Property wrapper trouble

Using Swift
1

I'm trying to make a property wrapper for accessing the string value of a control owned by a view controller. So far I have this:

@propertyWrapper struct ControlStringValue<Container> where Container: AnyObject
{
  private let container: Container
  private let keyPath: ReferenceWritableKeyPath<Container, NSControl?>
  
  var wrappedValue: String
  {
    get { return container[keyPath: keyPath]?.stringValue ?? "" }
    set { container[keyPath: keyPath]?.stringValue = newValue }
  }
}

class MyViewController: NSViewController
{
  @IBOutlet var someControl: NSControl!
  
  @ControlStringValue(container: self, keyPath: \MyViewController.someControl) var text: String
}

But I get this error:

error: Sandbox.playground:20:53: error: cannot convert value of type 'ReferenceWritableKeyPath<MyViewController, NSControl?>' to expected argument type 'ReferenceWritableKeyPath<_, NSControl?>'
@ControlStringValue(container: self, keyPath: \MyViewController.someControl) var text: String

So it seems like it's not fully inferring the generic type. What else can I do to nudge the compiler?

An alternative would be to store the control in the wrapper later, once the view controller is fully initialized, but that doesn't seem as clean.

2

There are two issues:

  1. The synthesised init is private
  2. You're passing self in a property initializer

You're getting a bogus error because of the first issue. If you remove container, you'll get the actual error:

'ControlStringValue' initializer is inaccessible due to 'private' protection level

Once you've fixed that, I think there's two ways to fix (2):

  1. You can use the _enclosingInstance static subscript, but keep in mind it's a prototype and can change behaviour or stop working as it's not official yet.

  2. You can assign self after the view loads.

Here's how you can do (2):

@propertyWrapper
struct ControlStringValue<Container: AnyObject> {
  private var container: Container?
  private let keyPath: ReferenceWritableKeyPath<Container, NSControl?>

  init(keyPath: ReferenceWritableKeyPath<Container, NSControl?>) {
    self.keyPath = keyPath
  }

  var wrappedValue: String {
    get { return container?[keyPath: keyPath]?.stringValue ?? "" }
    set { container?[keyPath: keyPath]?.stringValue = newValue }
  }

  var projectedValue: Container? {
	get { container }
	set { container = newValue }
  }
}

final class MyViewController: NSViewController {
  @IBOutlet var someControl: NSControl!
  @ControlStringValue(keyPath: \MyViewController.someControl) var text: String

  override func viewDidLoad() {
	super.viewDidLoad()
	$text = self
  }
}
3

Thanks! I think I'll go for the second option until enclosingInstance is official.

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