qmk_firmware/docs/feature_combo.md

Combos

The Combo feature is a chording type solution for adding custom actions. It lets you hit multiple keys at once and produce a different effect. For instance, hitting A and S within the combo term would hit ESC instead, or have it perform even more complex tasks.

To enable this feature, you need to add COMBO_ENABLE = yes to your rules.mk.

Additionally, in your config.h, you'll need to specify the number of combos that you'll be using, by adding #define COMBO_COUNT 1 (replacing 1 with the number that you're using). It is also possible to not define this and instead set the variable COMBO_LEN yourself. There's a trick where we don't need to think about this variable at all. More on this later.

Then, in your keymap.c file, you'll need to define a sequence of keys, terminated with COMBO_END, and a structure to list the combination of keys, and its resulting action.

const uint16_t PROGMEM test_combo1[] = {KC_A, KC_B, COMBO_END};
const uint16_t PROGMEM test_combo2[] = {KC_C, KC_D, COMBO_END};
combo_t key_combos[COMBO_COUNT] = {
    COMBO(test_combo1, KC_ESC),
    COMBO(test_combo2, LCTL(KC_Z)), // keycodes with modifiers are possible too!
};

This will send "Escape" if you hit the A and B keys, and Ctrl+Z when you hit the C and D keys.

As of PR#8591, it is possible to fire combos from ModTap keys and LayerTap keys. So in the above example you could have keys LSFT_T(KC_A) and LT(_LAYER, KC_B) and it would work. So Home Row Mods and Home Row Combos at same time is now a thing!

It is also now possible to overlap combos. Before, with the example below both combos would activate when all three keys were pressed. Now only the three key combo will activate.

const uint16_t PROGMEM test_combo1[] = {LSFT_T(KC_A), LT(_LAYER, KC_B), COMBO_END};
const uint16_t PROGMEM test_combo2[] = {LSFT_T(KC_A), LT(_LAYER, KC_B), KC_C, COMBO_END};
combo_t key_combos[COMBO_COUNT] = {
    COMBO(test_combo1, KC_ESC)
    COMBO(test_combo2, KC_TAB)
};

Executing more complex keycodes like ModTaps and LayerTaps is now also possible.

Examples

If you want to add a list, then you'd use something like this:

enum combos {
  AB_ESC,
  JK_TAB,
  QW_SFT,
  SD_LAYER,
};

const uint16_t PROGMEM ab_combo[] = {KC_A, KC_B, COMBO_END};
const uint16_t PROGMEM jk_combo[] = {KC_J, KC_K, COMBO_END};
const uint16_t PROGMEM qw_combo[] = {KC_Q, KC_W, COMBO_END};
const uint16_t PROGMEM sd_combo[] = {KC_S, KC_D, COMBO_END};

combo_t key_combos[COMBO_COUNT] = {
  [AB_ESC] = COMBO(ab_combo, KC_ESC),
  [JK_TAB] = COMBO(jk_combo, KC_TAB),
  [QW_SFT] = COMBO(qw_combo, KC_LSFT)
  [SD_LAYER] = COMBO(layer_combo, MO(_LAYER)),
};

For a more complicated implementation, you can use the process_combo_event function to add custom handling. Additionally, this example shows how you can leave COMBO_COUNT undefined.

enum combo_events {
  EM_EMAIL,
  BSPC_LSFT_CLEAR,
  COMBO_LENGTH
};
uint16_t COMBO_LEN = COMBO_LENGTH; // remove the COMBO_COUNT define and use this instead!

const uint16_t PROGMEM email_combo[] = {KC_E, KC_M, COMBO_END};
const uint16_t PROGMEM clear_line_combo[] = {KC_BSPC, KC_LSFT, COMBO_END};

combo_t key_combos[] = {
  [EM_EMAIL] = COMBO_ACTION(email_combo),
  [BSPC_LSFT_CLEAR] = COMBO_ACTION(clear_line_combo),
};
/* COMBO_ACTION(x) is same as COMBO(x, KC_NO) */

void process_combo_event(uint16_t combo_index, bool pressed) {
  switch(combo_index) {
    case EM_EMAIL:
      if (pressed) {
        SEND_STRING("john.doe@example.com");
      }
      break;
    case BSPC_LSFT_CLEAR:
      if (pressed) {
        tap_code16(KC_END);
        tap_code16(S(KC_HOME));
        tap_code16(KC_BSPC);
      }
      break;
  }
}

This will send "john.doe@example.com" if you chord E and M together, and clear the current line with Backspace and Left-Shift. You could change this to do stuff like play sounds or change settings.

It is worth noting that COMBO_ACTIONs are not needed anymore. As of PR#8591, it is possible to run your own custom keycodes from combos. Just define the custom keycode, program its functionality in process_record_user, and define a combo with COMBO(<key_array>, <your_custom_keycode>).

Keycodes

You can enable, disable and toggle the Combo feature on the fly. This is useful if you need to disable them temporarily, such as for a game. The following keycodes are available for use in your keymap.c

Keycode	Description
CMB_ON	Turns on Combo feature
CMB_OFF	Turns off Combo feature
CMB_TOG	Toggles Combo feature on and off

Advanced Configuration

These configuration settings can be set in your config.h file.

