SwiftUI callback when (de)selecting an item

Question

I have the following List:

List(selection: self.$selectionKeeper) {
    ForEach(self.items, id: \.self) { name in
        Text(name)
    }
}
.environment(\.editMode, .constant(.active))

How can I get a callback each time an item is (de)selected? I know I can listen for changes in selectionKeeper, but I thought there might be a better way

Answer 1

Yes, the easiest way is to use onChange (or onReceive in iOS 13) to monitor changes to selectionKeeper:

List(selection: self.$selectionKeeper) {
    ForEach(0..<5, id: \.self) { item in
        Text("\(item)")
    }
}
.environment(\.editMode, .constant(.active))
.onChange(of: selectionKeeper) { selectedItems in
    print(selectedItems)
}

If you want to do it in another way, you can create a custom binding:

struct ContentView: View {
    @State private var selectionKeeper = Set<Int>()
    
    var body: some View {
        List(selection: .init(
            get: {
                selectionKeeper
            },
            set: {
                selectionKeeper = $0
                selectionCallback($0)
            }
        )) {
            ForEach(0..<5, id: \.self) { item in
                Text("\(item)")
            }
        }
        .environment(\.editMode, .constant(.active))
    }
    
    func selectionCallback(_ selectedItems: Set<Int>) {
        print(selectedItems)
    }
}

You can also extract the binding as a computed property:

var listBinding: Binding<Set<Int>> {
    .init(
        get: {
            selectionKeeper
        },
        set: {
            selectionKeeper = $0
            selectionCallback($0)
        }
    )
}

List(selection: listBinding) { ... }

The onChange solution is simpler and more useful when you only need to react to changes to a specific variable. However, with a custom binding you get much more flexibility (you can change both get and set).