Drawing with what the parent holds

A child's view reads its own Model and the inputs the parent's view passes. A fact only the parent holds, such as whether a write the child started has been answered, reaches it through viewInputs in the placement config, which rewrites those inputs at every draw:

const Shown = Shelf.at(Notice, {
  // Whether the page is online is the page's to know; the notice draws it.
  viewInputs: (model, given) => ({ ...given, offline: !model.online }),
})

given is what Shown.view(model, h, inputs) was called with, and the function returns the inputs the child's view takes. It reads; it changes no Model. Crud.editor places its form this way, so the drawn form reads busy while its save is in flight.

A view that takes no inputs ((model, h)) refuses viewInputs as a type error: it would never be called, so what it adds would never show. A bundle made without a view takes it, since its view is the page's to draw: its inputs are NoView, not void.

Presets#

bundle.with(args) binds the args, so a placement gives none:

const PrefersDark = MediaQuery.with({ query: '(prefers-color-scheme: dark)' })
Page.place(PrefersDark, 'dark')

Page.place(bundle, field, config) is Page.at without a separate declaration, for a Message union built another way.