Combo Term

By default, the timeout for the Combos to be recognized is set to 50ms. This can be changed if accidental combo misfires are happening or if you're having difficulties pressing keys at the same time. For instance, #define COMBO_TERM 40 would set the timeout period for combos to 40ms.

Buffer and state sizes

If you're using long combos, or you have a lot of overlapping combos, you may run into issues with this, as the buffers may not be large enough to accommodate what you're doing. In this case, you can configure the sizes of the buffers used. Be aware, larger combo sizes and larger buffers will increase memory usage!

To configure the amount of keys a combo can be composed of, change the following:

Keys	Define to be set
6	#define EXTRA_SHORT_COMBOS
8	QMK Default
16	#define EXTRA_LONG_COMBOS
32	#define EXTRA_EXTRA_LONG_COMBOS

Defining EXTRA_SHORT_COMBOS combines a combo's internal state into just one byte. This can, in some cases, save some memory. If it doesn't, no point using it. If you do, you also have to make sure you don't define combos with more than 6 keys.

Processing combos has two buffers, one for the key presses, another for the combos being activated. Use the following options to configure the sizes of these buffers:

Define	Default
#define COMBO_KEY_BUFFER_LENGTH 8	8 (the key amount (EXTRA_)EXTRA_LONG_COMBOS gives)
#define COMBO_BUFFER_LENGTH 4	4

Modifier Combos

If a combo resolves to a Modifier, the window for processing the combo can be extended independently from normal combos. By default, this is disabled but can be enabled with #define COMBO_MUST_HOLD_MODS, and the time window can be configured with #define COMBO_HOLD_TERM 150 (default: TAPPING_TERM). With COMBO_MUST_HOLD_MODS, you cannot tap the combo any more which makes the combo less prone to misfires.

Per Combo Timing, Holding and Tapping

For each combo, it is possible to configure the time window it has to pressed in, if it needs to be held down, or if it needs to be tapped.

For example, tap-only combos are useful if any (or all) of the underlying keys is a Mod-Tap or a Layer-Tap key. When you tap the combo, you get the combo result. When you press the combo and hold it down, the combo doesn't actually activate. Instead the keys are processed separately as if the combo wasn't even there.

In order to use these features, the following configuration options and functions need to be defined. Coming up with useful timings and configuration is left as an exercise for the reader.

Config Flag	Function	Description
COMBO_TERM_PER_COMBO	uint16_t get_combo_term(uint16_t index, combo_t *combo)	Optional per-combo timeout window. (default: COMBO_TERM)
COMBO_MUST_HOLD_PER_COMBO	bool get_combo_must_hold(uint16_t index, combo_t *combo)	Controls if a given combo should fire immediately on tap or if it needs to be held. (default: false)
COMBO_MUST_TAP_PER_COMBO	bool get_combo_must_tap(uint16_t index, combo_t *combo)	Controls if a given combo should fire only if tapped within COMBO_HOLD_TERM. (default: false)

Examples:

uint16_t get_combo_term(uint16_t index, combo_t *combo) {
    // decide by combo->keycode
    switch (combo->keycode) {
        case KC_X:
            return 50;
    }

    // or with combo index, i.e. its name from enum.
    switch (index) {
        case COMBO_NAME_HERE:
            return 9001;
    }

    // And if you're feeling adventurous, you can even decide by the keys in the chord,
    // i.e. the exact array of keys you defined for the combo.
    // This can be useful if your combos have a common key and you want to apply the
    // same combo term for all of them.
    if (combo->keys[0] == KC_ENTER) { // if first key in the array is KC_ENTER
        return 150;
    }

    return COMBO_TERM;
}

bool get_combo_must_hold(uint16_t index, combo_t *combo) {
    // Same as above, decide by keycode, the combo index, or by the keys in the chord.

    if (KEYCODE_IS_MOD(combo->keycode) || 
        (combo->keycode >= QK_MOMENTARY && combo->keycode <= QK_MOMENTARY_MAX) // MO(kc) keycodes
        ) {
        return true;
    }

    switch (index) {
        case COMBO_NAME_HERE:
            return true;
    }

    return false;
}

bool get_combo_must_tap(uint16_t index, combo_t *combo) {
    // If you want all combos to be tap-only, just uncomment the next line
    // return true

    // If you want *all* combos, that have Mod-Tap/Layer-Tap/Momentary keys in its chord, to be tap-only, this is for you:
    uint16_t key;
    uint8_t idx = 0;
    while ((key = pgm_read_word(&combo->keys[idx])) != COMBO_END) {
        switch (key) {
            case QK_MOD_TAP...QK_MOD_TAP_MAX:
            case QK_LAYER_TAP...QK_LAYER_TAP_MAX:
            case QK_MOMENTARY...QK_MOMENTARY_MAX:
                return true;
        }
        idx += 1;
    }
    return false;

