SwiftUI load data on push view OR reset on pop view - ios

Question

Swift 5.2
I have a NavigationView, for a tree of Views joined by NavigationLinks. (No weirdness going on; no cycles or hopping between different parts of the tree - just views linking to subviews linking to subsubviews.) One of them is a configuration screen. I desire that when you arrive on the config screen (aka ConfigView), the configuration is loaded from disk. In subviews of ConfigView, you can modify different settings, and on ConfigView you can save the settings. However, I desire that if you pop ConfigView (leave, towards the root of the tree), your unsaved changes are discarded.
One of my initial thoughts was to load the changes in the ConfigView initializer. This doesn't work, because ConfigView() is called in the parent view, so exiting the ConfigView and returning to ConfigView keeps the same ConfigView between views (and thus the same dirty data). (...Or, the ConfigView is recreated multiple times without actually leaving it, causing the data to be erroneously reloaded. I tried to test the conditions under which it did either, just now, but ran into a crash I'm not going to fix today. Regardless, I tested it earlier today and it demonstrated one or both of the above behaviors.)
Perhaps load it in the body block? But that gets re-run e.g. when you return from a subview link, which would erroneously reload the data.
Perhaps load it in .onAppear? But that ALSO erroneously reloads the data when you return from a subview.
I considered perhaps going with the dual of my original intent, disposing of the dirty data when you pop the ConfigView (and reloading it whenever requested), but I haven't found any hooks I can use to be notified when e.g. the user hits the "back" button.
How do I get a clean copy of the data to be present on new entry to the ConfigView, and not lost until the ConfigView is popped?

Answer 1

You should have a persistence data somewhere in your app, and when passing data to a child view create a draft data (ObservableObject). Then, save the updated data to the persistence data whenever needed. This way, the child will only play with the draft data and your parent view will only show the persistence data.
There's a good example here: https://developer.apple.com/tutorials/swiftui/working-with-ui-controls
Please have a look at all of the tutorials made by Apple, they are very short and powerful samples.
I will paste the code from the link above to here just in case the code is lost sometime in the future.
struct ProfileHost: View {
#Environment(\.editMode) var mode
#EnvironmentObject var userData: UserData
#State var draftProfile = Profile.default
var body: some View {
VStack(alignment: .leading, spacing: 20) {
HStack {
if self.mode?.wrappedValue == .active {
Button("Cancel") {
self.draftProfile = self.userData.profile
self.mode?.animation().wrappedValue = .inactive
}
}
Spacer()
EditButton()
}
if self.mode?.wrappedValue == .inactive {
ProfileSummary(profile: userData.profile)
} else {
ProfileEditor(profile: $draftProfile)
.onAppear {
self.draftProfile = self.userData.profile
}
.onDisappear {
self.userData.profile = self.draftProfile
}
}
}
.padding()
}
}