How do I calculate a characteristic value in real-time ?

[Andrew Ong]

When my mobile phone reads my characteristic, I would like the characteristic to be calculated at the same time, and not before. i.e. like a video game that does not render the world until the camera looks at it. Is that possible? How do I implement it? 

Currently I have a read-only characteristic but it doesn't seem like my application code can react in time to give the mobile phone a value. I think the nRF can only get notification AFTER the value has been read, but not during. Is this correct? 

I can think of this workaround: 

• Make my characteristic a read/write, have the mobile phone write 1 to the nRF as a request, then have the nRF send a Notification back with the data. 

Is there a better way?

[pabigot]

Set rd_auth in the characteristic value ble_gatts_attr_md_t referenced by ble_gatts_attr_t::p_attr_md.  When a read request is received the soft device will generate a BLE_GATTS_EVT_RW_AUTHORIZE_REQUEST event; prior to (or as a side effect of) invoking sd_ble_gatts_rw_authorize_reply() in the handler you can update the value.  (edited to correct types).