SwiftUI: can I add on-init code to a view without overriding the default initializer?

I'm a very experienced programmer, but I'm completely new to Swift and SwiftUI. (I took a few of the intro courses in the Playgrounds app yesterday.)

The app I'm trying to write takes several length measurements as input, which is nontrivial, so I figured I should encapsulate that into its own view. Using [this answer][1] as inspiration, here's what I have so far:

struct LengthMeasurementField: View {
    var placeholder : String
    @Binding var binding : Measurement<UnitLength>
    @State var textValue = ""
    
    var body: some View {
        TextField(placeholder, text: $textValue)
            .fixedSize()
            .textFieldStyle(.roundedBorder)
            .onChange(of: textValue) { val in
            if let newValue = Double(val) {
                binding = Measurement(value: newValue, unit: UnitLength.inches)
            }
        }
    }
}

Which I then use as follows:

HStack {
    Text("Diameter")
    LengthMeasurementField(placeholderText: "Diameter", binding: $inputs.diameter)
}

This renders, but if inputs.diameter already has a value when the view is first rendered, that value is ignored -- the UI renders with just the placeholder text. This is because textValue is initialized as empty string and only updated via onChange.

I'd like to have textValue initialize with the string-formatted value from the binding. I tried this:

init() {
    textValue = String(format: "%f", binding.value)
}

But, of course, that eliminates the default initializer, so now my usage breaks. I then tried implementing the default initializer explicitly, but it doesn't compile, I think something special is happening because of the @Binding:

    init(placeholder: String, binding: Measurement<UnitLength>) {
        self.placeholder = placeholder
        self.binding = binding // Variable 'self.binding' used before being initialized
        self.textValue = String(format: "%f", binding.value) // Variable 'self.binding' used before being initialized
    } // Return from initializer without initializing all stored properties

This raises a few questions:

1. Why doesn't it compile? How can I fix it?
2. Is there a way for me to provide some on-init code for my view while still having access to the default initializer?

Side note: I know that I could do all this much more easily if I didn't use Measurement, and just had a field with a formatted Double in it. I plan to make the unit selectable via a dropdown, and won't be hardcoding it to .inches in the long term, so I think my current approach is probably going to be worthwhile. If you know otherwise, or if there's some built-in thing that already does this somewhere, I'd appreciate any advice you might have!

Your overriding of the synthesized initializer is the correct approach. You've just defined the wrong parameter types:

init(placeholder: String, binding: Binding<Measurement<UnitLength>>) {
                                   ^^^^^^^

You can get Xcode to write this for you by right-clicking the struct name, and selecting "Show Code Actions> Generate Memberwise Initializer."

To answer question 2 which kind of makes question irrelevant, you can use the view modifier .onAppear to set up textValue correctly in LengthMeasurementField

.onAppear {
    textValue = binding.formatted()
}