BindableObservableObject.swift

import SwiftUI
import Combine

typealias BindableObservableObject<T> = T where T: ObservableObject, T.ObjectWillChangePublisher == ObservableObjectPublisher

/// A type that exposes its writable properties as bindings.
extension ObservableObject where Self == BindableObservableObject<Self> {
    /// A type that acts as a lens that exposes bindings to the
    /// writable properties of this type.
    typealias Bindings = ExposedBindings<Self>

    /// A lens that exposes bindings to the writable properties
    /// of this instance.
    @MainActor
    var bindings: Bindings {
        Bindings(base: self)
    }
}

/// A type that acts as a lens that exposes bindings to the
/// writable properties of a base object.
@dynamicMemberLookup
struct ExposedBindings<Base> where Base == BindableObservableObject<Base> {
    /// The base object whose bindings are exposed.
    fileprivate let base: Base

    /// Returns a binding to the property at the given key path.
    subscript<Value>(dynamicMember keyPath: ReferenceWritableKeyPath<Base, Value>) -> Binding<Value> {
        Binding {
            base[keyPath: keyPath]
        } set: {
            base[keyPath: keyPath] = $0
            base.objectWillChange.send()
        }
    }

    /// Returns a lens that exposes the bindings of the object
    /// at the given key path.
    subscript<T>(dynamicMember keyPath: KeyPath<Base, T>) -> ExposedBindings<T> where T == BindableObservableObject<T> {
        ExposedBindings<T>(base: base[keyPath: keyPath])
    }
}