+        }
+        if lhs.component != rhs.component {
+            return false
+        }
+        return true
+    }
+}

submodules/ComponentFlow/Source/Base/Environment.swift

@@ -0,0 +1,219 @@
+import Foundation
+import UIKit
+
+public final class Empty: Equatable {
+    static let shared: Empty = Empty()
+
+    public static func ==(lhs: Empty, rhs: Empty) -> Bool {
+        return true
+    }
+}
+
+public class _Environment {
+    fileprivate var data: [Int: _EnvironmentValue] = [:]
+    var _isUpdated: Bool = false
+
+    func calculateIsUpdated() -> Bool {
+        if self._isUpdated {
+            return true
+        }
+        for (_, item) in self.data {
+            if let parentEnvironment = item.parentEnvironment, parentEnvironment.calculateIsUpdated() {
+                return true
+            }
+        }
+        return false
+    }
+
+    fileprivate func set<T: Equatable>(index: Int, value: EnvironmentValue<T>) {
+        if let current = self.data[index] {
+            self.data[index] = value
+            if current as! EnvironmentValue<T> != value {
+                self._isUpdated = true
+            }
+        } else {
+            self.data[index] = value
+            self._isUpdated = true
+        }
+    }
+}
+
+private enum EnvironmentValueStorage<T> {
+    case direct(T)
+    case reference(_Environment, Int)
+}
+
+public class _EnvironmentValue {
+    fileprivate let parentEnvironment: _Environment?
+
+    fileprivate init(parentEnvironment: _Environment?) {
+        self.parentEnvironment = parentEnvironment
+    }
+}
+
+@dynamicMemberLookup
+public final class EnvironmentValue<T: Equatable>: _EnvironmentValue, Equatable {
+    private var storage: EnvironmentValueStorage<T>
+
+    public var value: T {
+        switch self.storage {
+        case let .direct(value):
+            return value
+        case let .reference(environment, index):
+            return (environment.data[index] as! EnvironmentValue<T>).value
+        }
+    }
+
+    fileprivate init(_ value: T) {
+        self.storage = .direct(value)
+
+        super.init(parentEnvironment: nil)
+    }
+
+    fileprivate init(environment: _Environment, index: Int) {
+        self.storage = .reference(environment, index)
+
+        super.init(parentEnvironment: environment)
+    }
+
+    public static func ==(lhs: EnvironmentValue<T>, rhs: EnvironmentValue<T>) -> Bool {
+        if lhs === rhs {
+            return true
+        }
+        // TODO: follow the reference chain for faster equality checking
+        return lhs.value == rhs.value
+    }
+
+    public subscript<V>(dynamicMember keyPath: KeyPath<T, V>) -> V {
+        return self.value[keyPath: keyPath]
+    }
+}
+
+public class Environment<T>: _Environment {
+    private let file: StaticString
+    private let line: Int
+
+    public init(_ file: StaticString = #file, _ line: Int = #line) {
+        self.file = file
+        self.line = line
+    }
+}
+
+public extension Environment where T == Empty {
+    static let value: Environment<Empty> = {
+        let result = Environment<Empty>()
+        result.set(index: 0, value: EnvironmentValue(Empty()))
+        return result
+    }()
+}
+
+public extension Environment {
+    subscript(_ t1: T.Type) -> EnvironmentValue<T> where T: Equatable {
+        return EnvironmentValue(environment: self, index: 0)
+    }
+
+    subscript<T1, T2>(_ t1: T1.Type) -> EnvironmentValue<T1> where T == (T1, T2), T1: Equatable, T2: Equatable {
+        return EnvironmentValue(environment: self, index: 0)
+    }
+
+    subscript<T1, T2>(_ t2: T2.Type) -> EnvironmentValue<T2> where T == (T1, T2), T1: Equatable, T2: Equatable {
+        return EnvironmentValue(environment: self, index: 1)
+    }
+
+    subscript<T1, T2, T3>(_ t1: T1.Type) -> EnvironmentValue<T1> where T == (T1, T2, T3), T1: Equatable, T2: Equatable, T3: Equatable {
+        return EnvironmentValue(environment: self, index: 0)
