Use property wrappers in templates

[djbe]

Needed some help from @IanKeen to get it working, we got to this:

protocol AssetLoadable {
  associatedtype Asset
  static func loadAsset(named name: String) -> Asset
}
@propertyWrapper
struct AssetCatalogEntry<T: AssetLoadable> where T == T.Asset {
  let name: String
  var wrappedValue: T.Asset {
    T.loadAsset(named: name)
  }
  var projectedValue: AssetCatalogEntry<T> {
    self
  }
}
extension NSImage: AssetLoadable {
  static func loadAsset(named name: String) -> NSImage {
    guard let result = Bundle.main.image(forResource: name) else {
      preconditionFailure("Unable to load image asset '\(name)'")
    }
    return result
  }
}
extension NSDataAsset: AssetLoadable {
  static func loadAsset(named name: String) -> NSDataAsset {
    guard let result = NSDataAsset(name: name, bundle: .main) else {
      preconditionFailure("Unable to load data asset '\(name)'")
    }
    return result
  }
}

enum Asset {
  @AssetCatalogEntry(name: "Foo") static var foo: NSImage
  @AssetCatalogEntry(name: "Bar") static var bar: NSDataAsset
}

Users can then access an asset directly using Asset.foo, and if they want other properties such as the name, they can still get it using Asset.$foo.name.

First question

Do we add a new swift5.1 template, or somehow add it to the existing swift5 templates based on swift version? It would be a breaking change for all users, so probably best to go with swift5.1?

Second question

To which parsers/templates do we add these?

• Colors: swift5, not the literals template
• Fonts: NO as a font still needs a size. We can use a closure as a wrappedValue, but it'd be (CGFloat) -> Font, where the parameter is unclear.
• IB: maybe for storyboard scenes? Instead of using instantiate().
• XCAssets: will need some tweaks for color caching and other perf. related things we do now in the template

---

Once we decide on these topics, I'll open a dedicated issue for each template to discuss the implementation, before creating a PR.

References #603 (inconvenient access to assets) and #605 (PR for that issue).