Expand description
Module for interfacing with the device’s button inputs.
The GBA has two primary face buttons (A and B), two secondary face buttons (Select and Start), a 4-way directional pad (“D-pad”), and two shoulder buttons (L and R).
To get the state of all the buttons just read from
KEYINPUT. For consistency, you should usually
read the buttons only once per frame. Then use that same data for all user
input considerations across that entire frame. Otherwise, small fluctuations
in pressure can cause inconsistencies in the reading during a frame.
In addition to simply providing inputs, the buttons can also trigger a
hardware interrupt. Set the desired set of buttons that will trigger a key
interrupt with KEYCNT, and when that button
combination is pressed the key interrupt will be fired. Key interrupts
aren’t a good fit for standard inputs, but as a way to provide a single
extra special input it works okay. For example, this is generally how games
with a “soft reset” button combination do that. The key interrupt handler
sets a “reset requested” flag when the key interrupt occurs, and then the
main game loop checks the flag each frame and performs a soft reset instead
of the normal game simulation when the flag is set.