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?

Perhaps this discussion of State and ObservedObject can help aid your understanding of what they mean, and how to use them. — Sean Skelly
– Sean Skelly, Commented Apr 1, 2020 at 22:58

score 3 · Accepted Answer · 2020-04-02 00:16:19Z

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.

How are underscores "_" being used in this case? I haven't seen them be used like that before. Thanks!

Mamoon Yameen · Accepted Answer · 2023-02-28 12:20:17Z

This also happened when you are building the app for old OS iPhone for those Apple is not giving updates because things becomes deprecated. use for example

if #available(iOS 16.0, *) { } else { // Fallback on earlier versions }

on each single line where this warning is being shown. Note: Don't use it in the start of the body of class or struct use it where its needed.

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