Web BLE Characteristic startNotifications sometimes doesn't bind

Question

I'm using web BLE. I have based my code according to the example of the heart rate measurement.

Everything is working fine most of the time. But sometimes, even if the connection is successfully made, when I try to bind to the notification, it doesn't work.

The link is made in this function :

    _startNotifications(characteristicUuid) {
      let characteristic = this._characteristics.get(characteristicUuid);
      console.log(characteristic);
      return characteristic.startNotifications().then(() => characteristic);
    }

When everything is OK, I can see in the console that BluetoothRemoteGATTCharacteristic has a value : DataView(2) {} Otherwise, when it's not working it has a value : null

I would like to be able to retry automatically, if I detect that the value is null. But I'm not familiar with Promise (I think this is it) and console.log(characteristic.value) doesn't work here.

How would you approach this ?

Answer 1

What I ended up doing is "bypass" the issue. So it's a more algorithmic resolution than a pure Javascript one.

I didn't change the connection function, so it is still called like this :

device._startNotifications(some_uuid).then(handleHeartRateMeasurement)

I check everything in the handleHeartRateMeasurement function :

var ready = false;

function handleHeartRateMeasurement(heartRateMeasurement) {
  console.log("Hold on...");
  heartRateMeasurement.addEventListener("characteristicvaluechanged", event => {
    // Everytime the value change, this should be triggered
    // If it did not, variable "ready" will stay false
    ready = true;
    var value = device.parseValue(event.target.value);
    // Do something with value here
  });

  var check = function(){
    // If we have received data from characteristic, we are ready to go !
    if(ready === false){
      console.log("Device connected but not receiving data");

      // Stop the current notification subscription
      device.stopNotificationsHeartRateMeasurement();

      // Start a new one
      device._startNotifications(some_uuid).then(handleHeartRateMeasurement);

      setTimeout(check, 1000); // check again in a 1s
    }
    else{
      console.log("Device connected and receiving data");
    }
  }

  setTimeout(() => {
    check();
  }, 500);
}