att_server: allow individual service implementations to register for can send now callbacks

src/ble/att_server.c

@@ -95,6 +95,8 @@ static btstack_packet_callback_registration_t hci_event_callback_registration;
 static btstack_packet_callback_registration_t sm_event_callback_registration;
 static btstack_packet_handler_t att_client_packet_handler = NULL;
 
+static btstack_linked_list_t can_send_now_clients;
+
 static void att_handle_value_indication_notify_client(uint8_t status, uint16_t client_handle, uint16_t attribute_handle){
     if (!att_client_packet_handler) return;
     
@@ -345,12 +347,27 @@ static void att_server_handle_can_send_now(void){
 
     if (att_server_state == ATT_SERVER_REQUEST_RECEIVED_AND_VALIDATED){
         int sent = att_server_process_validated_request(att_connection.con_handle);
-        if (sent && att_client_waiting_for_can_send){
+        if (sent && (att_client_waiting_for_can_send || !btstack_linked_list_empty(&can_send_now_clients))){
             att_dispatch_server_request_can_send_now_event(att_connection.con_handle);
             return;
         }
     }
 
+    while (!btstack_linked_list_empty(&can_send_now_clients)){
+        // handle first client
+        btstack_context_callback_registration_t * client = (btstack_context_callback_registration_t*) can_send_now_clients;
+        btstack_linked_list_remove(&can_send_now_clients, (btstack_linked_item_t *) client);
+        client->callback(client->context);
+
+        // request again if needed
+        if (!att_dispatch_server_can_send_now(att_connection.con_handle)){
+            if (!btstack_linked_list_empty(&can_send_now_clients) || att_client_waiting_for_can_send){
+                att_dispatch_server_request_can_send_now_event(att_connection.con_handle);
+            }
+            return;
+        }
+    }
+
     if (att_client_waiting_for_can_send){
         att_client_waiting_for_can_send = 0;
         att_emit_can_send_now_event();
@@ -441,6 +458,16 @@ void att_server_request_can_send_now_event(hci_con_handle_t con_handle){
     att_dispatch_server_request_can_send_now_event(att_connection.con_handle);
 }
 
+void att_server_register_can_send_now_callback(btstack_context_callback_registration_t * callback_registration, hci_con_handle_t con_handle){
+    // check if can send already
+    if (att_dispatch_server_can_send_now(con_handle)){
+        callback_registration->callback(callback_registration->context);
+        return;
+    }
+    btstack_linked_list_add_tail(&can_send_now_clients, (btstack_linked_item_t*) callback_registration);
+    att_dispatch_server_request_can_send_now_event(con_handle);
+}
+
 int att_server_notify(hci_con_handle_t con_handle, uint16_t attribute_handle, uint8_t *value, uint16_t value_len){
     if (!att_dispatch_server_can_send_now(att_connection.con_handle)) return BTSTACK_ACL_BUFFERS_FULL;
 

src/ble/att_server.h

@@ -56,8 +56,9 @@ void att_server_init(uint8_t const * db, att_read_callback_t read_callback, att_
 
 /*
  * @brief register packet handler for ATT server events:
- *        - ATT_EVENT_MTU_EXCHANGE_COMPLETE 
+ *        - ATT_EVENT_CAN_SEND_NOW
  *        - ATT_EVENT_HANDLE_VALUE_INDICATION_COMPLETE
+ *        - ATT_EVENT_MTU_EXCHANGE_COMPLETE 
  * @param handler
  */
 void att_server_register_packet_handler(btstack_packet_handler_t handler);
@@ -77,6 +78,14 @@ int  att_server_can_send_packet_now(hci_con_handle_t con_handle);
  */
 void att_server_request_can_send_now_event(hci_con_handle_t con_handle);
 
+/** 
+ * @brief Request callback when sending is possible
+ * @note callback might happend during call to this function
+ * @param callback_registration to point to callback function and context information
+ * @param con_handle
+ */
+void att_server_register_can_send_now_callback(btstack_context_callback_registration_t * callback_registration, hci_con_handle_t con_handle);
+
 /*
  * @brief notify client about attribute value change
  * @param con_handle

src/btstack_defines.h

@@ -61,6 +61,13 @@ typedef struct {
     btstack_packet_handler_t callback;
 } btstack_packet_callback_registration_t;
 
+// context callback supporting multiple registrations
+typedef struct {
+  btstack_linked_item_t * item;
+  void (*callback)(void * context);
+  void * context;
+} btstack_context_callback_registration_t;
+
 /**
  * @brief 128 bit key used with AES128 in Security Manager
  */