[Docs] Add Sections and MO(layer)/TG(layer) Example (#6308)

* Add Sections and MO(layer)/TG(layer) Example

Major changes:
1. Added sub-section headings to the portion before the examples.
2. Added a new Example 6, that allows MO(layer) and TG(layer) functionality to be embedded within tap dance functions.

Minor Changes:
1. Edited some text to better fit with new sub-headings.

* Update feature_tap_dance.md

* Update feature_tap_dance.md

docs/feature_tap_dance.md

@@ -1,40 +1,56 @@
 # Tap Dance: A Single Key Can Do 3, 5, or 100 Different Things
 
-<!-- FIXME: Break this up into multiple sections -->
-
+## Introduction
 Hit the semicolon key once, send a semicolon. Hit it twice, rapidly -- send a colon. Hit it three times, and your keyboard's LEDs do a wild dance. That's just one example of what Tap Dance can do. It's one of the nicest community-contributed features in the firmware, conceived and created by [algernon](https://github.com/algernon) in [#451](https://github.com/qmk/qmk_firmware/pull/451). Here's how algernon describes the feature:
 
 With this feature one can specify keys that behave differently, based on the amount of times they have been tapped, and when interrupted, they get handled before the interrupter.
 
-To make it clear how this is different from `ACTION_FUNCTION_TAP`, let's explore a certain setup! We want one key to send `Space` on single tap, but `Enter` on double-tap.
+## Explanatory Comparison with `ACTION_FUNCTION_TAP`
+`ACTION_FUNCTION_TAP` can offer similar functionality to Tap Dance, but it's worth noting some important differences. To do this, let's explore a certain setup! We want one key to send `Space` on single-tap, but `Enter` on double-tap.
+
+With `ACTION_FUNCTION_TAP`, it is quite a rain-dance to set this up, and has the problem that when the sequence is interrupted, the interrupting key will be sent first. Thus, `SPC a` will result in `a SPC` being sent, if `SPC` and `a` are both typed within `TAPPING_TERM`. With the Tap Dance feature, that'll come out correctly as `SPC a` (even if both `SPC` and `a` are typed within the `TAPPING_TERM`.
+
+To achieve this correct handling of interrupts, the implementation of Tap Dance hooks into two parts of the system: `process_record_quantum()`, and the matrix scan. These two parts are explained below, but for now the point to note is that we need the latter to be able to time out a tap sequence even when a key is not being pressed. That way, `SPC` alone will time out and register after `TAPPING_TERM` time.
 
-With `ACTION_FUNCTION_TAP`, it is quite a rain-dance to set this up, and has the problem that when the sequence is interrupted, the interrupting key will be sent first. Thus, `SPC a` will result in `a SPC` being sent, if they are typed within `TAPPING_TERM`. With the tap dance feature, that'll come out as `SPC a`, correctly.
+## How to Use Tap Dance
+But enough of the generalities; lets look at how to actually use Tap Dance!
 
-The implementation hooks into two parts of the system, to achieve this: into `process_record_quantum()`, and the matrix scan. We need the latter to be able to time out a tap sequence even when a key is not being pressed, so `SPC` alone will time out and register after `TAPPING_TERM` time.
+First, you will need `TAP_DANCE_ENABLE=yes` in your `rules.mk`, because the feature is disabled by default. This adds a little less than 1k to the firmware size. 
 
-But lets start with how to use it, first!
+Optionally, you might want to set a custom `TAPPING_TERM` time by adding something like this in you `config.h`:
 
-First, you will need `TAP_DANCE_ENABLE=yes` in your `rules.mk`, because the feature is disabled by default. This adds a little less than 1k to the firmware size. Next, you will want to define some tap-dance keys, which is easiest to do with the `TD()` macro, that - similar to `F()`, takes a number, which will later be used as an index into the `tap_dance_actions` array.
+```
+#define TAPPING_TERM 175
+```
 
-This array specifies what actions shall be taken when a tap-dance key is in action. Currently, there are five possible options:
+The `TAPPING_TERM` time is the maximum time allowed between taps of your Tap Dance key, and is measured in milliseconds. For example, if you used the above `#define` statement and set up a Tap Dance key that sends `Space` on single-tap and `Enter` on double-tap, then this key will send `ENT` only if you tap this key twice in less than 175ms. If you tap the key, wait more than 175ms, and tap the key again you'll end up sending `SPC SPC` instead. 
+
+Next, you will want to define some tap-dance keys, which is easiest to do with the `TD()` macro, that - similar to `F()` - takes a number, which will later be used as an index into the `tap_dance_actions` array. 
+
+After this, you'll want to use the `tap_dance_actions` array to specify what actions shall be taken when a tap-dance key is in action. Currently, there are five possible options:
 
