Re-pad the view on re-render while keyboard continues to show

[wearhere]

The current implementation will only update the padding if the keyboard shows or hides. But what if the view re-renders, moving the first responder up or down, while the keyboard is showing?

This happens in my app, which is a tutorial for setting up a custom keyboard. My view is like this:

struct SwitchToKeyboardView: View {
    @EnvironmentObject var keyboardState: KeyboardState

    @State var dummyText = ""
    @State var dummyInputIsEditing = true
    
    @ViewBuilder
    var body: some View {
        VStack {
            if keyboardState.hasBeenOpened {
                Text("Great!").font(.title)
                Text("Searchboard is now ready")
                Text("to use in any app.")
            } else {
                Text("Tap and hold the ") + Text("Globe").bold() + Text(" icon")
                Text("Tap ") + Text("Searchboard").bold() + Text(" to switch keyboards")
            }
            // Use an invisible text view to bring up the keyboard and to
            // permit the keyboard to tell us that we're opened (see
            // `KeyboardState`). We don't use a text field since you can't
            // force those to become first responder
            // https://stackoverflow.com/a/56508132/495611 . Keep the text
            // view visible even after the user has switched, the first
            // time that they switch, so that they can play around with the
            // keyboard.
            TextView(text: $dummyText, isEditing: $dummyInputIsEditing).frame(width: 0, height: 0, alignment: .center)
        }
        .keyboardAdaptive()
    }
}

Where TextView is an instance of this. I won't get into the implementation of KeyboardState unless you like, but I have a mechanism where once my custom keyboard opens, it causes keyboardState.hasBeenOpened to become true, asynchronously with respect to the keyboard changing.

You may notice that the text above the field grows bigger when the keyboard has been opened, so the padding needs to increase. But it doesn't, because the update comes in after the keyboard has already changed.

---

Here's a PR that's a stab at re-rendering the ViewModifier when the keyboard height changes or when a client-provided publisher fires. In my case, that publisher is a binding to keyboardState.hasBeenOpened: I change my .keyboardAdaptive() invocation to look like

.keyboardAdaptive(rerender: keyboardState.$hasBeenOpened.map({ arg -> Any in
    arg
}).eraseToAnyPublisher())

There are a bunch of things I don't like about this approach, but I figured I'd put it up to start a conversation.

[wearhere]

Futzing with the types is really frustrating here considering I don't even care what the values are when self.rerender fires. Ideally, I wouldn't have to eraseToAnyPublisher nor map this to convert it to <Any, Never>, and the client wouldn't have to do likewise when passing in their publisher, but I'm not sure how to type things to accomplish that.

[wearhere]

I'm dividing by 2 and multiplying by 2 here for reasons described here. My keyboard is around 500pt tall, like I mention in the comments there.

[wearhere]

I think it would be really slick to convert this publisher into a CurrentValueSubject so that we didn't have to retain the return value separately, but not sure how to do that.

[wearhere]

We change how we're publishing the keyboard height so that KeyboardAdaptive can read the current value when re-rendering not in response to the keyboard changing, but rather in response to another condition.

I tried caching the keyboard height on the KeyboardAdaptive modifier e.g. in a @State property and it didn't work but rather got reset, I would assume when the view hierarchy re-renders.

[wearhere]

It would be really nice if we could just observe the frame of this translating, that's the real condition that should prompt us to re-pad, without the client having to do anything. Maybe something using KVO?