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 {
  private final class _Box: ObservableObject {
    let objectWillChange = ObservableObjectPublisher()

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

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

  @ObservedObject
  private var _box: _Box

  public init(wrappedValue value: Value) {
    // Substituted by the compiler with:
    // `self.__box = ObservedObject(wrappedValue: _Box(value: value))`
    self._box = _Box(value: value)
  }

  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._box = _Box(value: nil)
  }
}

@sindresorhus just noticed that you forked my gist. :) Check out the updated version, it contains a very cool upgrade for reactive streams.