Every canvas comes with an associated world. A client program can set the world, start the world’s clock, stop the world’s clock, and deal with tick events (the clock ticks) and keyboard inputs (keyevents).
Sets the initial value of viewport’s world to v.
For viewport, sets update-callback to be invoked to transform the world value every secs seconds. Only one callback is installed at a time.
Stops updating viewport’s world via a callback installed with set-on-tick-event.
Sets key-callback as the function to call whenever a key event is received for viewport. The key-callback is given a key event and the current world, and it produces an updated world.