How do I know that characteristic has been subscribed to?

Question

I've tried getting a notification of subscription from a bluetooth event handler like so: 
 
 
 ble_evt_dispatch() is being called from main: 
 static void ble_evt_dispatch(ble_evt_t * p_ble_evt)
 {
 on_ble_evt(p_ble_evt);
 ble_conn_params_on_ble_evt(p_ble_evt);
 ble_MY_SERVICE_on_ble_evt(&MY_SERVICE_handle, p_ble_evt); // <- My service's event handler
 }
 
 
 
 My service's event handler does some crude event matching: 
 void ble_MY_SERVICE_on_ble_evt(MY_SERVICE_handle_t *MY_SERVICE_handle, ble_evt_t *p_ble_evt)
 {
 switch (p_ble_evt->header.evt_id)
 {
 case BLE_GAP_EVT_CONNECTED:
 case BLE_GAP_EVT_DISCONNECTED:
 ....
 case ???: {
 ???
 } break;
 }
 }
 
 
 
 At this point I'm stuck. When subscribing to a characteristic, p_ble_evt->header.evt_id equals to 0x0050, but since enums don't have explicit values in the code, I don't know which event this corresponds to and how to handle it. 
 And if I try and bruteforce it by hardcoding 0x0050 event handling, p_ble_evt->evt.gattc_evt.params.hvx.type always equals to 0xF1, which doesn't make sense since this parameter is of 'type' BLE_GATT_HVX_TYPES which can only be 0x00, 0x01 or 0x02, so I must be doing something wrong since I think its just garbage data.

Vebjørn · Accepted Answer

I agree that it is a bit difficult to find out which event an ID corresponds to, but you can find out like this: 
 
 Open ble_ranges.h and scroll down to #define BLE_EVT_BASE 
 Look at your event id (0x0050) and find out which range this lies in. Scrolling a bit down in ble_ranges.h we see that BLE_GATTS_EVT_BASE starts at 0x50 and stops at 0x6F, so the id is a GATTS event. 
 Open ble_gatts.h and find out where BLE_GATTS_EVT_BASE is used 
 We see that BLE_GATTS_EVT_WRITE is defined as BLE_GATTS_EVT_BASE (0x50) and the succeeding events are increments of this. (if your code was 0x53 you would have to count down along the enum declarations, and would arrive at BLE_GATTS_EVT_HVC). 
 
 So, what you are looking for in your switch-case is BLE_GATTS_EVT_WRITE. What happens when a device subscribes to a characteristic, it writes 1 to the characteristic CCCD handle. There are examples of catching this in most of the projects in the SDK. If you look in ble_app_hrs for example, you can see the following in the ble_hrs_on_ble_evt function: 
 case BLE_GATTS_EVT_WRITE:
 on_write(p_hrs, p_ble_evt);
 break;
 
 Then the on_write function checks if it is a write to the cccd handle: 
 static void on_write(ble_hrs_t * p_hrs, ble_evt_t * p_ble_evt){

 ble_gatts_evt_write_t * p_evt_write = &p_ble_evt->evt.gatts_evt.params.write;

 if (p_evt_write->handle == p_hrs->hrm_handles.cccd_handle){
 on_hrm_cccd_write(p_hrs, p_evt_write);
 }
 }
 
 and lastly the on_hrm_cccd_write function checks if notification has been enabled or disabled, and you can handle this acccordingly, in that function

How do I know that characteristic has been subscribed to?

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