How to delay setting value of #State variable in SwiftUI - swift

Question

I want to create an application with a date picker. The initial value of the date picker (startDate) requires some computation performed in another class. (Let's say for example that this date is initially set to a customer's enrolment date in a service, which is retrieved from a database.) The user can leave that date as-is, or select a new date using the date picker.
I haven't been able to find a way of setting this initial value. The following simplified code illustrates the issue. The date picker shows the initial value of today's date rather than the desired date (here, Date.distantPast.) The date picker otherwise works fine.
Things I've tried:
If I set the value in the init clause (as shown below), it is ignored (no error reported, but the value is not written to startDate variable).
Trying to set the value of startDate inline with its declaration gives a compile-time error message ("Cannot use instance member 'db' within property initializer; property initializers run before 'self' is available".)
Wrapping the DataPicker in an if-statement so it won't not be displayed until the startDate has been computed makes no difference.
Is there any way to defer the setting of the initial date selection in a date picker until it can be computed?
import SwiftUI
struct ContentView: View {
private let db = DB()
#State private var startDate = Date()
var body: some View {
DatePicker("Start Date",
selection: $startDate,
displayedComponents: [.date])
}
init() {
startDate = db.theDate
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
class DB {
var theDate: Date
init() {
theDate = Date.distantPast
}
}

Answer 1

You can create a ViewModel to match your view. In the ViewModel you have more possibilities. For example, you can easily initialise date in the Init.
This would look something like this:
class ContentViewModel: ObservableObject {
private let db = DB()
#Published var date: Date
init() {
date = db.theDate
}
}
struct ContentView: View {
#ObservedObject var viewModel = ContentViewModel()
var body: some View {
DatePicker("Start Date",
selection: $viewModel.date,
displayedComponents: [.date])
}
}
Make sure that the ViewModel is an ObservableObject and that properties that can change use the property wrapper #Published.
If your app no longer needs to support iOS 13, you can also use #StateObject instead of #ObservedObject.