Web Bluetooth / Notifications (Async Await) Sample

Available in Chrome 55+ | View on GitHub | Browse Samples

The Web Bluetooth API lets websites discover and communicate with devices over the Bluetooth 4 wireless standard using the Generic Attribute Profile (GATT). It is currently partially implemented in Android M, Chrome OS, Linux, and Mac.

This sample illustrates the use of the Web Bluetooth API to start and stop characteristic notifications from a nearby Bluetooth Low Energy Device. You may want to try this demo with the BLE Peripheral Simulator App from the Google Play Store and check out the Notifications (Promises) sample.

Live Output


JavaScript Snippet

var myCharacteristic;

async function onStartButtonClick() {
  let serviceUuid = document.querySelector('#service').value;
  if (serviceUuid.startsWith('0x')) {
    serviceUuid = parseInt(serviceUuid);
  }

  let characteristicUuid = document.querySelector('#characteristic').value;
  if (characteristicUuid.startsWith('0x')) {
    characteristicUuid = parseInt(characteristicUuid);
  }

  try {
    log('Requesting Bluetooth Device...');
    const device = await navigator.bluetooth.requestDevice({
        filters: [{services: [serviceUuid]}]});

    log('Connecting to GATT Server...');
    const server = await device.gatt.connect();

    log('Getting Service...');
    const service = await server.getPrimaryService(serviceUuid);

    log('Getting Characteristic...');
    myCharacteristic = await service.getCharacteristic(characteristicUuid);

    await myCharacteristic.startNotifications();

    log('> Notifications started');
    myCharacteristic.addEventListener('characteristicvaluechanged',
        handleNotifications);
  } catch(error) {
    log('Argh! ' + error);
  }
}

async function onStopButtonClick() {
  if (myCharacteristic) {
    try {
      await myCharacteristic.stopNotifications();
      log('> Notifications stopped');
      myCharacteristic.removeEventListener('characteristicvaluechanged',
          handleNotifications);
    } catch(error) {
      log('Argh! ' + error);
    }
  }
}

function handleNotifications(event) {
  let value = event.target.value;
  let a = [];
  // Convert raw data bytes to hex values just for the sake of showing something.
  // In the "real" world, you'd use data.getUint8, data.getUint16 or even
  // TextDecoder to process raw data bytes.
  for (let i = 0; i < value.byteLength; i++) {
    a.push('0x' + ('00' + value.getUint8(i).toString(16)).slice(-2));
  }
  log('> ' + a.join(' '));
}