SwiftUI – A note about onAppear()

This morning I made a custom version of a picker for the events form. I needed a way for events to select a different timeline. The default picker in SwiftUI had some limitations so I set out to make my own. The only main difference is that it uses a sectioned list with timelines sorted into non-archived and archived sections. I made a binding variable to pass in the selected timeline. This allowed me to update the selected timeline from the picker view, then just close it when done.

struct TimelinePicker: View {

    @Binding var selectedTimeline: Timeline?
    @ObservedObject var dataSource = RADDataSource<Timeline>()
    @Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>

    var body : some View {
        List {

            ForEach(dataSource.loadSectionedDataSource(sort: self.getSort(), predicate: nil, sectionKeyPathName: "isArchived"), id: \.name) { listSection in

                Section(header:

                    HStack {
                        Text(listSection.name == "1" ? "Archived Timelines" : "Timelines")
                    }
                    .font(.system(.headline, design: .rounded))

                ) {
                     ForEach(self.dataSource.objects(inSection: listSection)) { timeline in
                        
                        Button(action: ({
                            self.selectedTimeline = timeline
                            self.presentationMode.wrappedValue.dismiss()
                        })) {
                            
                            HStack {
                                TimelineListRowView(timeline: timeline)
                                .foregroundColor(.primary)
                                Spacer()
                                if(self.selectedTimeline == timeline) {
                                    Image(systemName: "checkmark")
                                }
                            }
                            
                        }
                    }
                }
            }
        }
    }

    public func getSort() -> [NSSortDescriptor] {
        ...
    }
}

It just would not work though. I tried several ways of passing in the timeline, binding, observed object, environment object, etc. Nothing would work. Every time I closed the picker the initial value remained set on the events form.

Turns out that’s because I’m an idiot. I was using a call to onAppear to load data from the event record into a View Model class. This was running every time I closed the timeline picker. I would set the initial value, go to the picker, select a new value, and then close the picker which would call onAppear to set the initial value again.

The fix for this was super simple. I just added a new state variable called onAppearCalled and initialized it to false. Then I could check this condition in the function that populates my form when onAppear is called, setting it to true when it is called.

func populateOnAppear() {
    if(self.onAppearCalled == false) {
        ...
        self.onAppearCalled = true
    }
}