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SOLVED: Saving list item selection on main view change

Forums > SwiftUI

In my MacOS app MainView a define a NavigationView for my Students and Scores CoreData items as follows:

struct MainView: View {

var dataTypes = ["Students", "Repertoire"]
var dataIcons = ["person.2.fill", "music.note.list"]

  @State private var selectedData = 0

  var body: some View {

    NavigationView {
      List(0..<2, selection: $selectedData) { selection in
        Label(dataTypes[selection], systemImage: dataIcons[selection])
      }

      switch selectedData {
      case 0: StudentsView()
      case 1: WorksView()
      default: StudentsView()
      }

My StudentsView, for example, is as follows:

struct StudentsView: View {
   var body: some View {
      FilteredStudentsView()

And its correspondant FilteredStudentsView:

struct FilteredStudentsView: View {
@FetchRequest var studentsFetchRequest: FetchedResults<Student>

   var body: some View {
      List(studentsFetchRequest, id: \.id) { student in
          NavigationLink(destination: StudentDetailView(student: student)) {
            StudentCellView(student: student)
          }

As you can see, a typical navigationView-List outline.

I'd like to keep selected item on StudentsView when I change to Works on MainView, and back again (and viceversa). Now doing so I find an empty detailed view until I select –again– one item from the list.

I've tried with @Bindings and so, but haven´t found the correct way to make it work.

Any suggestions? Thanks you all in advance.

2      

Hi,

You should be able to do something like i did below, hopefully it works for you.

struct FilteredStudentsView: View {
    @AppStorage("lastSelection") var lastSelection: Int = 0
    @State var students = [Student(), Student(), Student(), Student(), Student()]
    @State var selection: UUID?

    var body: some View {
        List(students, id: \.id, selection: $selection) { student in
            NavigationLink(destination: StudentDetailView(student: student)) {
                VStack {
                    Text("\(student.id)")
                    Text(student.name)
                }

            }
            onChange(of: selection) { newValue in
                lastSelection = students.firstIndex(where: {$0.id == newValue}) ?? students.count
            }
            .onAppear {
                if lastSelection != students.count {
                    selection = students[lastSelection].id
                }
            }
        }
    }
}

2      

Thank you! Two comments about this.

Solution suggested above by @Hectorcrdna really doesn't. It throws no error at all, but doesn't save/reveal last selected student after changing to another view.

I tryed a very similar solution as suggested above by @Hectorcrdna, but with uuidString property os Students and .first property of Students array. This works! and both saves last selected item, and shows it when coming back to the view:

struct FilteredStudentsView: View {

@AppStorage("selectedStudentId") var selectedStudentId = ""
@State private var selectedStudent: Student?

  var body: some View {

  List(students, id: \.self, selection: $selectedStudent) { student in
          NavigationLink(destination: StudentDetailView(student: student)) {
            StudentCellView(student: student)
          }

          .onChange(of: selectedStudent, perform: { selection in
            selectedStudentId = selection?.id?.uuidString ?? ""
          })

          .onAppear{
            if studentsFetchRequest.first != nil {
              selectedStudent = students.first { $0.id?.uuidString == selectedStudentId } // <-- Throws an error
            }
          }

BUT now line marked throws an error on console, on run time, and app doesn't get displayed:

Ignoring exception: Unable to install constraint on view. Does the constraint reference something from outside the subtree of the view? That's illegal.

I supposse it's due to "forcing" the "instalation on the view" of that "constraint". That is: selecting the Student which holds that ID. I know the sentence "Does the constraint reference something from outside the subtree of the view?" is the clue to solve this issue, but I can't figure out exactly how.

Any further suggestions? Thanks again!

2      

Where is students declared? I can't see it in this view and perhaps to this the error message is referring to.

2      

Students is not really an array. It's:

@FetchRequest var studentsFetchRequest: FetchedResults<Student>

Declared on this same view (I've modified references to clarify). This is due to the necesitty of initialiting my request each time with filterings:

init(query: String, marked: Bool, orderedBy: String) {
    let queryPredicate = NSPredicate(format: "(name CONTAINS[cd] %@) || (relatedInstrument.name CONTAINS[cd] %@) || (relatedTutor.name CONTAINS[cd] %@)", query, query, query)
    let markedPredicate = NSPredicate(format: "marked = %@", marked as NSNumber)
    let compoundPredicate = NSCompoundPredicate(type: .and, subpredicates: [queryPredicate, markedPredicate])

    let sortDescriptor: NSSortDescriptor
    switch orderedBy {
    case "student":
      sortDescriptor = NSSortDescriptor(key: "name", ascending: true)
    case "instrument":
      sortDescriptor = NSSortDescriptor(key: "relatedInstrument.name", ascending: true)
    case "course":
      sortDescriptor = NSSortDescriptor(key: "relatedCourse.level", ascending: true)
    case "tutor":
      sortDescriptor = NSSortDescriptor(key: "relatedTutor.name", ascending: true)
    default:
      sortDescriptor = NSSortDescriptor(key: "name", ascending: true)
    }

    switch marked {
    case true:
      if query != "" {
        _studentsFetchRequest = FetchRequest<Student>(sortDescriptors: [sortDescriptor], predicate: compoundPredicate)
      } else {
        _studentsFetchRequest = FetchRequest<Student>(sortDescriptors: [sortDescriptor], predicate: markedPredicate)
      }
    case false:
      if query != "" {
        _studentsFetchRequest = FetchRequest<Student>(sortDescriptors:[sortDescriptor], predicate: queryPredicate)
      } else {
        _studentsFetchRequest = FetchRequest<Student>(sortDescriptors: [sortDescriptor])
      }
    }
  }

Could it interfere with this? In fact, previously selected student could not be been displayed at all due to filterings change... mmm...

2      

Solved! :-)

Just transforming fetchRequest to an array solved the problem:

selectedStudent = Array(studentsFetchRequest).first { $0.id?.uuidString == selectedStudentId }

Now everything working ok!

2      

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