Add @ObservedObject implementation (#171)

This pulls a fork of OpenCombine that can be compiled with the same SwiftWasm snapshot we use in `main`.

The only caveat is that this doesn't work for `ObservableObject`s that are subclasses of other classes with `@Published` properties. This issue needs to be fixed in [the Runtime library fork](https://github.com/MaxDesiatov/Runtime). Since this is a rare case, and fixing it wouldn't change this `@ObservedObject` implementation, I think it's ready for review as is.

Package.resolved

@@ -10,12 +10,21 @@
           "version": null
         }
       },
+      {
+        "package": "OpenCombine",
+        "repositoryURL": "https://github.com/MaxDesiatov/OpenCombine.git",
+        "state": {
+          "branch": "observable-object",
+          "revision": "3c3a181acad7ab44a64d7c41140eb843222bb2aa",
+          "version": null
+        }
+      },
       {
         "package": "Runtime",
         "repositoryURL": "https://github.com/MaxDesiatov/Runtime.git",
         "state": {
           "branch": "wasi-build",
-          "revision": "a617ead8a125a97e69d6100e4d27922006e82e0a",
+          "revision": "a9309b4822d6dd0e4a8e92351ee9e3d210e19b4e",
           "version": null
         }
       }

Package.swift

@@ -26,6 +26,7 @@ let package = Package(
     // .package(url: /* package url */, from: "1.0.0"),
     .package(url: "https://github.com/kateinoigakukun/JavaScriptKit.git", .revision("47f2bb1")),
     .package(url: "https://github.com/MaxDesiatov/Runtime.git", .branch("wasi-build")),
+    .package(url: "https://github.com/MaxDesiatov/OpenCombine.git", .branch("observable-object")),
   ],
   targets: [
     // Targets are the basic building blocks of a package. A target can define
@@ -34,7 +35,7 @@ let package = Package(
     // in packages which this package depends on.
     .target(
       name: "TokamakCore",
-      dependencies: ["Runtime"]
+      dependencies: ["OpenCombine", "Runtime"]
     ),
     .target(
       name: "TokamakDemo",

Sources/TokamakCore/MountedViews/MountedCompositeView.swift

@@ -15,6 +15,7 @@
 //  Created by Max Desiatov on 03/12/2018.
 //
 
+import OpenCombine
 import Runtime
 
 final class MountedCompositeView<R: Renderer>: MountedView<R>, Hashable {
@@ -31,6 +32,7 @@ final class MountedCompositeView<R: Renderer>: MountedView<R>, Hashable {
   private let parentTarget: R.TargetType
 
   var state = [Any]()
+  var subscriptions = [AnyCancellable]()
   var environmentValues: EnvironmentValues
 
   init(_ view: AnyView, _ parentTarget: R.TargetType,

Sources/TokamakCore/ObservedObject.swift

@@ -0,0 +1,53 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import OpenCombine
+
+public typealias ObservableObject = OpenCombine.ObservableObject
+public typealias Published = OpenCombine.Published
+
+protocol ObservedProperty {
+  var objectWillChange: AnyPublisher<(), Never> { get }
+}
+
+@propertyWrapper
+public struct ObservedObject<ObjectType>: ObservedProperty where ObjectType: ObservableObject {
+  @dynamicMemberLookup
+  public struct Wrapper {
+    let root: ObjectType
+    public subscript<Subject>(
+      dynamicMember keyPath: ReferenceWritableKeyPath<ObjectType, Subject>
+    ) -> Binding<Subject> {
+      .init(
+        get: {
+          self.root[keyPath: keyPath]
+        }, set: {
+          self.root[keyPath: keyPath] = $0
+        }
+      )
+    }
+  }
+
+  public var wrappedValue: ObjectType { projectedValue.root }
+
+  public init(wrappedValue: ObjectType) {
+    projectedValue = Wrapper(root: wrappedValue)
+  }
+
+  public let projectedValue: Wrapper
+
+  var objectWillChange: AnyPublisher<(), Never> {
+    wrappedValue.objectWillChange.map { _ in }.eraseToAnyPublisher()
+  }
+}

Sources/TokamakCore/StackReconciler.swift

@@ -40,17 +40,20 @@ public final class StackReconciler<R: Renderer> {
     rootView.mount(with: self)
   }
 
-  func queueUpdate(
+  private func queueStateUpdate(
     for mountedView: MountedCompositeView<R>,
     id: Int,
     updater: (inout Any) -> ()
   ) {
-    let scheduleReconcile = queuedRerenders.isEmpty
-
     updater(&mountedView.state[id])
+    queueUpdate(for: mountedView)
+  }
+
+  private func queueUpdate(for mountedView: MountedCompositeView<R>) {
+    let shouldSchedule = queuedRerenders.isEmpty
     queuedRerenders.insert(mountedView)
 
