BLE - no callback if client enables notifications (CCCD callbacks)

[beegee-tokyo]

Hardware:

Board: ESP32 dev board
Core Installation version: 1.0.4
IDE name: Platform.io
Flash Frequency: 40Mhz
PSRAM enabled: no
Upload Speed: 460800
Computer OS: Windows 7

Description:

@chegewara
I am working on a BLE UART connection between Android and ESP32.
After connection established, it takes some time before the Android app is enabling notification on the UartTX characteristic. On the ESP32 I found no way to get informed if and when the BLE client enabled notifications. If the ESP32 notifies data immediately after a connection is established, the data is lost, because the Android app is not notified about it.
What I would need is a callback (or flag to test) that tells me that the Android app has enabled notifications on the characteristic. On nRF52 Arduino is a callback for the CCCD requests from the client.
Maybe I am blind, but I went through the BLE library up and down, but I cannot find anything that would give me this information.

Sketch: BLE Uart example

#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

BLEServer *pServer = NULL;
BLECharacteristic *pTxCharacteristic;
bool deviceConnected = false;
bool oldDeviceConnected = false;
uint8_t txValue = 0;

// See the following for generating UUIDs:
// https://www.uuidgenerator.net/

#define SERVICE_UUID "6E400001-B5A3-F393-E0A9-E50E24DCCA9E" // UART service UUID
#define CHARACTERISTIC_UUID_RX "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TX "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"

class MyServerCallbacks : public BLEServerCallbacks
{
	void onConnect(BLEServer *pServer)
	{
		deviceConnected = true;
	};

	void onDisconnect(BLEServer *pServer)
	{
		deviceConnected = false;
	}
};

class MyCallbacks : public BLECharacteristicCallbacks
{
	void onWrite(BLECharacteristic *pCharacteristic)
	{
		std::string rxValue = pCharacteristic->getValue();

		if (rxValue.length() > 0)
		{
			Serial.println("*********");
			Serial.print("Received Value: ");
			for (int i = 0; i < rxValue.length(); i++)
				Serial.print(rxValue[i]);

			Serial.println();
			Serial.println("*********");
		}
	}
};

void setup()
{
	Serial.begin(115200);

	// Create the BLE Device
	BLEDevice::init("UART Service");

	// Create the BLE Server
	pServer = BLEDevice::createServer();
	pServer->setCallbacks(new MyServerCallbacks());

	// Create the BLE Service
	BLEService *pService = pServer->createService(SERVICE_UUID);

	// Create a BLE Characteristic
	pTxCharacteristic = pService->createCharacteristic(
		CHARACTERISTIC_UUID_TX,
		BLECharacteristic::PROPERTY_NOTIFY);

	pTxCharacteristic->addDescriptor(new BLE2902());

	BLECharacteristic *pRxCharacteristic = pService->createCharacteristic(
		CHARACTERISTIC_UUID_RX,
		BLECharacteristic::PROPERTY_WRITE);

	pRxCharacteristic->setCallbacks(new MyCallbacks());

	// Start the service
	pService->start();

	// Start advertising
	pServer->getAdvertising()->start();
	Serial.println("Waiting a client connection to notify...");
}

void loop()
{

	if (deviceConnected)
	{
                // ============================================
                // First write might fail because client has not yet enabled notifications
                // ============================================
		pTxCharacteristic->setValue(&txValue, 1);
		pTxCharacteristic->notify();
		txValue++;
		delay(10); // bluetooth stack will go into congestion, if too many packets are sent
	}

	// disconnecting
	if (!deviceConnected && oldDeviceConnected)
	{
		delay(500);					 // give the bluetooth stack the chance to get things ready
		pServer->startAdvertising(); // restart advertising
		Serial.println("start advertising");
		oldDeviceConnected = deviceConnected;
	}
	// connecting
	if (deviceConnected && !oldDeviceConnected)
	{
		// do stuff here on connecting
		oldDeviceConnected = deviceConnected;
	}
}```

### Debug Messages:

No crash, just missing a callback

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[STALE_SET] This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

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[GianCann]

@beegee-tokyo have you find a solution?

[adabru]

Don't know since when, but now you can use descriptor callbacks, see https://github.com/espressif/arduino-esp32/blob/master/libraries/BLE/src/BLEDescriptor.h :

class SubscribeCallbacks: public BLEDescriptorCallbacks {
  public:
    void onWrite(BLEDescriptor* pDescriptor)"
} subscribeCallback;

...

BLEDescriptor* pCCCD = new BLE2902();
pCCCD->setCallbacks(&subscribeCallback);
pCharacteristic->addDescriptor(pCCCD);

...

void SubscribeCallbacks::onWrite(BLEDescriptor* pDescriptor) {
  // do what you want
}