+    }
+
+    subscript<T1, T2, T3>(_ t2: T2.Type) -> EnvironmentValue<T2> where T == (T1, T2, T3), T1: Equatable, T2: Equatable, T3: Equatable {
+        return EnvironmentValue(environment: self, index: 1)
+    }
+
+    subscript<T1, T2, T3>(_ t3: T3.Type) -> EnvironmentValue<T3> where T == (T1, T2, T3), T1: Equatable, T2: Equatable, T3: Equatable {
+        return EnvironmentValue(environment: self, index: 2)
+    }
+
+    subscript<T1, T2, T3, T4>(_ t1: T1.Type) -> EnvironmentValue<T1> where T == (T1, T2, T3, T4), T1: Equatable, T2: Equatable, T3: Equatable, T4: Equatable {
+        return EnvironmentValue(environment: self, index: 0)
+    }
+
+    subscript<T1, T2, T3, T4>(_ t2: T2.Type) -> EnvironmentValue<T2> where T == (T1, T2, T3, T4), T1: Equatable, T2: Equatable, T3: Equatable, T4: Equatable {
+        return EnvironmentValue(environment: self, index: 1)
+    }
+
+    subscript<T1, T2, T3, T4>(_ t3: T3.Type) -> EnvironmentValue<T3> where T == (T1, T2, T3, T4), T1: Equatable, T2: Equatable, T3: Equatable, T4: Equatable {
+        return EnvironmentValue(environment: self, index: 2)
+    }
+
+    subscript<T1, T2, T3, T4>(_ t4: T4.Type) -> EnvironmentValue<T4> where T == (T1, T2, T3, T4), T1: Equatable, T2: Equatable, T3: Equatable, T4: Equatable {
+        return EnvironmentValue(environment: self, index: 3)
+    }
+}
+
+@resultBuilder
+public struct EnvironmentBuilder {
+    static var _environment: _Environment?
+    private static func current<T>(_ type: T.Type) -> Environment<T> {
+        return self._environment as! Environment<T>
+    }
+
+    public struct Partial<T: Equatable> {
+        fileprivate var value: EnvironmentValue<T>
+    }
+
+    public static func buildBlock() -> Environment<Empty> {
+        let result = self.current(Empty.self)
+        result.set(index: 0, value: EnvironmentValue(Empty.shared))
+        return result
+    }
+
+    public static func buildExpression<T: Equatable>(_ expression: T) -> Partial<T> {
+        return Partial<T>(value: EnvironmentValue(expression))
+    }
+
+    public static func buildExpression<T: Equatable>(_ expression: EnvironmentValue<T>) -> Partial<T> {
+        return Partial<T>(value: EnvironmentValue(expression.value))
+    }
+
+    public static func buildBlock<T1: Equatable>(_ t1: Partial<T1>) -> Environment<T1> {
+        let result = self.current(T1.self)
+        result.set(index: 0, value: t1.value)
+        return result
+    }
+
+    public static func buildBlock<T1: Equatable, T2: Equatable>(_ t1: Partial<T1>, _ t2: Partial<T2>) -> Environment<(T1, T2)> {
+        let result = self.current((T1, T2).self)
+        result.set(index: 0, value: t1.value)
+        result.set(index: 1, value: t2.value)
+        return result
+    }
+
+    public static func buildBlock<T1: Equatable, T2: Equatable, T3: Equatable>(_ t1: Partial<T1>, _ t2: Partial<T2>, _ t3: Partial<T3>) -> Environment<(T1, T2, T3)> {
+        let result = self.current((T1, T2, T3).self)
+        result.set(index: 0, value: t1.value)
+        result.set(index: 1, value: t2.value)
+        result.set(index: 2, value: t3.value)
+        return result
+    }
+
+    public static func buildBlock<T1: Equatable, T2: Equatable, T3: Equatable, T4: Equatable>(_ t1: Partial<T1>, _ t2: Partial<T2>, _ t3: Partial<T3>, _ t4: Partial<T4>) -> Environment<(T1, T2, T3, T4)> {
+        let result = self.current((T1, T2, T3, T4).self)
+        result.set(index: 0, value: t1.value)
+        result.set(index: 1, value: t2.value)
+        result.set(index: 2, value: t3.value)
+        result.set(index: 3, value: t4.value)
+        return result
+    }
+}
+
+@propertyWrapper
+public struct ZeroEquatable<T>: Equatable {
+    public var wrappedValue: T
+
+    public init(_ wrappedValue: T) {
+        self.wrappedValue = wrappedValue
+    }
+
+    public static func ==(lhs: ZeroEquatable<T>, rhs: ZeroEquatable<T>) -> Bool {
+        return true
+    }
+}