SwiftUI Binding Edits Wrong TextField after Item Reordered - swift

Question

Xcode 13 beta 5, iOS 14, macOS 11.6
I have a parent SwiftUI view that lists some children. Each child is bound to an NSViewRepresentable. Everything works and I can edit the values as expected. But once I reorder the items in the list and edit a field, it edits the wrong field. It appears that the binding remains intact from the previous item order.
Here's what that looks like:
Here's the parent:
struct ParentView: View {
#StateObject var model = ThingModel.shared
var body: some View {
VStack{
ForEach($model.things){ $thing in
ChildView(thing: $thing)
//Reorder
.onDrag{
model.draggedThing = thing
return NSItemProvider(object: NSString())
}
}
Text("Value: \(model.value)").font(.title)
}
.frame(width:300, height: 200)
}
}
...and here's the child view:
struct ChildView: View {
#Binding var thing: Thing
#StateObject var model = ThingModel.shared
var body: some View{
HStack{
GrowingField(text: $thing.text, submit: {
model.value = thing.text
print(thing.text)
})
Text(" = ")
.opacity(0.4)
}
.padding(10)
.onDrop(of: [UTType.text], delegate: ThingReorderDelegate(hoveredThing: thing))
}
}
Last of all, here is the NSViewRepresentable which is called GrowingField. For simplicity, I have omitted the NSTextField subclass.
struct GrowingField: NSViewRepresentable{
#Binding var text: String
var submit:(() -> Void)? //Hit enter
func makeNSView(context: Context) -> NSTextField {
let textField = NSTextField()
textField.delegate = context.coordinator
textField.stringValue = text
return textField
}
func updateNSView(_ nsView: NSTextField, context: Context) {
nsView.stringValue = text
context.coordinator.textBinding = $text
}
//Delegates
func makeCoordinator() -> Coordinator {
Coordinator(self)
}
class Coordinator: NSObject, NSTextFieldDelegate {
let parent: GrowingField
var textBinding : Binding<String>?
init(_ field: GrowingField) {
self.parent = field
}
func controlTextDidChange(_ obj: Notification) {
guard let textField = obj.object as? NSTextField else { return }
self.textBinding?.wrappedValue = textField.stringValue
}
//Listen for certain keyboard keys
func control(_ control: NSControl, textView: NSTextView, doCommandBy commandSelector: Selector) -> Bool {
switch commandSelector{
case #selector(NSStandardKeyBindingResponding.insertNewline(_:)):
//- Enter -
parent.submit?()
textView.window?.makeFirstResponder(nil) //Blur cursor
return true
default:
return false
}
}
}
}
Why does the binding to the NSViewRepresentable not follow the field after it is reordered?
Here is a sample project to download and try it out.

Answer 1

I believe the issue (bug?) is with the ForEach-generated binding.
If you forego the generated binding and create your own, everything seems to work as expected.
Added to the ThingModel:
func bindingForThing(id: String) -> Binding<Thing> {
.init {
self.things.first { $0.id == id }!
} set: { newThing in
self.things = self.things.map { $0.id == id ? newThing : $0 }
}
}
And the ParentView:
ForEach(model.things){ thing in
ChildView(thing: model.bindingForThing(id: thing.id))