Convert a #State into a Publisher - swift

Question

I want to use a #State variable both for the UI and for computing a value.
For example, let's say I have a TextField bound to #State var userInputURL: String = "https://". How would I take that userInputURL and connect it to a publisher so I can map it into a URL.
Pseudo code:
$userInputURL.publisher()
.compactMap({ URL(string: $0) })
.flatMap({ URLSession(configuration: .ephemeral).dataTaskPublisher(for: $0).assertNoFailure() })
.eraseToAnyPublisher()

Answer 1

You can't convert #state to publisher, but you can use ObservableObject instead.
import SwiftUI
final class SearchStore: ObservableObject {
#Published var query: String = ""
func fetch() {
$query
.map { URL(string: $0) }
.flatMap { URLSession.shared.dataTaskPublisher(for: $0) }
.sink { print($0) }
}
}
struct ContentView: View {
#StateObject var store = SearchStore()
var body: some View {
VStack {
TextField("type something...", text: $store.query)
Button("search") {
self.store.fetch()
}
}
}
}

Answer 2

You can also use onChange(of:) to respond to #State changes.
struct MyView: View {
#State var userInputURL: String = "https://"
var body: some View {
VStack {
TextField("search here", text: $userInputURL)
}
.onChange(of: userInputURL) { _ in
self.fetch()
}
}
func fetch() {
print("changed", userInputURL)
// ...
}
}
Output:
changed https://t
changed https://ts
changed https://tsr
changed https://tsrs
changed https://tsrst

Answer 3

The latest beta has changed how variables are published so I don't think that you even want to try. Making ObservableObject classes is pretty easy but you then want to add a publisher for your own use:
class ObservableString: Combine.ObservableObject, Identifiable {
let id = UUID()
let objectWillChange = ObservableObjectPublisher()
let publisher = PassthroughSubject<String, Never>()
var string: String {
willSet { objectWillChange.send() }
didSet { publisher.send(string) }
}
init(_ string: String = "") { self.string = string }
}
Instead of #State variables you use #ObservableObject and remember to access the property string directly rather than use the magic that #State uses.

Answer 4

After iOS 14.0, you can access to Publisher.
struct MyView: View {
#State var text: String?
var body: some View {
Text(text ?? "")
.onReceive($text.wrappedValue.publisher) { _ in
let publisher1: Optional<String>.Publisher = $text.wrappedValue.publisher
// ... or
let publisher2: Optional<String>.Publisher = _text.wrappedValue.publisher
}
}
}