 * `ACTION_TAP_DANCE_DOUBLE(kc1, kc2)`: Sends the `kc1` keycode when tapped once, `kc2` otherwise. When the key is held, the appropriate keycode is registered: `kc1` when pressed and held, `kc2` when tapped once, then pressed and held.
 * `ACTION_TAP_DANCE_DUAL_ROLE(kc, layer)`: Sends the `kc` keycode when tapped once, or moves to `layer`. (this functions like the `TO` layer keycode).
 * `ACTION_TAP_DANCE_FN(fn)`: Calls the specified function - defined in the user keymap - with the final tap count of the tap dance action.
 * `ACTION_TAP_DANCE_FN_ADVANCED(on_each_tap_fn, on_dance_finished_fn, on_dance_reset_fn)`: Calls the first specified function - defined in the user keymap - on every tap, the second function when the dance action finishes (like the previous option), and the last function when the tap dance action resets.
 * `ACTION_TAP_DANCE_FN_ADVANCED_TIME(on_each_tap_fn, on_dance_finished_fn, on_dance_reset_fn, tap_specific_tapping_term)`: This functions identically to the `ACTION_TAP_DANCE_FN_ADVANCED` function, but uses a custom tapping term for it, instead of the predefined `TAPPING_TERM`.
 
-The first option is enough for a lot of cases, that just want dual roles. For example, `ACTION_TAP_DANCE_DOUBLE(KC_SPC, KC_ENT)` will result in `Space` being sent on single-tap, `Enter` otherwise.
+The first option is enough for a lot of cases, that just want dual roles. For example, `ACTION_TAP_DANCE_DOUBLE(KC_SPC, KC_ENT)` will result in `Space` being sent on single-tap, `Enter` otherwise. 
 
 !> Keep in mind that only [basic keycodes](keycodes_basic.md) are supported here. Custom keycodes are not supported.
 
-And that's the bulk of it!
+Similar to the first option, the second option is good for simple layer-switching cases.
+
+For more complicated cases, use the third or fourth options (examples of each are listed below). 
+
+Finally, the fifth option is particularly useful if your non-Tap-Dance keys start behaving weirdly after adding the code for your Tap Dance keys. The likely problem is that you changed the `TAPPING_TERM` time to make your Tap Dance keys easier for you to use, and that this has changed the way your other keys handle interrupts.
 
-And now, on to the explanation of how it works!
+## Implementation Details
+Well, that's the bulk of it! You should now be able to work through the examples below, and to develop your own Tap Dance functionality. But if you want a deeper understanding of what's going on behind the scenes, then read on for the explanation of how it all works!
 
-The main entry point is `process_tap_dance()`, called from `process_record_quantum()`, which is run for every keypress, and our handler gets to run early. This function checks whether the key pressed is a tap-dance key. If it is not, and a tap-dance was in action, we handle that first, and enqueue the newly pressed key. If it is a tap-dance key, then we check if it is the same as the already active one (if there's one active, that is). If it is not, we fire off the old one first, then register the new one. If it was the same, we increment the counter and the timer.
+The main entry point is `process_tap_dance()`, called from `process_record_quantum()`, which is run for every keypress, and our handler gets to run early. This function checks whether the key pressed is a tap-dance key. If it is not, and a tap-dance was in action, we handle that first, and enqueue the newly pressed key. If it is a tap-dance key, then we check if it is the same as the already active one (if there's one active, that is). If it is not, we fire off the old one first, then register the new one. If it was the same, we increment the counter and reset the timer.
 
-This means that you have `TAPPING_TERM` time to tap the key again, you do not have to input all the taps within that timeframe. This allows for longer tap counts, with minimal impact on responsiveness.
+This means that you have `TAPPING_TERM` time to tap the key again; you do not have to input all the taps within a single `TAPPING_TERM` timeframe. This allows for longer tap counts, with minimal impact on responsiveness.
 
 Our next stop is `matrix_scan_tap_dance()`. This handles the timeout of tap-dance keys.
 
