Web Bluetooth / Write Descriptor 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 (Async Await) sample.

Live Output


JavaScript Snippet

var myDescriptor;

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);
  }

  log('Requesting any Bluetooth Device...');
  navigator.bluetooth.requestDevice({
   // filters: [...] <- Prefer filters to save energy & show relevant devices.
      acceptAllDevices: true,
      optionalServices: [serviceUuid]})
  .then(device => {
    log('Connecting to GATT Server...');
    return device.gatt.connect();
  })
  .then(server => {
    log('Getting Service...');
    return server.getPrimaryService(serviceUuid);
  })
  .then(service => {
    log('Getting Characteristic...');
    return service.getCharacteristic(characteristicUuid);
  })
  .then(characteristic => {
    log('Getting Descriptor...');
    return characteristic.getDescriptor('gatt.characteristic_user_description');
  })
  .then(descriptor => {
    document.querySelector('#writeButton').disabled =
        !descriptor.characteristic.properties.write;
    myDescriptor = descriptor;
    log('Reading Descriptor...');
    return descriptor.readValue();
  })
  .then(value => {
    let decoder = new TextDecoder('utf-8');
    log('> Characteristic User Description: ' + decoder.decode(value));
  })
  .catch(error => {
    document.querySelector('#writeButton').disabled = true;
    log('Argh! ' + error);
  });
}

function onWriteButtonClick() {
  if (!myDescriptor) {
    return;
  }
  let encoder = new TextEncoder('utf-8');
  let value = document.querySelector('#description').value;
  log('Setting Characteristic User Description...');
  myDescriptor.writeValue(encoder.encode(value))
  .then(_ => {
    log('> Characteristic User Description changed to: ' + value);
  })
  .catch(error => {
    log('Argh! ' + error);
  });
}