What state am I modifying during view update? - ios

Question

I have a CoreData object Day which I am editing within this view, when in "editing" mode the card allows for a Picker view and then a TextField to allow user input. These all work as expected, however when I tap "done" which runs the saveEditToDay() function which is just saving those into the Day variable. This crashes with Thread 1: EXC_BREAKPOINT (code=1, subcode=0x1cffc77e0) and gives the hint Modifying state during view update, this will cause undefined behavior.. I am not clear what state I modifying in this case?
Here is the view:
import SwiftUI
struct EditCard: View {
#Binding var day: Day?
var timeOfDay: Time
#State var isEditing: Bool = false
#State var selectedRating = 0
#State var description = ""
var ratings = Rating.getAllRatings()
// MARK: - Body
var body: some View {
VStack(alignment: .leading) {
HStack {
Text(timeOfDay.rawValue).font(.headline)
Spacer()
Button(action: {
if self.isEditing { self.saveEditToDay() }
self.isEditing.toggle()
}, label: { self.isEditing ? Text("Done") : Text("Edit") })
.foregroundColor(Color("PrimaryColour"))
}
Divider()
VStack(alignment: .leading) {
Text("RATING")
.font(.caption)
.foregroundColor(Color(UIColor.systemGray))
if isEditing {
Picker(selection: $selectedRating, label: Text("")) {
ForEach(0 ..< ratings.count) {
Text(self.ratings[$0]).tag([$0])
}
}
} else {
Text(day!.getRating(for: timeOfDay))
}
}.padding(.trailing)
VStack(alignment: .leading) {
Text("REASON")
.font(.caption)
.foregroundColor(Color(UIColor.systemGray))
if isEditing {
TextField(description, text: $description)
} else {
Text(description)
.multilineTextAlignment(.leading)
.lineLimit(nil)
}
}
}
.padding()
.background(Color(UIColor.secondarySystemGroupedBackground))
.cornerRadius(10)
.onAppear { self.setSelected() }
.onDisappear{ self.saveEditToDay(); print("Toodles") }
}
// MARK: - Functions
func setSelected() {
guard let day = day else { return }
self.selectedRating = ratings.firstIndex(of: day.getRating(for: timeOfDay))!
self.description = day.getDescription(for: timeOfDay)
}
func saveEditToDay() {
guard let day = day else { return }
day.setRating(rating: ratings[selectedRating], for: timeOfDay)
day.setDescription(description: description, for: timeOfDay)
}
}
Here at the Extensions to the NSManagedObject that i'm calling:
func setRating(rating: String, for time: Time) {
switch time {
case .morning: self.morning = rating;
case .afternoon: self.afternoon = rating;
case .evening: self.evening = rating;
}
}
func setDescription(description: String, for time: Time) {
switch time {
case .morning: self.morningDescription = description;
case .afternoon: self.afternoonDescription = description;
case .evening: self.eveningDescription = description;
}
}

Answer 1

You are calling setSelected() in onApear(), which modifies two of your variables marked as #State: selectedRating and description. This means that as the view gets drawn or redrawn you are changing the variables that define what is being drawn halfway through the process.
onApear() is a good place to trigger a background refresh, but not modify any #State variables instantly.
You probably should write an initialiser that takes binding to a Day and Time sets the other two variables just like you do in setSelected().