The Leader Key: A New Kind of Modifier

If you've ever used Vim, you know what a Leader key is. If not, you're about to discover a wonderful concept. :) Instead of hitting Alt+Shift+W for example (holding down three keys at the same time), what if you could hit a sequence of keys instead? So you'd hit our special modifier (the Leader key), followed by W and then C (just a rapid succession of keys), and something would happen.

That's what KC_LEAD does. Here's an example:

Pick a key on your keyboard you want to use as the Leader key. Assign it the keycode KC_LEAD. This key would be dedicated just for this -- it's a single action key, can't be used for anything else.
Include the line #define LEADER_TIMEOUT 300 in your config.h. This sets the timeout for the KC_LEAD key. Specifically, when you press the KC_LEAD key, you only have a certain amount of time to complete the Leader Key sequence. The 300 here sets that to 300ms, and you can increase this value to give you more time to hit the sequence. But any keys pressed during this timeout are intercepted and not sent, so you may want to keep this value low.
- By default, this timeout is how long after pressing KC_LEAD to complete your entire sequence. This may be very low for some people. So you may want to increase this timeout. Optionally, you may want to enable the LEADER_PER_KEY_TIMING option, which resets the timeout after each key is tapped. This allows you to maintain a low value here, but still be able to use the longer sequences. To enable this option, add #define LEADER_PER_KEY_TIMING to your config.h.
Within your matrix_scan_user function, add something like this:

LEADER_EXTERNS();

void matrix_scan_user(void) {
  LEADER_DICTIONARY() {
    leading = false;
    leader_end();

    SEQ_ONE_KEY(KC_F) {
      // Anything you can do in a macro.
      SEND_STRING("QMK is awesome.");
    }
    SEQ_TWO_KEYS(KC_D, KC_D) {
      SEND_STRING(SS_LCTL("a") SS_LCTL("c"));
    }
    SEQ_THREE_KEYS(KC_D, KC_D, KC_S) {
      SEND_STRING("https://start.duckduckgo.com\n");
    }
    SEQ_TWO_KEYS(KC_A, KC_S) {
      register_code(KC_LGUI);
      register_code(KC_S);
      unregister_code(KC_S);
      unregister_code(KC_LGUI);
    }
  }
}

As you can see, you have a few function. You can use SEQ_ONE_KEY for single-key sequences (Leader followed by just one key), and SEQ_TWO_KEYS, SEQ_THREE_KEYS up to SEQ_FIVE_KEYS for longer sequences.

Each of these accepts one or more keycodes as arguments. This is an important point: You can use keycodes from any layer on your keyboard. That layer would need to be active for the leader macro to fire, obviously.

Adding Leader Key Support in the `rules.mk`

To add support for Leader Key you simply need to add a single line to your keymap's rules.mk:

LEADER_ENABLE = yes

Per Key Timing on Leader keys

Rather than relying on an incredibly high timeout for long leader key strings or those of us without 200wpm typing skills, we can enable per key timing to ensure that each key pressed provides us with more time to finish our stroke. This is incredibly helpful with leader key emulation of tap dance (read: multiple taps of the same key like C, C, C).

In order to enable this, place this in your config.h:

#define LEADER_PER_KEY_TIMING

After this, it's recommended that you lower your LEADER_TIMEOUT to something less that 300ms.

#define LEADER_TIMEOUT 250

Now, something like this won't seem impossible to do without a 1000MS leader key timeout:

SEQ_THREE_KEYS(KC_C, KC_C, KC_C) {
  SEND_STRING("Per key timing is great!!!");
}

Strict Key Processing

By default, the Leader Key feature will filter the keycode out of Mod-Tap and Layer Tap functions when checking for the Leader sequences. That means if you're using LT(3, KC_A), it will pick this up as KC_A for the sequence, rather than LT(3, KC_A), giving a more expected behavior for newer users.

While, this may be fine for most, if you want to specify the whole keycode (eg, LT(3, KC_A) from the example above) in the sequence, you can enable this by added #define LEADER_KEY_STRICT_KEY_PROCESSING to your config.h file. This will then disable the filtering, and you'll need to specify the whole keycode.

Customization

The Leader Key feature has some additional customization to how the Leader Key feature works. It has two functions that can be called at certain parts of the process. Namely leader_start() and leader_end().

The leader_start() function is called when you tap the KC_LEAD key, and the leader_end() function is called when either the leader sequence is completed, or the leader timeout is hit.

You can add these functions to your code (keymap.c usually) to add feedback to the Leader sequences (such as beeping or playing music).

void leader_start(void) {
  // sequence started
}

void leader_end(void) {
  // sequence ended (no success/failuer detection)
}

Example

This example will play the Mario "One Up" sound when you hit KC_LEAD to start the Leader Sequence, and will play "All Star" if it completes successfully or "Rick Roll" you if it fails.

bool did_leader_succeed;
#ifdef AUDIO_ENABLE
float leader_start[][2] = SONG(ONE_UP_SOUND );
float leader_succeed[][2] = SONG(ALL_STAR);
float leader_fail[][2] = SONG(RICK_ROLL);
#endif
LEADER_EXTERNS();

void matrix_scan_user(void) {
  LEADER_DICTIONARY() {
    did_leader_succeed = leading = false;

    SEQ_ONE_KEY(KC_E) {
      // Anything you can do in a macro.
      SEND_STRING(SS_LCTL(SS_LSFT("t")));
      did_leader_succeed = true;
    } else 
    SEQ_TWO_KEYS(KC_E, KC_D) {
      SEND_STRING(SS_LGUI("r") "cmd\n" SS_LCTL("c"));
      did_leader_succeed = true;
    }
    leader_end();
  }
}

void leader_start(void) {
#ifdef AUDIO_ENABLE
    PLAY_SONG(leader_start);
#endif
}

void leader_end(void) {
  if (did_leader_succeed) {
#ifdef AUDIO_ENABLE
    PLAY_SONG(leader_succeed);
#endif
  } else {
#ifdef AUDIO_ENABLE
    PLAY_SONG(leader_fail);
#endif
  }
}

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