Follows the submodule: on_touch_down and friends become onTouchDown, the last
snake_case left in the surface now that handlers are table fields rather than
globals.
main.lua declares SlateApp, this card's contract with its apps, and every app
composes what it fills -- PaintApp is SlateApp plus TouchHandlers, Home is
SlateApp alone -- which is also the only static record of the features an app
needs.
evan
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