How does a callback function get revoke itself eg. Connect function get revoked when there is new connection

Question

Further to my previous question here

Can anyone help me understand how the callback function gets revoked itself? The connect function, disconnect function.

BLE custom UUID code source here

Another example is BLEUART which have callback functions:

/*********************************************************************
 This is an example for our nRF52 based Bluefruit LE modules
 Pick one up today in the adafruit shop!
 Adafruit invests time and resources providing this open source code,
 please support Adafruit and open-source hardware by purchasing
 products from Adafruit!
 MIT license, check LICENSE for more information
 All text above, and the splash screen below must be included in
 any redistribution
*********************************************************************/
/*
 * This sketch demonstrate the central API(). A additional bluefruit
 * that has bleuart as peripheral is required for the demo.
 */
#include <bluefruit.h>
BLEClientBas clientBas; // battery client
BLEClientDis clientDis; // device information client
BLEClientUart clientUart; // bleuart client
void setup()
{
 Serial.begin(115200);
// while ( !Serial ) delay(10); // for nrf52840 with native usb
 Serial.println("Bluefruit52 Central BLEUART Example");
 Serial.println("-----------------------------------\n");
 
 // Initialize Bluefruit with maximum connections as Peripheral = 0, Central = 1
 // SRAM usage required by SoftDevice will increase dramatically with number of connections
 Bluefruit.begin(0, 1);
 
 Bluefruit.setName("Bluefruit52 Central");
 // Configure Battery client
 clientBas.begin(); 
 // Configure DIS client
 clientDis.begin();
 // Init BLE Central Uart Serivce
 clientUart.begin();
 clientUart.setRxCallback(bleuart_rx_callback);
 // Increase Blink rate to different from PrPh advertising mode
 Bluefruit.setConnLedInterval(250);
 // Callbacks for Central
 Bluefruit.Central.setConnectCallback(connect_callback);
 Bluefruit.Central.setDisconnectCallback(disconnect_callback);
 /* Start Central Scanning
 * - Enable auto scan if disconnected
 * - Interval = 100 ms, window = 80 ms
 * - Don't use active scan
 * - Start(timeout) with timeout = 0 will scan forever (until connected)
 */
 Bluefruit.Scanner.setRxCallback(scan_callback);
 Bluefruit.Scanner.restartOnDisconnect(true);
 Bluefruit.Scanner.setInterval(160, 80); // in unit of 0.625 ms
 Bluefruit.Scanner.useActiveScan(false);
 Bluefruit.Scanner.start(0); // // 0 = Don't stop scanning after n seconds
}
/**
 * Callback invoked when scanner pick up an advertising data
 * @param report Structural advertising data
 */
void scan_callback(ble_gap_evt_adv_report_t* report)
{
 // Check if advertising contain BleUart service
 if ( Bluefruit.Scanner.checkReportForService(report, clientUart) )
 {
 Serial.print("BLE UART service detected. Connecting ... ");
 // Connect to device with bleuart service in advertising
 Bluefruit.Central.connect(report);
 }else
 { 
 // For Softdevice v6: after received a report, scanner will be paused
 // We need to call Scanner resume() to continue scanning
 Bluefruit.Scanner.resume();
 }
}
/**
 * Callback invoked when an connection is established
 * @param conn_handle
 */
void connect_callback(uint16_t conn_handle)
{
 Serial.println("Connected");
 Serial.print("Dicovering Device Information ... ");
 if ( clientDis.discover(conn_handle) )
 {
 Serial.println("Found it");
 char buffer[32+1];
 
 // read and print out Manufacturer
 memset(buffer, 0, sizeof(buffer));
 if ( clientDis.getManufacturer(buffer, sizeof(buffer)) )
 {
 Serial.print("Manufacturer: ");
 Serial.println(buffer);
 }
 // read and print out Model Number
 memset(buffer, 0, sizeof(buffer));
 if ( clientDis.getModel(buffer, sizeof(buffer)) )
 {
 Serial.print("Model: ");
 Serial.println(buffer);
 }
 Serial.println();
 }else
 {
 Serial.println("Found NONE");
 }
 Serial.print("Dicovering Battery ... ");
 if ( clientBas.discover(conn_handle) )
 {
 Serial.println("Found it");
 Serial.print("Battery level: ");
 Serial.print(clientBas.read());
 Serial.println("%");
 }else
 {
 Serial.println("Found NONE");
 }
 Serial.print("Discovering BLE Uart Service ... ");
 if ( clientUart.discover(conn_handle) )
 {
 Serial.println("Found it");
 Serial.println("Enable TXD's notify");
 clientUart.enableTXD();
 Serial.println("Ready to receive from peripheral");
 }else
 {
 Serial.println("Found NONE");
 
 // disconnect since we couldn't find bleuart service
 Bluefruit.disconnect(conn_handle);
 } 
}
/**
 * Callback invoked when a connection is dropped
 * @param conn_handle
 * @param reason is a BLE_HCI_STATUS_CODE which can be found in ble_hci.h
 */
void disconnect_callback(uint16_t conn_handle, uint8_t reason)
{
 (void) conn_handle;
 (void) reason;
 
 Serial.print("Disconnected, reason = 0x"); Serial.println(reason, HEX);
}
/**
 * Callback invoked when uart received data
 * @param uart_svc Reference object to the service where the data 
 * arrived. In this example it is clientUart
 */
void bleuart_rx_callback(BLEClientUart& uart_svc)
{
 Serial.print("[RX]: ");
 
 while ( uart_svc.available() )
 {
 Serial.print( (char) uart_svc.read() );
 }
 Serial.println();
}
void loop()
{
 if ( Bluefruit.Central.connected() )
 {
 // Not discovered yet
 if ( clientUart.discovered() )
 {
 // Discovered means in working state
 // Get Serial input and send to Peripheral
 if ( Serial.available() )
 {
 delay(2); // delay a bit for all characters to arrive
 
 char str[20+1] = { 0 };
 Serial.readBytes(str, 20);
 
 clientUart.print( str );
 }
 }
 }
}

The following helps set the callback functions:

clientUart.setRxCallback(bleuart_rx_callback);
...
Bluefruit.Central.setConnectCallback(connect_callback);
...
Bluefruit.Central.setDisconnectCallback(disconnect_callback);

Tracing back the libraries I found :

void BLEPeriph::setConnectCallback( ble_connect_callback_t fp )
{
 _connect_cb = fp;
}
void BLEPeriph::setDisconnectCallback( ble_disconnect_callback_t fp )
{
 _disconnect_cb = fp;
}

But I can't understand how this initializes the calling of the connect or disconnect function while getting a new connection or break in connection respectively.

Answer 1

All a callback function assignment does is link an internal pointer to the function that you provide.

Any time the callback is needed to be called the code just uses the pointer that your function has been assigned to.

Thus _connect_cb and connect_callback() are one and the same thing. Any time connect_callback() would be expected to be used _connect_cb() is used instead.

It is, in effect, just creating an internal alias to your code then using that alias to call your code when it needs to.

• Just doin Gods work: Okay I got the pointer assigning part clear and _connect_cb have assigned value of the function connect_callback(). But the one more thing I need to clarify is where did I call _connect_cb in my main code? When there is new connection it get revoke itself. So I want to know is it defined somewhere in the library?
• Majenko: You don't call anything. That's the whole point of a callback - it's the library that calls it. Somewhere in the library there will be a call to _connect_cb().
• Juraj: @JustdoinGodswork, you call BLE functions in loop(). any of them can call the callback
• Just doin Gods work: Okay got it, I'll try searching the connect_cb() in the libraries.