swiftui_helper_property_wrapper.swift

import Combine
import SwiftUI

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
@propertyWrapper
public struct Model<Value>: DynamicProperty {
  final class _Box: ObservableObject {
    let objectWillChange = ObservableObjectPublisher()

    var value: Value {
      willSet {
        objectWillChange.send()
      }
    }

    init(value: Value) {
      self.value = value
    }
  }

  let _subscription: AnyCancellable?

  @ObservedObject
  var _box: _Box

  public init(wrappedValue value: Value) {
    self._subscription = .none
    self._box = _Box(value: value)
  }

  public init<T>(
    publisher: T,
    defaultValue: Value
  ) where T: Publisher, T.Output == Value, T.Failure == Never {
    let box = _Box(value: defaultValue)
    self._subscription = publisher.assign(to: \.value, on: box)
    self._box = box
  }

  public var wrappedValue: Value {
    get {
      _box.value
    }
    nonmutating set {
      _box.value = newValue
    }
  }

  public var projectedValue: Binding<Value> {
    $_box.value
  }

  public mutating func update() {
    __box.update()
  }
}

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
extension Model where Value: ExpressibleByNilLiteral {
  public init() {
    self.init(wrappedValue: nil)
  }

  public init<T>(
    publisher: T
  ) where T: Publisher, T.Output == Value, T.Failure == Never {
    self.init(publisher: publisher, defaultValue: nil)
  }
}