@@ -397,3 +413,111 @@ qk_tap_dance_action_t tap_dance_actions[] = {
 ```
 
 Wrap each tapdance keycode in `TD()` when including it in your keymap, e.g. `TD(ALT_LP)`.
+
+### Example 6: Using tap dance for momentary-layer-switch and layer-toggle keys
+
+Tap Dance can be used to mimic MO(layer) and TG(layer) functionality. For this example, we will set up a key to function as `KC_QUOT` on single-tap, as `MO(_MY_LAYER)` on single-hold, and `TG(_MY_LAYER)` on double-tap.
+
+The first step is to include the following code towards the beginning of your `keymap.c`:
+
+```
+typedef struct {
+  bool is_press_action;
+  int state;
+} tap;
+
+//Define a type for as many tap dance states as you need
+enum {
+  SINGLE_TAP = 1,
+  SINGLE_HOLD = 2,
+  DOUBLE_TAP = 3
+};
+
+enum {
+  QUOT_LAYR = 0     //Our custom tap dance key; add any other tap dance keys to this enum 
+};
+
+//Declare the functions to be used with your tap dance key(s)
+
+//Function associated with all tap dances
+int cur_dance (qk_tap_dance_state_t *state);
+
+//Functions associated with individual tap dances
+void ql_finished (qk_tap_dance_state_t *state, void *user_data);
+void ql_reset (qk_tap_dance_state_t *state, void *user_data);
+
+//Declare variable to track which layer is active
+int active_layer;
+```
+
+The above code is similar to that used in previous examples. The one point to note is that you need to declare a variable to keep track of what layer is currently the active layer. We'll see why shortly.
+
+Towards the bottom of your `keymap.c`, include the following code:
+
+```
+//Update active_layer
+uint32_t layer_state_set_user(uint32_t state) {
+  switch (biton32(state)) {
+    case 1:
+      active_layer = 1;
+      break;
+    case 2:
+      active_layer = 2;
+      break;
+    case 3:
+      active_layer = 3;
+      break;
+    default:
+      active_layer = 0;
+      break;
+  }
+  return state;
+}
+
+//Determine the current tap dance state
+int cur_dance (qk_tap_dance_state_t *state) {
+  if (state->count == 1) {
+    if (!state->pressed) {return SINGLE_TAP;}
+    else return SINGLE_HOLD;
+  } else if (state->count == 2) {return DOUBLE_TAP;}
+  else return 8;
+}
+
+//Initialize tap structure associated with example tap dance key
+static tap ql_tap_state = {
+  .is_press_action = true,
+  .state = 0
+};
+
+//Functions that control what our tap dance key does
+void ql_finished (qk_tap_dance_state_t *state, void *user_data) {
+  ql_tap_state.state = cur_dance(state);
+  switch (ql_tap_state.state) {
+    case SINGLE_TAP: tap_code(KC_QUOT); break;
+    case SINGLE_HOLD: layer_on(_MY_LAYER); break;
+    case DOUBLE_TAP: 
+      if (active_layer==_MY_LAYER) {layer_off(_MY_LAYER);}
+      else layer_on(_MY_LAYER);
+  }
+}
+
+void ql_reset (qk_tap_dance_state_t *state, void *user_data) {
+  if (ql_tap_state.state==SINGLE_HOLD) {layer_off(_MY_LAYER);}
+  ql_tap_state.state = 0;
+}
+
+//Associate our tap dance key with its functionality
+qk_tap_dance_action_t tap_dance_actions[] = {
+  [QUOT_LAYR] = ACTION_TAP_DANCE_FN_ADVANCED_TIME(NULL, ql_finished, ql_reset, 275)
+};
+```
+
+The is where the real logic of our tap dance key gets worked out. Since `layer_state_set_user()` is called on any layer switch, we use it to update `active_layer`. Our example is assuming that your `keymap.c` includes 4 layers, so adjust the switch statement here to fit your actual number of layers.
+
+The use of `cur_dance()` and `ql_tap_state` mirrors the above examples.
+
+The `case:SINGLE_TAP` in `ql_finished` is similar to the above examples. The `case:SINGLE_HOLD` works in conjunction with `ql_reset()` to switch to `_MY_LAYER` while the tap dance key is held, and to switch away from `_MY_LAYER` when the key is released. This mirrors the use of `MO(_MY_LAYER)`. The `case:DOUBLE_TAP` works by checking whether `_MY_LAYER` is the active layer, and toggling it on or off accordingly. This mirrors the use of `TG(_MY_LAYER)`.
+
+`tap_dance_actions[]` works similar to the above examples. Note that I used `ACTION_TAP_DANCE_FN_ADVANCED_TIME()` instead of `ACTION_TAP_DANCE_FN_ADVANCED()`. This is because I like my `TAPPING_TERM` to be short (~175ms) for my non-tap-dance keys but find that this is too quick for me to reliably complete tap dance actions - thus the increased time of 275ms here.
+
+Finally, to get this tap dance key working, be sure to include `TD(QUOT_LAYR)` in your `keymaps[]`.