feat(behaviors): &key_repeat behavior + tests.

* Add new `&key_repeat` behavior that captures and re-sends
  the most recently triggered keycode.

Closes: #853

app/src/behaviors/behavior_key_repeat.c

@@ -0,0 +1,124 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define DT_DRV_COMPAT zmk_behavior_key_repeat
+
+#include <device.h>
+#include <drivers/behavior.h>
+#include <logging/log.h>
+#include <zmk/behavior.h>
+
+#include <zmk/event_manager.h>
+#include <zmk/events/keycode_state_changed.h>
+
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#if DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT)
+
+struct behavior_key_repeat_config {
+    uint8_t index;
+    uint8_t usage_pages_count;
+    uint16_t usage_pages[];
+};
+
+struct behavior_key_repeat_data {
+    struct zmk_keycode_state_changed last_keycode_pressed;
+    struct zmk_keycode_state_changed current_keycode_pressed;
+};
+
+static int on_key_repeat_binding_pressed(struct zmk_behavior_binding *binding,
+                                         struct zmk_behavior_binding_event event) {
+    const struct device *dev = device_get_binding(binding->behavior_dev);
+    struct behavior_key_repeat_data *data = dev->data;
+
+    if (data->last_keycode_pressed.usage_page == 0) {
+        return ZMK_BEHAVIOR_OPAQUE;
+    }
+
+    memcpy(&data->current_keycode_pressed, &data->last_keycode_pressed,
+           sizeof(struct zmk_keycode_state_changed));
+    data->current_keycode_pressed.timestamp = k_uptime_get();
+
+    ZMK_EVENT_RAISE(new_zmk_keycode_state_changed(data->current_keycode_pressed));
+
+    return ZMK_BEHAVIOR_OPAQUE;
+}
+
+static int on_key_repeat_binding_released(struct zmk_behavior_binding *binding,
+                                          struct zmk_behavior_binding_event event) {
+    const struct device *dev = device_get_binding(binding->behavior_dev);
+    struct behavior_key_repeat_data *data = dev->data;
+
+    if (data->current_keycode_pressed.usage_page == 0) {
+        return ZMK_BEHAVIOR_OPAQUE;
+    }
+
+    data->current_keycode_pressed.timestamp = k_uptime_get();
+    data->current_keycode_pressed.state = false;
+
+    ZMK_EVENT_RAISE(new_zmk_keycode_state_changed(data->current_keycode_pressed));
+    return ZMK_BEHAVIOR_OPAQUE;
+}
+
+static const struct behavior_driver_api behavior_key_repeat_driver_api = {
+    .binding_pressed = on_key_repeat_binding_pressed,
+    .binding_released = on_key_repeat_binding_released,
+};
+
+static int key_repeat_keycode_state_changed_listener(const zmk_event_t *eh);
+
+ZMK_LISTENER(behavior_key_repeat, key_repeat_keycode_state_changed_listener);
+ZMK_SUBSCRIPTION(behavior_key_repeat, zmk_keycode_state_changed);
+
+static const struct device *devs[DT_NUM_INST_STATUS_OKAY(DT_DRV_COMPAT)];
+
+static int key_repeat_keycode_state_changed_listener(const zmk_event_t *eh) {
+    struct zmk_keycode_state_changed *ev = as_zmk_keycode_state_changed(eh);
+    if (ev == NULL || !ev->state) {
+        return ZMK_EV_EVENT_BUBBLE;
+    }
+
+    for (int i = 0; i < DT_NUM_INST_STATUS_OKAY(DT_DRV_COMPAT); i++) {
+        const struct device *dev = devs[i];
+        if (dev == NULL) {
+            continue;
+        }
+
+        struct behavior_key_repeat_data *data = dev->data;
+        const struct behavior_key_repeat_config *config = dev->config;
+
+        for (int u = 0; u < config->usage_pages_count; u++) {
+            if (config->usage_pages[u] == ev->usage_page) {
+                memcpy(&data->last_keycode_pressed, ev, sizeof(struct zmk_keycode_state_changed));
+                break;
+            }
+        }
+    }
+
+    return ZMK_EV_EVENT_BUBBLE;
+}
+
+static int behavior_key_repeat_init(const struct device *dev) {
+    const struct behavior_key_repeat_config *config = dev->config;
+    devs[config->index] = dev;
+    return 0;
+}
+
+#define KR_INST(n)                                                                                 \
+    static struct behavior_key_repeat_data behavior_key_repeat_data_##n = {};                      \
+    static struct behavior_key_repeat_config behavior_key_repeat_config_##n = {                    \
+        .index = n,                                                                                \
+        .usage_pages = DT_INST_PROP(n, usage_pages),                                               \
+        .usage_pages_count = DT_INST_PROP_LEN(n, usage_pages),                                     \
+    };                                                                                             \
+    DEVICE_DT_INST_DEFINE(n, behavior_key_repeat_init, device_pm_control_nop,                      \
+                          &behavior_key_repeat_data_##n, &behavior_key_repeat_config_##n,          \
+                          APPLICATION, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT,                        \
+                          &behavior_key_repeat_driver_api);
+
+DT_INST_FOREACH_STATUS_OKAY(KR_INST)
+
+#endif