Extract and Convert NSData from BLE to Float in Swift

Question

I'm developing an IOS App that handles a BlueTooth SensorTag. That SensorTag is based on the TI BLE SensorTag, but we had some of the Sensors removed.

In the sourcecode of the original IOS App from TI the XYZ-Values are calculated like follows with KXTJ9_RANGE defined as 1.0 in my implementation and KXTJ9 is the Accelerometer built on the SensorTag

+(float) calcXValue:(NSData *)data {
  char scratchVal[data.length];
  [data getBytes:&scratchVal length:3];
  return ((scratchVal[0] * 1.0) / (64 / KXTJ9_RANGE));
}

The data comes as hexadecimal like "fe850d" and by the method will be cut in 3 parts. Now i'm trying to convert this method to swift, but i get the wrong numbers back

e.g. fe should return something around 0.02 what the objective C Code does

My Swift Code so far

class Sensor: NSObject {
  let Range: Float = 1.0
  var data: NSData
  var bytes: [Byte] = [0x00, 0x00, 0x00]

  init(data: NSData) {
    self.data = data
    data.getBytes(&bytes, length: data.length)
  }

  func calcXValue()->Float {
    return ((Float(bytes[0]) * 1.0) / (64.0 / Range))
  }
...
}

I believe problem must lie around my Float(bytes[0]) because that makes 254 out of fe whereas scratchVal[0] in ObjectiveC is around 64

But my main problem is, i was all new with IOS programming when i had to begin with this project, so i chose to use Swift to learn and code the app with it.

Right now i use the original Objective C Code from TI to use with our SensorTag, but i would prefer using Swift for every part in the App.

Answer 1

On all current iOS and OS X platforms, char is a signed quantity, so that the input fe is treated as a negative number.

Byte on the other hand is an alias for UInt8 which is unsigned. Use [Int8] array instead to get the same behaviour as in your Objective-C code.