-    guard scheduleReconcile else { return }
+    guard shouldSchedule else { return }
 
     scheduler { [weak self] in self?.updateStateAndReconcile() }
   }
@@ -63,30 +66,58 @@ public final class StackReconciler<R: Renderer> {
     queuedRerenders.removeAll()
   }
 
-  func render(compositeView: MountedCompositeView<R>) -> some View {
+  private func setupState(
+    id: Int,
+    for property: PropertyInfo,
+    of compositeView: MountedCompositeView<R>
+  ) {
     // swiftlint:disable force_try
-    let info = try! typeInfo(of: compositeView.view.type)
-    let stateProperties = info.properties.filter { $0.type is ValueStorage.Type }
-
-    for (id, stateProperty) in stateProperties.enumerated() {
-      // `ValueStorage` properties were already filtered out, so safe to assume the value's type
-      // swiftlint:disable:next force_cast
-      var state = try! stateProperty.get(from: compositeView.view.view) as! ValueStorage
+    // `ValueStorage` property already filtered out, so safe to assume the value's type
+    // swiftlint:disable:next force_cast
+    var state = try! property.get(from: compositeView.view.view) as! ValueStorage
 
-      if compositeView.state.count == id {
-        compositeView.state.append(state.anyInitialValue)
-      }
+    if compositeView.state.count == id {
+      compositeView.state.append(state.anyInitialValue)
+    }
 
+    if state.getter == nil || state.setter == nil {
       state.getter = { compositeView.state[id] }
 
       // Avoiding an indirect reference cycle here: this closure can be
       // owned by callbacks owned by view's target, which is strongly referenced
       // by the reconciler.
       state.setter = { [weak self, weak compositeView] newValue in
         guard let view = compositeView else { return }
-        self?.queueUpdate(for: view, id: id) { $0 = newValue }
+        self?.queueStateUpdate(for: view, id: id) { $0 = newValue }
+      }
+    }
+    try! property.set(value: state, on: &compositeView.view.view)
+  }
+
+  private func setupSubscription(
+    for property: PropertyInfo,
+    of compositeView: MountedCompositeView<R>
+  ) {
+    // `ObservedProperty` property already filtered out, so safe to assume the value's type
+    // swiftlint:disable:next force_cast
+    let observed = try! property.get(from: compositeView.view.view) as! ObservedProperty
+
+    observed.objectWillChange.sink { [weak self] _ in
+      self?.queueUpdate(for: compositeView)
+    }.store(in: &compositeView.subscriptions)
+  }
+
+  func render(compositeView: MountedCompositeView<R>) -> some View {
+    let info = try! typeInfo(of: compositeView.view.type)
+
+    let needsSubscriptions = compositeView.subscriptions.isEmpty
+
+    for (id, property) in info.properties.enumerated() {
+      if property.type is ValueStorage.Type {
+        setupState(id: id, for: property, of: compositeView)
+      } else if needsSubscriptions && property.type is ObservedProperty.Type {
+        setupSubscription(for: property, of: compositeView)
       }
-      try! stateProperty.set(value: state, on: &compositeView.view.view)
     }
 
     let result = compositeView.view.bodyClosure(compositeView.view.view)

Sources/TokamakDOM/Views/HTML.swift

@@ -24,6 +24,9 @@ public typealias View = TokamakCore.View
 public typealias AnyView = TokamakCore.AnyView
 public typealias EmptyView = TokamakCore.EmptyView
 public typealias State = TokamakCore.State
+public typealias ObservableObject = TokamakCore.ObservableObject
+public typealias Published = TokamakCore.Published
+public typealias ObservedObject = TokamakCore.ObservedObject
 
 protocol AnyHTML {
   var innerHTML: String? { get }

Sources/TokamakDemo/Counter.swift

@@ -21,16 +21,22 @@ import SwiftUI
 import TokamakDOM
 #endif
 
-public struct Counter: View {
-  @State public var count: Int
+final class Count: ObservableObject {
+  @Published var value: Int
+
+  init(value: Int) { self.value = value }
+}
+
+struct Counter: View {
+  @ObservedObject var count: Count
 
   let limit: Int
 
   public var body: some View {
-    if count < limit {
+    if count.value < limit {
       VStack {
-        Button("Increment") { count += 1 }
-        Text("\(count)")
+        Button("Increment") { count.value += 1 }
+        Text("\(count.value)")
       }
       .onAppear { print("Counter.VStack onAppear") }
       .onDisappear { print("Counter.VStack onDisappear") }

Sources/TokamakDemo/TokamakDemo.swift

@@ -29,7 +29,7 @@ struct TokamakDemoView: View {
       }
       VStack {
         Group {
-          Counter(count: 5, limit: 15)
+          Counter(count: Count(value: 5), limit: 15)
             .padding()
             .background(Color(red: 0.9, green: 0.9, blue: 0.9, opacity: 1.0))
             .border(Color.red, width: 3)