Good SwiftUI/Combine?

Sample.swift

struct ContentView: View {
    @State private var input = ""
    private var formatter: NumberFormatter

    init () {
        formatter = NumberFormatter()
        formatter.numberStyle = .decimal
    }

    private func transform(string input: String) -> String {
        let value = 3 * (formatter.number(from: input)?.doubleValue ?? 0)
        return formatter.string(from: NSNumber(value: value)) ?? ""
    }

    var body: some View {
        VStack(alignment: .leading) {
            HStack {
                Text("Input: ")
                TextField("Type here", text: self.$input)
            }
            HStack {
                Text("Output:")
                Text(self.transform(string: self.input))
            }
        }.padding()
    }
}