Can't get notify to work in Web Bluetooth API

Question

I'm developing a software who will use Web Bluetooth API to connect to a BT to Serial Adapter. It seems to have support for Write and Notify. But I can't get nofity to work.

The event is never fired. Right now I'm testing in Canary on Mac.

Thanks Anders

My code for search/pair and add events:

var readCharacteristic;
var writeCharacteristic;
var serialBLEService = 'ba920001-d666-4d59-b141-05960b3a4ff7';
var txChar = 'ba920002-d666-4d59-b141-05960b3a4ff7';
var rxChar = 'ba920003-d666-4d59-b141-05960b3a4ff7';

$scope.writeToSerial = function (valueToWrite) {
                var tmpValue = $('input').val();

                valueToWrite = utf8AbFromStr(tmpValue);

                writeCharacteristic.writeValue(valueToWrite)
                    .then( a => {
                        alert("Written: " + valueToWrite);
                    })
                    .catch(function (error) {
                        // And of course: error handling!
                        console.error('Something went wrong!', error);
                    });
            }

function handleCharacteristicValueChanged(event) {
                var value = event.target.value;
                console.log('Received ' + value);
            }

$scope.searchForBTDevices = function() {
                navigator.bluetooth.requestDevice({
                        filters: [{ services: [serialBLEService] }],
                        optionalServices: [
                            serialBLEService, rxChar, txChar, configChar
                        ]

                    })
                    .then(device => {
                        return device.gatt.connect();

                    })
                    .then(server => {
                        return server.getPrimaryService(serialBLEService);
                    })
                    .then(service => {
                        return service.getCharacteristics();
                    })
                    .then(characteristics => {
                        $scope.btResult += '>> Found Characteristics!\n';
                        $scope.$apply();
                        var queue = Promise.resolve();
                        characteristics.forEach(characteristic => {
                            switch (characteristic.uuid) {

                            case rxChar:

                                readCharacteristic = characteristic;
                                readCharacteristic.addEventListener('characteristicvaluechanged',
                                    handleCharacteristicValueChanged);

                                break;

                            case txChar:
                                writeCharacteristic = characteristic;
                                break;


                            }
                        });

                    })
                    .catch(error => {
                        $scope.btResult = '>> Error:  ' + error;
                        $scope.$digest();
                    });
            };

Answer 1

Yes, you're absolutely right. There was 2 errors first in baud rate against my serial to BT adapter which made that I didn't send what I thought I was. And the other was that startnotifications was not called.

Thanks Anders

Answer 2

According to https://developers.google.com/web/updates/2015/07/interact-with-ble-devices-on-the-web#receive_gatt_notifications, it looks like you need to call characteristic.startNotifications() to let the browser know you want to receive GATT notifications:

navigator.bluetooth.requestDevice({ filters: [{ services: ['heart_rate'] }] })
.then(device => device.gatt.connect())
.then(server => server.getPrimaryService('heart_rate'))
.then(service => service.getCharacteristic('heart_rate_measurement'))
.then(characteristic => characteristic.startNotifications())
.then(characteristic => {
  characteristic.addEventListener('characteristicvaluechanged',
                                  handleCharacteristicValueChanged);
  console.log('Notifications have been started.');
})
.catch(error => { console.log(error); });

function handleCharacteristicValueChanged(event) {
  var value = event.target.value;
  console.log('Received ' + value);
  // TODO: Parse Heart Rate Measurement value.
  // See https://github.com/WebBluetoothCG/demos/blob/gh-pages/heart-rate-sensor/heartRateSensor.js
}