}

Variable Length Combos

If you leave COMBO_COUNT undefined in config.h, it allows you to programmatically declare the size of the Combo data structure and avoid updating COMBO_COUNT. Instead a variable called COMBO_LEN has to be set. It can be set with something similar to the following in keymap.c: uint16_t COMBO_LEN = sizeof(key_combos) / sizeof(key_combos[0]); or by adding COMBO_LENGTH as the last entry in the combo enum and then uint16_t COMBO_LEN = COMBO_LENGTH; as such:

enum myCombos {
    ...,
    COMBO_LENGTH
};
uint16_t COMBO_LEN = COMBO_LENGTH;

Regardless of the method used to declare COMBO_LEN, this also requires to convert the combo_t key_combos[COMBO_COUNT] = {...}; line to combo_t key_combos[] = {...};.

Combo timer

Normally, the timer is started on the first key press and then reset on every subsequent key press within the COMBO_TERM. Inputting combos is relaxed like this, but also slightly more prone to accidental misfires.

The next two options alter the behaviour of the timer.

#define COMBO_STRICT_TIMER

With COMBO_STRICT_TIMER, the timer is started only on the first key press. Inputting combos is now less relaxed; you need to make sure the full chord is pressed within the COMBO_TERM. Misfires are less common but if you type multiple combos fast, there is a chance that the latter ones might not activate properly.

#define COMBO_NO_TIMER

By defining COMBO_NO_TIMER, the timer is disabled completely and combos are activated on the first key release. This also disables the "must hold" functionalities as they just wouldn't work at all.

Customizable key releases

By defining COMBO_PROCESS_KEY_RELEASE and implementing the function bool process_combo_key_release(uint16_t combo_index, combo_t *combo, uint8_t key_index, uint16_t keycode), you can run your custom code on each key release after a combo was activated. For example you could change the RGB colors, activate haptics, or alter the modifiers.

You can also release a combo early by returning true from the function.

Here's an example where a combo resolves to two modifiers, and on key releases the modifiers are unregistered one by one, depending on which key was released.

enum combos {
  AB_MODS,
  COMBO_LENGTH
};
uint16_t COMBO_LEN = COMBO_LENGTH;

const uint16_t PROGMEM ab_combo[] = {KC_A, KC_B, COMBO_END};

combo_t key_combos[] = {
  [AB_MODS] = COMBO(ab_combo, LCTL(KC_LSFT)),
};

bool process_combo_key_release(uint16_t combo_index, combo_t *combo, uint8_t key_index, uint16_t keycode) {
    switch (combo_index) {
        case AB_MODS:
            switch(keycode) {
                case KC_A:
                    unregister_mods(MOD_MASK_CTRL);
                    break;
                case KC_B:
                    unregister_mods(MOD_MASK_SHIFT);
                    break;
            }
            return false; // do not release combo
    }
    return false;
}

Layer independent combos

If you, for example, use multiple base layers for different key layouts, one for QWERTY, and another one for Colemak, you might want your combos to work from the same key positions on all layers. Defining the same combos again for another layout is redundant and takes more memory. The solution is to just check the keycodes from one layer.

With #define COMBO_ONLY_FROM_LAYER _LAYER_A the combos' keys are always checked from layer _LAYER_A even though the active layer would be _LAYER_B.

User callbacks

In addition to the keycodes, there are a few functions that you can use to set the status, or check it:

Function	Description
combo_enable()	Enables the combo feature
combo_disable()	Disables the combo feature, and clears the combo buffer
combo_toggle()	Toggles the state of the combo feature
is_combo_enabled()	Returns the status of the combo feature state (true or false)

Dictionary Management

Having 3 places to update when adding new combos or altering old ones does become cumbersome when you have a lot of combos. We can alleviate this with some magic! ... If you consider C macros magic. First, you need to add VPATH += keyboards/gboards to your rules.mk. Next, include the file g/keymap_combo.h in your keymap.c.

!> This functionality uses the same process_combo_event function as COMBO_ACTION macros do, so you cannot use the function yourself in your keymap. Instead, you have to define the cases of the switch statement by themselves within inject.h, which g/keymap_combo.h will then include into the function.

Then, write your combos in combos.def file in the following manner:

//   name     result    chord keys
COMB(AB_ESC,   KC_ESC,   KC_A, KC_B)
COMB(JK_TAB,   KC_TAB,   KC_J, KC_K)
COMB(JKL_SPC,  KC_SPC,   KC_J, KC_K, KC_L)
COMB(BSSL_CLR, KC_NO,    KC_BSPC, KC_LSFT) // using KC_NO as the resulting keycode is the same as COMBO_ACTION before.
COMB(QW_UNDO,  C(KC_Z),  KC_Q, KC_W)
SUBS(TH_THE,   "the",    KC_T, KC_H) // SUBS uses SEND_STRING to output the given string.
...

Now, you can update only one place to add or alter combos. You don't even need to remember to update the COMBO_COUNT or the COMBO_LEN variables at all. Everything is taken care of. Magic!

For small to huge ready made dictionaries of combos, you can check out http://combos.gboards.ca/.