Web Bluetooth / Write Descriptor (Async Await) Sample

Available in Chrome 58+ | 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 write to the descriptor "Characteristic User Description" on 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 Write Descriptor (Promises) sample.

Live Output


JavaScript Snippet

var myDescriptor;

async function onReadButtonClick() {
  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 any Bluetooth Device...');
    const device = await navigator.bluetooth.requestDevice({
     // filters: [...] <- Prefer filters to save energy & show relevant devices.
        acceptAllDevices: true, optionalServices: [serviceUuid]});

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

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

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

    log('Getting Descriptor...');
    myDescriptor = await characteristic.getDescriptor('gatt.characteristic_user_description');

    document.querySelector('#writeButton').disabled =
        !characteristic.properties.write;

    log('Reading Descriptor...');
    const value = await myDescriptor.readValue();

    let decoder = new TextDecoder('utf-8');
    log('> Characteristic User Description: ' + decoder.decode(value));
  } catch(error) {
    document.querySelector('#writeButton').disabled = true;
    log('Argh! ' + error);
  }
}

async function onWriteButtonClick() {
  if (!myDescriptor) {
    return;
  }
  let encoder = new TextEncoder('utf-8');
  let value = document.querySelector('#description').value;
  try {
    log('Setting Characteristic User Description...');
    await myDescriptor.writeValue(encoder.encode(value));

    log('> Characteristic User Description changed to: ' + value);
  } catch(error) {
    log('Argh! ' + error);
  }
}