Add @bindable for two-way bindings into observable models

Author: colemancda

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 }
+        )
+    }
+}