BLE changing which UUID characteristic to write to

Question

I'm very new to coding using Swift so I'm hoping someone can help me out with an easy answer.

I'm basically trying to send a characteristic UUID into a write function and switch the write function to write to that UUID. It will just keep using the original one I had. Is there an easy way to do this? I have all the peripherals, services, characteristics discovered and am connected to the correct one already. I just can't get it to switch to the correct characteristic UUID. Any help would be awesome! I am a firmware/hardware developer so Swift and Xcode is not an everyday thing for me yet.

func writePosition(position: UInt16, characteristicUUID: CBUUID) {

    var position_low: UInt8 = 0
    var position_high: UInt8 = 0
    var position16: UInt16 = 0;

    position_low  = (UInt8) (position)  // (position)
    position_high = (UInt8)((position >> 8))


    // See if characteristic has been discovered before writing to it
    if self.positionCharacteristic == nil {
        return
    }
    var bytes:[UInt8] = [0x00, 0x00]

    bytes[1] = 0x01  //low byte is [1], high byte is [0] so reversed from LightBlue app

    bytes[1] = position_low  //low byte is [1], high byte is [0] so reversed from LightBlue app
    bytes[0] = position_high  //low byte is [1], high byte is [0] so reversed from LightBlue app



    let uuidsForBTService:  [CBUUID] = [characteristicUUID]
    //new code to try and set the correct UUID automatically
    for service in peripheral!.services
    {
        if service.UUID == BLEServiceUUID
        {
          peripheral!.discoverCharacteristics(uuidsForBTService, forService: service as! CBService)
        }

    }


    //find the correct characteristic to set to and then write to it
    //go through each characteristic and look at its UUID then if it matches, set us to that one so we write to it later on
    for characteristic in self.peripheral!.services {
        if characteristic.UUID == characteristicUUID//Position1PWMCharUUID
        {
            self.positionCharacteristic = (characteristic as! CBCharacteristic)
            peripheral!.setNotifyValue(true, forCharacteristic: characteristic as! CBCharacteristic)

            // Send notification that Bluetooth is connected and all required characteristics are discovered
            self.sendBTServiceNotificationWithIsBluetoothConnected(true)
        }
    }

    //end of code to set the UUID


    // Need a mutable var to pass to writeValue function
    //var positionValue = position
   //let data = NSData(bytes: &positionValue, length: sizeof(UInt16))
    let data = NSData(bytes: bytes, length: 2)
   self.peripheral?.writeValue(data, forCharacteristic: self.positionCharacteristic, type: CBCharacteristicWriteType.WithResponse)



}

Answer 1

Keep in mind that in the second for loop, where you are looking for a CBCharacteristic you are still iterating the services from the peripheral and not the characteristic from your ble service (a peripheral has services and a service has characteristics).

The following code is to get the CBCharacteristic you need.

var theService: CBService?

// Find service
for service in peripheral.services as! [CBService] {
    if service.UUID == BLEServiceUUID {
        theService = service

        break
    }
}

var theCharacteristic: CBCharacteristic?

// Find characteristic
for characteristic in theService?.characteristics as! [CBCharacteristic] {
    if characteristic.UUID == characteristicUUID {
        theCharacteristic = characteristic

        break
    }
}

// Write
if let characteristic = theCharacteristic {
    peripheral.writeValue(data, forCharacteristic: characteristic, type: .WithResponse)
}

In your code you are always writing to self.positionCharacteristic. Just write your data to the CBCharacteristic you just find.

Also if you need more help I wrote a simple swift class that manages a central connection, read and write tasks.

Hope it helps!