如何在 Swift 的绑定中解开可选值？

Question

I'm building an app using SwiftUI and would like a way to convert a Binding<Value?> to a Binding<Value >.

In my app I have an AvatarView which knows how to render an image for a particular user.

struct AvatarView: View {
  @Binding var userData: UserData

  ...
}

My app holds a ContentView that owns two bindings: a dictionary of users by id, and the id of the user whose avatar we should be showing.

struct ContentView: View {
  @State var userById: Dictionary<Int, UserData>
  @State var activeUserId: Int

  var body: some View {
    AvatarView(userData: $userById[activeUserId])
  }
}

Problem: the above code doesn't combine because $userById[activeUserId] is of type Binding<UserData?> and AvatarView takes in a Binding<UserData> .

Things I tried...

• $userById[activeUserId]! doesn't work because it's trying to unwrap a Binding<UserData?> . You can only unwrap an Optional , not a Binding<Optional> .

• $(userById[activeUserId]!) doesn't work for reasons that I don't yet understand, but I think something about $ is resolved at compile time so you can't seem to prefix arbitrary expressions with $ .

Answer 1

You can use this initialiser , which seems to handle this exact case - converting Binding<T?> to Binding<T>? :

var body: some View {
    AvatarView(userData: Binding($userById[activeUserId])!)
}

I have used ! to force unwrap, just like in your attempts, but you could unwrap the nil however you want. The expression Binding($userById[activeUserId]) is of type Binding<UserData>? .