Using Combine to parse phone number String - swift

Question

I'm trying to wrap my mind around how Combine works. I believe I'm doing something wrong when I use the .assign operator to mutate the #Published property I'm operating on. I've read the documentation on Publishers, Subscribers, and Operators. But I'm a bit loose on where exactly to create the Publisher if I don't want it to be a function call.
import SwiftUI
import Combine
struct PhoneNumberField: View {
let title: String
#ObservedObject var viewModel = ViewModel()
var body: some View {
TextField(title,text: $viewModel.text)
}
class ViewModel: ObservableObject {
#Published var text: String = ""
private var disposables = Set<AnyCancellable>()
init() {
$text.map { value -> String in
self.formattedNumber(number: value)
}
//something wrong here
.assign(to: \.text, on: self)
.store(in: &disposables)
}
func formattedNumber(number: String) -> String {
let cleanPhoneNumber = number.components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
let mask = "+X (XXX) XXX-XXXX"
var result = ""
var index = cleanPhoneNumber.startIndex
for ch in mask where index < cleanPhoneNumber.endIndex {
if ch == "X" {
result.append(cleanPhoneNumber[index])
index = cleanPhoneNumber.index(after: index)
} else {
result.append(ch)
}
}
return result
}
}
}
struct PhoneNumberParser_Previews: PreviewProvider {
static var previews: some View {
PhoneNumberField(title: "Phone Number")
}
}

Answer 1

Use .receive(on:):
$text.map { self.formattedNumber(number: $0) }
.receive(on: DispatchQueue.main)
.sink(receiveValue: { [weak self] value in
self?.text = value
})
.store(in: &disposables)
This will allow you to listen to changes of the text variable and update it in the main queue. Using main queue is necessary if you want to update #Published variables read by some View.
And to avoid having a retain cycle (self -> disposables -> assign -> self) use sink with a weak self.