Switch programs in Mac with one key tap.

keymap-pre.c

#include QMK_KEYBOARD_H

#ifndef KEYMAP_PRE_CUSTOM
#define KEYMAP_PRE_CUSTOM

uint16_t cmd_tab_timer = 16960;     
uint16_t cmd_tab_wait_delay = 700;   
bool in_state = false;


enum custom_keycodes {          
  SWITCH_APPS = SAFE_RANGE,
};


void switch_apps(void) {
  register_code(KC_LGUI);
  tap_code(KC_TAB);

  register_code(KC_LALT);
  unregister_code(KC_LGUI);
  unregister_code(KC_LALT);
}

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  switch (keycode) {
    case SWITCH_APPS:
      if (record->event.pressed) {
        if (in_state ) {
          cmd_tab_wait_delay = 400;
          tap_code(KC_TAB);
        } else {
          in_state = true;
          cmd_tab_wait_delay = 900;
          register_code(KC_LGUI);
          tap_code(KC_TAB);
        }
        cmd_tab_timer = timer_read();
      } 
    break;
  }
  return true;
}

void matrix_scan_user(void) { 
  if ( in_state && timer_read() - cmd_tab_timer > cmd_tab_wait_delay) {
    in_state = false;
    register_code(KC_LALT);
    unregister_code(KC_LGUI);
    unregister_code(KC_LALT);
  }
}


#endif