Add leader_add_user callback (#24266)

docs/features/leader_key.md

@@ -154,6 +154,21 @@ User callback, invoked when the leader sequence ends.
 
 ---
 
+### `bool leader_add_user(uint16_t keycode)` {#api-leader-add-user}
+
+User callback, invoked when a keycode is added to the leader sequence.
+
+#### Arguments {#api-leader-add-user-arguments}
+
+ - `uint16_t keycode`  
+   The keycode to added to the leader sequence.
+
+#### Return Value {#api-leader-add-user-return}
+
+`true` to finish the key sequence, `false` to continue.
+
+---
+
 ### `void leader_start(void)` {#api-leader-start}
 
 Begin the leader sequence, resetting the buffer and timer.

quantum/leader.c

@@ -21,6 +21,10 @@ __attribute__((weak)) void leader_start_user(void) {}
 
 __attribute__((weak)) void leader_end_user(void) {}
 
+__attribute__((weak)) bool leader_add_user(uint16_t keycode) {
+    return false;
+}
+
 void leader_start(void) {
     if (leading) {
         return;
@@ -61,6 +65,9 @@ bool leader_sequence_add(uint16_t keycode) {
     leader_sequence[leader_sequence_size] = keycode;
     leader_sequence_size++;
 
+    if (leader_add_user(keycode)) {
+        leader_end();
+    }
     return true;
 }
 

quantum/leader.h

@@ -21,6 +21,15 @@ void leader_start_user(void);
  */
 void leader_end_user(void);
 
+/**
+ * \brief User callback, invoked when a keycode is added to the leader sequence.
+ *
+ * \param keycode The keycode added to the leader sequence.
+ *
+ * \return `true` to finish the key sequence, `false` to continue.
+ */
+bool leader_add_user(uint16_t keycode);
+
 /**
  * Begin the leader sequence, resetting the buffer and timer.
  */