How to send BLE commands using CoreBluetooth?

Question

I want to connect to ohaus scale using ohaus bluetooth interface:

https://dmx.ohaus.com/WorkArea/showcontent.aspx?id=4294974224

I succeeded to connect and receive the CBCharacteristic with uuid 2456e1b9-26e2-8f83-e744-f34f01e9d703.

I want to get the weight of the scale. Here is what I do:

//send command to the scale
let data = "IP"
let valueString = (data as NSString).data(using: String.Encoding.utf8.rawValue)
self.scalePeripheral.writeValue(valueString!, for: scaleCharchteristic, type: CBCharacteristicWriteType.withResponse)

I should expect to receive the scale weight with the delegate function:

func peripheral(_ peripheral: CBPeripheral, didWriteValueFor characteristic: CBCharacteristic,error: Error?){
    guard 
        let characteristicValue = characteristic.value,
        let characteristicASCIIValue = NSString(data: characteristicValue, encoding: String.Encoding.utf8.rawValue)
    else {
        return
    }
    print("\((characteristicASCIIValue as String))")
}

But I receive empty value.

Also I don't receive any data to peripheralDidUpdateValueForCharacteristic.

What may be the problem?

Here is the reference of the bluetooth commands:

Answer 1

You can't just access characteristic.value you need to either:

• Perform a readValue(for:) of the characteristic and wait for a callback to peripheral(_:didUpdateValueFor:error) or

• Enable characteristic notifications via setNotifyValue(for:)

Not all characteristics support notifications, but if it is supported, it is the best approach to use, rather than polling.

Since it appears that your device is emulating a serial port over GATT, it is quite likely that notifications are supported (Check the properties of your characteristic for notify or indicate support).