Function useAppLevels

OS Service Hook for an app's internal levels — the list, the detail, the modal over it.

Two things have to happen for back to work inside an app, and they are independent, so either can be forgotten and one usually was:

1. A ladder that closes the deepest open level instead of leaving.
2. Claiming the back action, because the shell owns Backspace and pre-empts anything wired only to <Screen onback>.

Notes and Contacts each shipped step 1 without step 2, so Backspace left the app from a detail view. Here they are the same call: there is no way to describe the levels and not claim the key.

Levels are given deepest first and are read when back is pressed, not when this is called, so open and close see current state. close should undo only its own level — Messages reset twelve fields from one rung and lost the conversation list's scroll position doing it.

const app = useAppLevels({
  appId: 'notes',
  title: 'Notes',
  onback,
  levels: [
    { open: () => isEditing, close: () => (isEditing = false), title: 'Edit Note' },
    { open: () => !!selected, close: () => (selected = null), title: () => selected!.title }
  ]
});
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<Screen title={app.title} onback={app.back}> then needs nothing else.