Add some explanations on consumed modifiers

This should hopefully clarify this somewhat subtle point to the
uninitiated users.

Signed-off-by: Ran Benita <ran234@gmail.com>

1 files changed, 43 insertions, 2 deletions

diff --git a/xkbcommon/xkbcommon.h b/xkbcommon/xkbcommon.h
index 65c4dcc..f8b3d8b 100644
--- a/xkbcommon/xkbcommon.h
+++ b/xkbcommon/xkbcommon.h
@@ -1331,12 +1331,53 @@ xkb_state_mod_indices_are_active(struct xkb_state *state,
  * active, so it is not consumed by this translation.
  *
  * It may be desireable for some application to not reuse consumed modifiers
- * for further processing, e.g. for hotkeys or keyboard shortcuts.
+ * for further processing, e.g. for hotkeys or keyboard shortcuts. To
+ * understand why, consider some requirements from a standard shortcut
+ * mechanism, and how they are implemented:
+ *
+ * 1. The shortcut's modifiers must match exactly to the state. For example,
+ *    it is possible to bind separate actions to \<Alt\>\<Tab\> and to
+ *    \<Alt\>\<Shift\>\<Tab\>. Further, if only \<Alt\>\<Tab\> is bound to
+ *    an action, pressing \<Alt\>\<Shift\>\<Tab\> should not trigger the
+ *    shortcut.
+ *    Effectively, this means that the modifiers are compared using the
+ *    equality operator (==).
+ * 2. Only relevant modifiers are considered for the matching. For example,
+ *    Caps Lock and Num Lock should not generally affect the matching, e.g.
+ *    when matching \<Alt\>\<Tab\> against the state, it does not matter
+ *    whether Num Lock is active or not. These relevant, or significant,
+ *    modifiers usually include Alt, Control, Shift, Super and similar.
+ *    Effectively, this means that non-significant modifiers are masked out,
+ *    before doing the comparison as described above.
+ * 3. The matching must be independent of the layout/keymap. For example,
+ *    the \<Plus\> (+) symbol is found on the first level on some layouts,
+ *    and requires holding Shift on others. If you simply bind the action
+ *    to the \<Plus\> keysym, it would work for the unshifted kind, but
+ *    not for the others, because the match against Shift would fail. If
+ *    you bind the action to \<Shift\>\<Plus\>, only the shifted kind would
+ *    work. So what is needed is to recognize that Shift is used up in the
+ *    translation of the keysym itself, and therefore should not be included
+ *    in the matching.
+ *    Effectively, this means that consumed modifiers (Shift in this example)
+ *    are masked out as well, before doing the comparison.
+ *
+ * To summarize, this is how the matching would be performed:
+ * @code
+ *   (keysym == shortcut_keysym) &&
+ *   ((state_modifiers & ~consumed_modifiers & significant_modifiers) == shortcut_modifiers)
+ * @endcode
+ *
+ * @c state_modifiers are the modifiers reported by
+ * xkb_state_mod_index_is_active() and similar functions.
+ * @c consumed_modifiers are the modifiers reported by
+ * xkb_state_mod_index_is_consumed().
+ * @c significant_modifiers are decided upon by the application/toolkit/user;
+ * it is up to them to decide whether these are configurable or hard-coded.
  *
  * @returns 1 if the modifier is consumed, 0 if it is not.  If the modifier
  * index is not valid in the keymap, returns -1.
  *
- * @sa xkb_state_mod_mask_remove_consumend()
+ * @sa xkb_state_mod_mask_remove_consumed()
  * @memberof xkb_state
  */
 int