Keys¶

Reference: the keys a reader presses and the constant each has in sextile.keys. Movement is two-dimensional, and each direction has a letter and the arrow a BBC keyboard sends for it.

       W                     up          the frames of this item
  A    ·    D           left    right    the items either side
       S                   down

Moving about¶

Key	Arrow	Constant	Moves to
`W`	↑	`PREVIOUS_FRAME`	the previous frame of this item
`S`, `#`	↓	`NEXT_FRAME`, `HASH`	the next frame of this item
`A`	←	`PREVIOUS_ITEM`	the item before this one
`D`	→	`NEXT_ITEM`	the item after this one

The arrows send 0x08, 0x09, 0x0A, 0x0B (LEFT, RIGHT, DOWN, UP) once the 7E1 line has taken the eighth bit off the BBC’s cursor codes; TAB sends 0x09 too, so it reads as RIGHT. Whether an arrow means the same as its letter is the page’s to say, because W is West and S is South on a page with a coordinate field: ARROW_FOR maps each letter to its arrow, and with_arrows offers both on a page where they agree.

The session’s own commands¶

Keyed as *, the payload, then #. These the session answers itself rather than handing to a service.

Keyed	Constant	Does
`*0#`	`BACK`	returns to the page just left
`*00#`	`REDISPLAY`	redraws the current frame
`*09#`	`REFRESH`	rebuilds the page from its handler

BACK, REDISPLAY and REFRESH are the payloads 0, 00 and 09, not the 0 key a frame offers to go home — that is layout.HOME_KEY, a keypress on a frame rather than a command.

Typing and editing¶

Key	Constant	Does
`*`	`CANCEL`	cancels a part-typed request; a second begins again
`DELETE` (`0x7f`)	`RUB_OUT`	rubs out a character being typed; on a page, an ordinary key a field may answer

Turning one form into another¶

with_arrows(pressed) adds each key’s arrow to a set of movement keys; as_letter(key) turns a pressed arrow back into the letter it stands for; with_arrow_choices(choices) gives each arrow the destination its letter has; frame_moves(has_previous=, has_next=) is the set a page of several frames answers. ARROW_KEYS and LETTER_FOR are the byte-to-arrow and arrow-to-letter maps behind them.

Why the two spellings: the arrows are as period as anything here, and a reader uses whichever comes to hand; WASD postdates viewdata but reads as a compass, which is why a guide page can draw it.