DispatchQueue triggering foreach loop twice with SwiftUI - swift

Question

I'm having a weird behavior with my code.
There is View with text that is getting its text values from the User object.
If I take the reference out and leave it with just String everything is good, but with it the foreach loop that is getting this value runs twice.
Also, if I put the assignment out from DispatchQueue it runs once but the view isn't updating itself (because its a #Published var).
Even if I remove #Published it still run twice.
I'm sure there is a simple explanation for this behavior but I really don't know what could make this happen.
Better explanation in code with comments:
The view:
struct MenuGroupBoxList: View {
#EnvironmentObject var session: SessionStore
var columns = [
GridItem(.adaptive(minimum: 100, maximum: 200), spacing: 8),
GridItem(.adaptive(minimum: 100, maximum: 200), spacing: 8)
]
var body: some View {
LazyVGrid(columns: columns, spacing: 8) {
ForEach(MenuTitles.allCases, id:\.self) { item in
if item == .sentShifts{
GroupColumn(menuItem: item, unit: Text(session.user?.totalShiftConsts.description ?? "0"))
}else if item == .weekdays{
//Deleting this line: session.user?.totalShifts.description. will run the loop once.
GroupColumn(menuItem: item, unit: Text(session.user?.totalShifts.description ?? "0"))
}else{
GroupColumn(menuItem: item, unit: Text("0"))
}
}
}.background(Color(UIColor.systemGroupedBackground))
}
}
Where the foreach loop:
enum ChooseWeekdays{
case current, next
}
class WeekdaysModel: Identifiable, ObservableObject{
private var session = SessionStore.shared
private var date = Date()
#Published var week = [Day]()
#Published var isFetching = false
init(_ weekdays: ChooseWeekdays){
switch weekdays {
case .current:
currentWeekdaysFetch()
case .next:
nextWeekdaysFetch()
}
}
private func currentWeekdaysFetch(){
DispatchQueue.main.async {
self.isFetching = true
var count = 0
for (index, symbol) in self.date.weekDaySymbols.enumerated(){
let day = Day(date: self.date.currentWeek[index], isSelected: false, hasShifts: false, daySymbol: symbol, dayNumber: self.date.currentWeekNumbers[index])
day.setShifts(morning: self.session.employeesList, middle: self.session.employeesList, evening: self.session.employeesList)
if index == Date().getCurrentDayIndex(){
day.isSelected = true
}
count = count + day.shifts.compactMap({$0.employees.contains(where: {$0.uid == self.session.user?.uid})}).count
self.week.append(day)
self.isFetching = false
//Printing to see if loop run twice.
print("Index:", index)
}
//OR - take this line out from DispatchQueue, will run once but wouldn't update the UI because totalShifts is a #Published var
self.session.user?.totalShifts = count
}
}
}
The results I'm getting in print is Index: 0...6 twice.
Where WeekdaysModel created:
struct WeekdaysView: View {
#ObservedObject var viewModel = WeekdaysModel(.current)
var body: some View {
ScrollView{
WeekRow(model: viewModel)
ForEach(viewModel.week){ day in
ShiftTypesView(day: day)
}
}
.navigationTitle(LocalizedStringKey("Weekdays"))
.background(Color(UIColor.systemGroupedBackground))
.edgesIgnoringSafeArea(.bottom)
}
}
There is another initiation in a different file but different enum (WeekdaysModel(.next))

Answer 1

seems a common case on SwiftUI.
I did experiment similar behaviour filling cells of a tableview being called twice for every cell: it seems swiftUI fills every viewed elements twice (I assume to satisfy rotation)
As suggested fill a data structure ONCE out of swiftUI "building of views"
(your data will be read anyway twice, but no double expensive calls to DB or network)
This behaviour is very different from std UiKit and can be frustrating if network is involved (calls will be issued twice... double network completions, errors on servers (for example double login and so on...)