Dynamic reading of BLE characteristic?

Hello,

I've been playing around with BLE with Espruino Puck, and it's been quite pleasant so far. I've got a question and I can't find the answer- which means it's probably staring me in the face.

I'm experimenting/learning and used NRF.setServices to create a characteristic and I can read the value from it using Nordic nRF connect on my phone. I can also use NRF.updateServices to update the value, and if I use notify the value automatically updates on the phone screen. Yay: Bluetooth success!

My question: Suppose (hypothetically) I turned the puck into a BLE thermometer. This might be used really infrequently, so I don't want to use setInterval with updateServices as this would keep taking temperature readings which were never used, and use more power than necessary.

I only want to do the temperature reading (or whatever power hungry thing) when the data is requested by the phone.

I want something like an 'onRead' function for the characteristic, much as there is onWrite. So my code would be something like

onRead : function() { return doReallyPowerHungryThingThatReturnsAValue() }

I can't find out how to do such a thing. Is this possible or is it a limitation of BLE/The Nordic SDK? (I can imagine some sort of timing restriction in BLE or something - what if my function took a long time to execute.).

I'm grateful for any teachings!

Thanks,

Colin

Example code:

NRF.setServices({
  "047b0001-37e3-457d-3763-2a9b2d24cbb2" : {
    "047b0002-37e3-457d-3763-2a9b2d24cbb2": {
      maxLen : 10,
      value : "Unknown",
      readable : true,
      writable : true,
      notify : true,
      onWrite : function(evt) {
        var text = String.fromCharCode.apply(String, evt.data);
        console.log("Message received: ",text);
      },
    }
  }
}, { uart : false });

function updateTemperature() {
  NRF.updateServices({
    "047b0001-37e3-457d-3763-2a9b2d24cbb2" : {
      "047b0002-37e3-457d-3763-2a9b2d24cbb2" : {
        value : E.getTemperature().toString(),
        notify : true,
      }
    }
  });
}