WriteValue

ViewController

fileprivate var ledMask: UInt8    = 0
fileprivate let digitalBits       = 2

func setDigitalOutput(_ index: Int, on: Bool, characteristic  :CBCharacteristic) {
  
  let shift = UInt(index) * UInt(digitalBits)
  var mask = ledMask
  
  if on {
    mask = mask | UInt8(1 << shift)
  }
    else {
    mask = mask & ~UInt8(1 << shift)
  }
  
  let data = Data(bytes: [mask])
    
   self.peripheral?.writeValue(data, for: characteristic, type: .withResponse)
   ledMask = mask
}