@Bindable for observable models

AndroidSwiftUICore/Sources/AndroidSwiftUICore/Bindable.swift

@@ -0,0 +1,39 @@
+//
+//  Bindable.swift
+//  AndroidSwiftUICore
+//
+//  `@Bindable` wraps a reference to an `@Observable` model and projects a
+//  two-way `Binding` into any of its properties via `$model.property`. Because
+//  it wraps an existing reference (from `@State`, `@Environment`, or an init
+//  parameter), it needs no state storage of its own — writes go straight
+//  through the reference, and the model's observation drives re-evaluation.
+//
+
+@propertyWrapper
+@dynamicMemberLookup
+public struct Bindable<Value: AnyObject> {
+
+    public var wrappedValue: Value
+
+    public init(wrappedValue: Value) {
+        self.wrappedValue = wrappedValue
+    }
+
+    public init(_ wrappedValue: Value) {
+        self.wrappedValue = wrappedValue
+    }
+
+    public var projectedValue: Bindable<Value> { self }
+
+    /// Projects a binding to a property of the wrapped reference. A reference
+    /// keypath means the set writes through the object without reassigning it.
+    public subscript<Subject>(
+        dynamicMember keyPath: ReferenceWritableKeyPath<Value, Subject>
+    ) -> Binding<Subject> {
+        let object = wrappedValue
+        return Binding<Subject>(
+            get: { object[keyPath: keyPath] },
+            set: { object[keyPath: keyPath] = $0 }
+        )
+    }
+}

AndroidSwiftUICore/Tests/AndroidSwiftUICoreTests/ControlTests.swift

@@ -9,7 +9,7 @@ import Testing
 #if canImport(Observation)
 import Observation
 
-@Observable final class ObservableModel { var counter = 0 }
+@Observable final class ObservableModel { var counter = 0; var name = "" }
 #endif
 
 @Suite("Controls and environment")
@@ -109,5 +109,33 @@ struct ControlTests {
         node = host.evaluate()
         #expect(node.props["text"] == .string("count 1"))
     }
+
+    @Test("@Bindable projects a two-way binding into an observable model")
+    func bindableProjection() {
+        let model = ObservableModel()
+        @Bindable var bound = model
+        let binding = $bound.name
+        binding.wrappedValue = "hi"
+        #expect(model.name == "hi")
+        #expect(binding.wrappedValue == "hi")
+    }
+
+    @Test("A TextField bound via @Bindable writes back to the model")
+    func bindableTextField() {
+        struct Screen: View {
+            @Bindable var model: ObservableModel
+            var body: some View { TextField("Name", text: $model.name) }
+        }
+        let model = ObservableModel()
+        let host = ViewHost(Screen(model: model))
+        var node = host.evaluate()
+        #expect(node.props["text"] == .string(""))
+        if case .int(let id)? = node.props["onChange"] {
+            host.callbacks.invokeString(Int64(id), "Coleman")
+        }
+        #expect(model.name == "Coleman")
+        node = host.evaluate()
+        #expect(node.props["text"] == .string("Coleman"))
+    }
     #endif
 }

Demo/App.swiftpm/Sources/BindablePlaygrounds.swift

@@ -0,0 +1,55 @@
+#if canImport(AndroidSwiftUI)
+import AndroidSwiftUI
+#else
+import SwiftUI
+#endif
+
+import Observation
+
+@Observable
+final class ProfileModel {
+    var name = ""
+    var subscribed = false
+    var volume = 0.5
+}
+
+struct BindablePlayground: View {
+    @State private var model = ProfileModel()
+    var body: some View {
+        ScrollView {
+            BindableForm(model: model)
+        }
+    }
+}
+
+/// `@Bindable` takes the model by reference and projects `$model.property`
+/// bindings into it; writing a bound control mutates the observable, which
+/// re-evaluates the views that read it.
+struct BindableForm: View {
+    @Bindable var model: ProfileModel
+    var body: some View {
+        VStack(alignment: .leading, spacing: 0) {
+            Example("Bound TextField") {
+                VStack(alignment: .leading, spacing: 8) {
+                    TextField("Name", text: $model.name)
+                    Text(model.name.isEmpty ? "No name yet" : "Hello, \(model.name)")
+                }
+            }
+            Example("Bound Toggle") {
+                VStack(alignment: .leading, spacing: 8) {
+                    Toggle("Subscribed", isOn: $model.subscribed)
+                    Text(model.subscribed ? "Subscribed" : "Not subscribed")
+                }
+            }
+            Example("Bound Slider") {
+                VStack(alignment: .leading, spacing: 8) {
+                    Slider(value: $model.volume, in: 0...1)
+                    Text("Volume: \(Int(model.volume * 100))%")
+                }
+            }
+            Example("Shared model") {
+                Text("All three controls above bind into one @Observable model.")
+            }
+        }
+    }
+}

Demo/App.swiftpm/Sources/Catalog.swift

@@ -46,5 +46,6 @@ struct CatalogEntry: Identifiable {
         CatalogEntry(id: "state", title: "State", screen: AnyCatalogScreen(StatePlayground())),
         CatalogEntry(id: "environment", title: "Environment", screen: AnyCatalogScreen(EnvironmentPlayground())),
         CatalogEntry(id: "observable", title: "Observable", screen: AnyCatalogScreen(ObservablePlayground())),
+        CatalogEntry(id: "bindable", title: "Bindable", screen: AnyCatalogScreen(BindablePlayground())),
     ]
 }