How do I use an existing property in a property wrapper when self hasn't been initialized? (SwiftUI)

Question

I have a struct with two variables inside property wrappers. One of the variables is supposed to be computed from the other. When I try to do this, I get the following error:

Cannot use instance member 'name' within property initializer; property initializers run before 'self' is available.

I tried assigning a temporary value to these variables, and then re-assigning them within a custom init() function, but that doesn't seem to work ether. I made a simplified version of the code to see if I could isolate the issue.

import SwiftUI

struct Person {
    @State var name: String = ""
    @State var nameTag: NameTag = NameTag(words: "")
    
    init(name: String) {
        // not changing name and nameTag
        self.name = name
        nameTag = NameTag(words: "Hi, my name is \(name).")
    }
}


class NameTag {
    var words: String
    
    init(words: String) {
        self.words = words
    }
}


var me = Person(name: "Myself")
// still set to initial values
me.name
me.nameTag.words

I noticed that when I changed nameTag to an @ObservedObject, rather than @State, it was able to be re-assigned correctly. Although I don't believe I can change name to @ObservedObject. Could anyone tell me what I'm doing wrong?

Answer 1

To use property wrappers in initializers, you use the variable names with preceding underscores.

And with State, you use init(initialValue:).

struct Person {
  @State var name: String
  @State var nameTag: NameTag

  init(name: String) {
    _name = .init(initialValue: name)
    _nameTag = .init( initialValue: .init(words: name) )
  }
}

Here's what a @State property really looks like, as your tear down levels of syntactic sugar:

name
_name.wrappedValue
$name.wrappedValue
_name.projectedValue.wrappedValue

You can't use the underscore-name outside of the initial type definition.