Add shared storage to property wrappers

Evolution Pitches
71

I am sorry if I missed this (I haven't had a chance to read through all the posts above, so maybe this is out of scope or unsupported) but I was wondering if it would be possible to assign a shared value to all instances declared in a given type? For example, I might have a @Preference property wrapper and I want to inject an instance of some type that conforms to my PreferencesType protocol into all instances of @Preference in a given type. Right now, you need to inject it into each property wrapper manually, something like:

struct S {
  @Preference(key: "some_key_1") var key1: Bool
  @Preference(key: "some_key_2") var key2: Int
  @Preference(key: "some_key_3") var key3: String
  
  init(defaults: PreferencesType = UserDefaults.shared) {
    self._key1 = .init(defaults: defaults)
    self._key2 = .init(defaults: defaults)
    self._key3 = .init(defaults: defaults)
  }
  
  // Alternatively
  init(defaults: PreferencesType = UserDefaults.shared) {
    self.defaults = defaults
    configureWrappers()
  }

  private func configureWrappers() {
    $key1.defaults = defaults
    $key2.defaults = defaults
    $key3.defaults = defaults
  }
}

It would be nice if you could do something like this instead:

// Assuming I have declared @shared var defaults: UserDefaultsProtocol? in my property wrapper
init(defaults: UserDefaultsProtocol = UserDefaults.shared) {
   self.$preferencesShared.defaults = defaults
}

Effectively, I want to inject a value (provided to me via an initializer) into all instances of the property wrapper within a given type (and not globally i.e. into every instance throughout the app) to avoid having to write a lot of boilerplate.