Web Bluetooth / Read Characteristic Value Changed Sample

Available in Chrome 48+ | 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 read battery level and be notified of changes from a nearby Bluetooth Device advertising Battery information with Bluetooth Low Energy. You may want to try this demo with the BLE Peripheral Simulator App from the Google Play Store and check out the Read Characteristic Value Changed Sample (Async Await) sample.

Here, we use the characteristicvaluechanged event listener to handle reading battery level characteristic value. This event listener will optionally handle upcoming notifications as well.

Live Output


JavaScript Snippet

var bluetoothDevice;
var batteryLevelCharacteristic;

function onReadBatteryLevelButtonClick() {
  return (bluetoothDevice ? Promise.resolve() : requestDevice())
  .then(connectDeviceAndCacheCharacteristics)
  .then(_ => {
    log('Reading Battery Level...');
    return batteryLevelCharacteristic.readValue();
  })
  .catch(error => {
    log('Argh! ' + error);
  });
}

function requestDevice() {
  log('Requesting any Bluetooth Device...');
  return navigator.bluetooth.requestDevice({
   // filters: [...] <- Prefer filters to save energy & show relevant devices.
      acceptAllDevices: true,
      optionalServices: ['battery_service']})
  .then(device => {
    bluetoothDevice = device;
    bluetoothDevice.addEventListener('gattserverdisconnected', onDisconnected);
  });
}

function connectDeviceAndCacheCharacteristics() {
  if (bluetoothDevice.gatt.connected && batteryLevelCharacteristic) {
    return Promise.resolve();
  }

  log('Connecting to GATT Server...');
  return bluetoothDevice.gatt.connect()
  .then(server => {
    log('Getting Battery Service...');
    return server.getPrimaryService('battery_service');
  })
  .then(service => {
    log('Getting Battery Level Characteristic...');
    return service.getCharacteristic('battery_level');
  })
  .then(characteristic => {
    batteryLevelCharacteristic = characteristic;
    batteryLevelCharacteristic.addEventListener('characteristicvaluechanged',
        handleBatteryLevelChanged);
    document.querySelector('#startNotifications').disabled = false;
    document.querySelector('#stopNotifications').disabled = true;
  });
}

/* This function will be called when `readValue` resolves and
 * characteristic value changes since `characteristicvaluechanged` event
 * listener has been added. */
function handleBatteryLevelChanged(event) {
  let batteryLevel = event.target.value.getUint8(0);
  log('> Battery Level is ' + batteryLevel + '%');
}

function onStartNotificationsButtonClick() {
  log('Starting Battery Level Notifications...');
  batteryLevelCharacteristic.startNotifications()
  .then(_ => {
    log('> Notifications started');
    document.querySelector('#startNotifications').disabled = true;
    document.querySelector('#stopNotifications').disabled = false;
  })
  .catch(error => {
    log('Argh! ' + error);
  });
}

function onStopNotificationsButtonClick() {
  log('Stopping Battery Level Notifications...');
  batteryLevelCharacteristic.stopNotifications()
  .then(_ => {
    log('> Notifications stopped');
    document.querySelector('#startNotifications').disabled = false;
    document.querySelector('#stopNotifications').disabled = true;
  })
  .catch(error => {
    log('Argh! ' + error);
  });
}

function onResetButtonClick() {
  if (batteryLevelCharacteristic) {
    batteryLevelCharacteristic.removeEventListener('characteristicvaluechanged',
        handleBatteryLevelChanged);
    batteryLevelCharacteristic = null;
  }
  // Note that it doesn't disconnect device.
  bluetoothDevice = null;
  log('> Bluetooth Device reset');
}

function onDisconnected() {
  log('> Bluetooth Device disconnected');
  connectDeviceAndCacheCharacteristics()
  .catch(error => {
    log('Argh! ' + error);
  });
}