qmk_firmware/users/drashna/callbacks.md

# Custom Userspace Callback Functions

Specifically QMK works by using customized callback functions for everything. This allows for multiple levels of customization. 

`matrix_scan` calls `matrix_scan_quantum`, which calls `matrix_scan_kb`, which calls `matrix_scan_user`. 
`process_record` calls a bunch of stuff, but eventually calls `process_record_kb` which calls `process_record_user`
The same goes for `matrix_init`, `layer_state_set`, `led_set`, and a few other functions.  

All (most) `_user` functions are handled here, in the userspace instead.  To allow keyboard specific configuration, I've created `_keymap` functions that can be called by the keymap.c files instead.

This allows for keyboard specific configuration while maintaining the ability to customize the board.

My [Ergodox EZ Keymap](https://github.com/qmk/qmk_firmware/blob/master/layouts/community/ergodox/drashna/keymap.c) is a good example of this, as it uses the LEDs as modifier indicators.

You can see a list of these files in [callbacks.c](callbacks.c), or a shortend list here 

```c
__attribute__((weak)) void matrix_init_keymap(void) {}
void                       matrix_init_user(void) {
    matrix_init_keymap();
}

__attribute__((weak)) void keyboard_post_init_keymap(void) {}
void                       keyboard_post_init_user(void) {
    keyboard_post_init_keymap();
}

__attribute__((weak)) void matrix_scan_keymap(void) {}
void                       matrix_scan_user(void) {
    matrix_scan_keymap();
}

__attribute__ ((weak)) bool process_record_keymap(uint16_t keycode, keyrecord_t *record) { return true; }
bool                        process_record_user(uint16_t keycode, keyrecord_t *record) {
    if (!process_record_keymap(keycode, record)) { return false; }
    return true;
}

__attribute__((weak)) layer_state_t layer_state_set_keymap(layer_state_t state) { return state; }
layer_state_t                       layer_state_set_user(layer_state_t state) {
    state = layer_state_set_keymap(state);
    return state;
}

__attribute__ ((weak)) void led_set_keymap(uint8_t usb_led) {}
void                        led_set_user(uint8_t usb_led) {
    led_set_keymap(usb_led);
}

__attribute__ ((weak)) void suspend_power_down_keymap(void) {}
void                        suspend_power_down_user(void) {
    suspend_power_down_keymap();
}

__attribute__ ((weak)) void suspend_wakeup_init_keymap(void) {}
void                        suspend_wakeup_init_user(void) {
    suspend_wakeup_init_keymap();
}


__attribute__ ((weak)) void shutdown_keymap(void) {}
void                        shutdown_user (void) {
    shutdown_keymap();
}

__attribute__ ((weak)) void eeconfig_init_keymap(void) {}
void                   eeconfig_init_user(void) {
    eeconfig_update_user(0);
    eeconfig_init_keymap();
}
```