Why does this SwiftUI Picker code not work?

Question

Xcode: 11.2.1

The code below is for a simple form with two components; a Picker (to select a letter) and a Text (to display the selected letter). The code compiles and runs but, when a letter is selected it does not appear in the "Selected..." text. In addition, Xcode displays a (spurious?) run time warning the first time (only) a letter is selected..

struct ContentView: View {

    @State private var letter = ""
    private let letters = ["Alpha", "Bravo", "Charlie"]

    var body: some View {
        NavigationView {
            Form {
                Picker("Select a letter", selection: $letter) {
                    ForEach(0..<letters.count) {
                        Text(self.letters[$0])
                    }
                }
                Text("Selected letter: \(letter)")
            }
            .navigationBarTitle("Main Menu")
        }
    }
}

For info: the run time warning:

[TableView] Warning once only: UITableView was told to layout its visible cells and other contents without being in the view hierarchy (the table view or one of its superviews has not been added to a window). This may cause bugs by forcing views inside the table view to load and perform layout without accurate information (e.g. table view bounds, trait collection, layout margins, safe area insets, etc), and will also cause unnecessary performance overhead due to extra layout passes. Make a symbolic breakpoint at UITableViewAlertForLayoutOutsideViewHierarchy to catch this in the debugger and see what caused this to occur, so you can avoid this action altogether if possible, or defer it until the table view has been added to a window. Table view: <_TtC7SwiftUIP33_BFB370BA5F1BADDC9D83021565761A4925UpdateCoalescingTableView: 0x7f8b33872600; baseClass = UITableView; frame = (0 0; 414 896); clipsToBounds = YES; autoresize = W+H; gestureRecognizers = ; layer = ; contentOffset: {0, -140}; contentSize: {414, 205.33333333333337}; adjustedContentInset: {140, 0, 34, 0}; dataSource: <_TtGC7SwiftUIP10$10f5cd23419ListCoreCoordinatorGVS_20SystemListDataSourceOs5Never_GOS_19SelectionManagerBoxS2___: 0x7f8b33407a70>>

Martijn Pieters · Accepted Answer · 2022-08-23 14:31:52Z

5

ForEach types inside Picker should be aligned with selection type.

Here is a corrected code that should work for you:

@State private var letter = ""
private let letters = ["Alpha", "Bravo", "Charlie"]

var body: some View {
    NavigationView {
        Form {
            Picker("Select a letter", selection: $letter) {
                ForEach(letters, id: \.self) { option in
                    Text(option)
                }
            }
            Text("Selected letter: \(letter)")
        }
        .navigationBarTitle("Main Menu")
    }
}

edited Aug 23, 2022 at 14:31

Martijn Pieters

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answered Nov 15, 2019 at 16:43

Asperi

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1 Comment

Vince O'Sullivan Over a year ago

Thanks, that worked. However, I'm still getting the runtime warning. My guess is that it is being generated because the text field is being updated even though it is not visible while the Picker is displayed.

Vince O'Sullivan · Accepted Answer · 2019-11-17 09:01:21Z

1

Just for the record, I'm answering my own question based on the accepted answer above and my increased knowledge as a result of that answer. In the question I was not selecting a letter from the array but the index of the letter in the array. So, based on the insight, an alternative working solution might be:

struct ContentView: View {

    @State private var letterIndex = 0
    private let letters = ["Alpha", "Bravo", "Charlie"]

    var body: some View {
        NavigationView {
            Form {
                Picker("Select a letter", selection: $letterIndex) {
                    ForEach(0..<letters.count) { index in
                        Text(self.letters[index])
                    }
                }
                Text("Selected letter: \(letters[letterIndex])")
        }
        .navigationBarTitle("Main Menu")
    }
}

answered Nov 17, 2019 at 9:01

Vince O'Sullivan

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1 Comment

user3069232 Over a year ago

So this version doesn't get the warning? Its a question?

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