add ble exposition

config/boards/shields/corney/corney.conf

@@ -8,4 +8,7 @@ CONFIG_ZMK_MOUSE_TICK_DURATION=8
 # Optional: Adjust mouse movement speed (default is 500)
 CONFIG_ZMK_MOUSE_DEFAULT_SPEED=500
 
+# Expose active layer over GATT
+CONFIG_ZMK_GATT_LAYER_EXPOSITION=y
+
 ZMK_KEYBOARD_NAME="Corney"

docs/gatt-layer-exposition.md

@@ -14,3 +14,11 @@ This document defines the custom GATT service and characteristic used to expose
 ## Properties
 - Read
 - Notify
+
+## Configuration
+- Kconfig: `CONFIG_ZMK_GATT_LAYER_EXPOSITION`
+- This repo enables the feature in `config/boards/shields/corney/corney.conf`.
+
+## Validation
+- Connect with a BLE GATT browser and read the Layer Number characteristic to confirm it returns the active layer index.
+- Subscribe to notifications, switch layers on the keyboard, and verify notifications contain the updated layer index.

openspec/changes/add-gatt-layer-exposition/tasks.md

@@ -1,6 +1,6 @@
 ## 1. Implementation
 - [x] 1.1 Define the GATT service/characteristic UUIDs and data format for layer number exposure
-- [ ] 1.2 Add a read characteristic that returns the current active layer number
-- [ ] 1.3 Add notify support and emit updates on layer change when clients subscribe
-- [ ] 1.4 Add configuration and documentation for enabling/disabling the feature
-- [ ] 1.5 Add tests or validation steps for read/notify behavior
+- [x] 1.2 Add a read characteristic that returns the current active layer number
+- [x] 1.3 Add notify support and emit updates on layer change when clients subscribe
+- [x] 1.4 Add configuration and documentation for enabling/disabling the feature
+- [x] 1.5 Add tests or validation steps for read/notify behavior

zephyr/CMakeLists.txt

@@ -0,0 +1,3 @@
+zephyr_library()
+zephyr_library_sources_ifdef(CONFIG_ZMK_GATT_LAYER_EXPOSITION src/gatt_layer_exposition.c)
+

zephyr/Kconfig

@@ -0,0 +1,8 @@
+menuconfig ZMK_GATT_LAYER_EXPOSITION
+    bool "Expose active layer number over GATT"
+    depends on ZMK_BLE && BT_GATT
+    help
+      Adds a custom GATT service/characteristic that exposes the currently
+      active layer number as a readable byte and optionally notifies on
+      layer changes.
+

zephyr/module.yml

@@ -1,3 +1,5 @@
 build:
+  cmake: zephyr
+  kconfig: zephyr/Kconfig
   settings:
     board_root: .

zephyr/src/gatt_layer_exposition.c

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2026 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/bluetooth/gatt.h>
+#include <zephyr/bluetooth/uuid.h>
+#include <zephyr/kernel.h>
+
+#include <stdint.h>
+#include <zmk/event_manager.h>
+#include <zmk/events/layer_state_changed.h>
+#include <zmk/keymap.h>
+
+#include <zephyr/logging/log.h>
+
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#define ZMK_GATT_LAYER_SERVICE_UUID                                                               \
+    BT_UUID_DECLARE_128(BT_UUID_128_ENCODE(0x715d81e1, 0x377d, 0x4f26, 0xa678, 0xa506675d99ec))
+
+#define ZMK_GATT_LAYER_CHAR_UUID                                                                  \
+    BT_UUID_DECLARE_128(BT_UUID_128_ENCODE(0xe87c518a, 0xf323, 0x4dd7, 0x9dd5, 0x0991add1c01b))
+
+enum {
+    LAYER_SVC_ATTR_PRIMARY,
+    LAYER_SVC_ATTR_CHAR_DECL,
+    LAYER_SVC_ATTR_CHAR_VALUE,
+    LAYER_SVC_ATTR_CHAR_CCC,
+    LAYER_SVC_ATTR_COUNT,
+};
+
+static bool layer_notify_enabled;
+static uint8_t last_layer_index = UINT8_MAX;
+
+static uint8_t current_layer_index(void) {
+    return (uint8_t)zmk_keymap_highest_layer_active();
+}
+
+static ssize_t read_layer(struct bt_conn *conn, const struct bt_gatt_attr *attr, void *buf,
+                          uint16_t len, uint16_t offset) {
+    uint8_t layer = current_layer_index();
+    return bt_gatt_attr_read(conn, attr, buf, len, offset, &layer, sizeof(layer));
+}
+
+static void layer_ccc_cfg_changed(const struct bt_gatt_attr *attr, uint16_t value) {
+    ARG_UNUSED(attr);
+    layer_notify_enabled = (value == BT_GATT_CCC_NOTIFY);
+}
+
+BT_GATT_SERVICE_DEFINE(layer_svc, BT_GATT_PRIMARY_SERVICE(ZMK_GATT_LAYER_SERVICE_UUID),
+                       BT_GATT_CHARACTERISTIC(ZMK_GATT_LAYER_CHAR_UUID,
+                                              BT_GATT_CHRC_READ | BT_GATT_CHRC_NOTIFY,
+                                              BT_GATT_PERM_READ, read_layer, NULL, NULL),
+                       BT_GATT_CCC(layer_ccc_cfg_changed, BT_GATT_PERM_READ | BT_GATT_PERM_WRITE));
+
+static void notify_layer_change(uint8_t layer_index) {
+    if (!layer_notify_enabled) {
+        return;
+    }
+
+    int err = bt_gatt_notify(NULL, &layer_svc.attrs[LAYER_SVC_ATTR_CHAR_VALUE], &layer_index,
+                             sizeof(layer_index));
+    if (err) {
+        LOG_DBG("Layer notify failed (%d)", err);
+    }
+}
+
+static int layer_state_changed_listener(const zmk_event_t *eh) {
+    if (as_zmk_layer_state_changed(eh) == NULL) {
+        return -ENOTSUP;
+    }
+
+    uint8_t layer_index = current_layer_index();
+    if (layer_index == last_layer_index) {
+        return 0;
+    }
+
+    last_layer_index = layer_index;
+    notify_layer_change(layer_index);
+    return 0;
+}
+
+ZMK_LISTENER(layer_gatt, layer_state_changed_listener);
+ZMK_SUBSCRIPTION(layer_gatt, zmk_layer_state_changed);