Support binds against `@Published` properties

[helje5]

This is a funny side effect when using the inline view accessor like so:

class CowsOverview: ViewController {
  @Published var search = ""
  var view: some View {
    TextField("Search it", $search) // <= doesn't work
  }

This does work when using the ViewController via the environment, e.g.:

class CowsOverview: ViewController {
  @Published var search = ""
  struct ContentView: View {
    @EnvironmentObject var viewController : CowsOverview
    TextField("Search it", $viewController.search)
  }
}

This is because the $ accesses the projectedValue of the property wrapper - @Published in the first case, and @EnvironmentObject in the second. And while @EnvironmentObject gives you a Binding (and then that goes on traversing keypathes), the @Published gives you the Combine Publisher for the property.

There are a few ways to solve this, either using a trampoline providing the Bindings, maybe like:

var `$`: Trampoline<Self, ... key path stuff>

or maybe just adding a .bind() to @Published, like:

extension Published {
  var bind : Binding<Value> { ... }
}

Maybe someone has additional ideas. Generally it is not a huge deal, as one often ends up w/ real view structs anyways.

[helje5]

BTW:

  var `$`     : String { "Hello" } // works
  var `$self` : String { "Hello" } // errors out

[helje5]

The feature/bindable-published branch has an implementation of a BindablePublished property wrapper, which seems to work OK. What doesn't work OK is aliasing it to @State from within the framework:

class Page: ViewController {
  #if true // works
    @BindablePublished var title = "Hello"
  #elseif true // works
    typealias State = BindablePublished
    @State var title = "Hello"
  #elseif false // fails, if alias is in ViewController protocol
    @State var title = "Hello"
  #endif

  var view: some View {
    TextField("Title", $title)
  }
}

[helje5]

This is another, less recognisable, variant:

TextField("Title", bind.title)

extension ViewController {
  
  var bind : Binder<Self> { Binder(self) }

  // Works, but also requires backticks on the callsite
  var `$` : Binder<Self> { Binder(self) }
}

@dynamicMemberLookup
struct Binder<O: ObservableObject> {
  
  let instance : O
  
  init(_ instance: O) { self.instance = instance }
  
  public subscript<Subject>(dynamicMember keyPath: ReferenceWritableKeyPath<O, Subject>)
         -> Binding<Subject>
  {
    Binding<Subject>(
      get: { instance[keyPath:keyPath] },
      set: { value in instance[keyPath: keyPath] = value }
    )
  }
}