SwiftUI @State var initialization issue

SwiftUI doesn't allow you to change @State in the initializer but you can initialize it.

Remove the default value and use _fullText to set @State directly instead of going through the property wrapper accessor.

@State var fullText: String // No default value of ""

init(letter: String) {
    _fullText = State(initialValue: list[letter]!)
}

The top answer is an anti-pattern that will cause pain down the road, when the dependency changes (letter) and your state will not update accordingly.

One should never use State(initialValue:) or State(wrappedValue:) to initialize state in a View's init. In fact, State should only be initialized inline, like so:

@State private var fullText: String = "The value"

If that's not feasible, use @Binding, @ObservedObject, a combination between @Binding and @State or even a custom DynamicProperty

More about this here.

It's not an issue nowadays to set a default value of the @State variables inside the init method. But you MUST just get rid of the default value which you gave to the state and it will work as desired:

,,,
    @State var fullText: String // 👈 No default value should be here

    init(letter: String) {
        self.fullText = list[letter]!
    }

    var body: some View {
        TextField("", text: $fullText)
    }
}

Working demo:

Depending on the case, you can initialize the State in different ways:

// With default value

@State var fullText: String = "XXX"

// Not optional value and without default value

@State var fullText: String

init(x: String) {
    fullText = x
}

// Optional value and without default value

@State var fullText: String

init(x: String) {
    _fullText = State(initialValue: x)
}

The answer of Bogdan Farca is right for this case but we can't say this is the solution for the asked question because I found there is the issue with the Textfield in the asked question. Still we can use the init for the same code So look into the below code it shows the exact solution for asked question.

struct StateFromOutside: View {
    let list = [
        "a": "Letter A",
        "b": "Letter B",
        // ...
    ]
    @State var fullText: String = ""

    init(letter: String) {
        self.fullText = list[letter]!
    }

    var body: some View {
        VStack {
            Text("\(self.fullText)")
            TextField("Enter some text", text: $fullText)
        }
    }
}

And use this by simply calling inside your view

struct ContentView: View {
    var body: some View {
        StateFromOutside(letter: "a")
    }
}

You can create a view model and initiate the same as well :

 class LetterViewModel: ObservableObject {

     var fullText: String
     let listTemp = [
         "a": "Letter A",
         "b": "Letter B",
         // ...
     ]

     init(initialLetter: String) {
         fullText = listTemp[initialLetter] ?? ""
     }
 }

 struct LetterView: View {

     @State var viewmodel: LetterViewModel

     var body: some View {
    
         TextField("Enter text", text: $viewmodel.fullText)
     }
 }

And then call the view like this:

 struct ContentView: View {

     var body: some View {

           LetterView(viewmodel: LetterViewModel(initialLetter: "a"))
     }
 }

By this you would also not have to call the State instantiate method.

To initialize a @State variable in SwiftUI through the init method of a struct, you need to understand that @State properties are managed by SwiftUI and cannot be directly set within the initializer. Instead, you can use a workaround by utilizing a separate property for the initial value and then assigning it to the @State variable within the body of the view. Here is how you can achieve this:

struct StateFromOutside: View {
    let list = [
        "a": "Letter A",
        "b": "Letter B",
        // ...
    ]
    
    // Private property to hold the initial value
    private var initialText: String
    
    // @State variable to be used in the view
    @State private var fullText: String = ""
    
    init(letter: String) {
        self.initialText = list[letter] ?? ""
    }
    
    var body: some View {
        // Assign initial value to @State variable if it's not already set
        VStack {
            TextField("Enter text", text: $fullText)
                .onAppear {
                    if self.fullText.isEmpty {
                        self.fullText = self.initialText
                    }
                }
        }
    }
}

Explanation Private Property for Initial Value: A private property initialText is added to hold the initial value passed through the initializer. Setting the @State Variable: In the body of the view, use the onAppear modifier to set the @State variable fullText to initialText only if it hasn't been set already. This ensures that fullText gets its initial value when the view appears.