Characteristic.value from Bluetooth reading in Swift

Question

I'm trying to read and convert into String a reading from a BT device.

<CBCharacteristic: 0x1700a2e80, UUID = 2A9D, properties = 0x20, value = <02ac08e1 07010a14 0029>, notifying = YES>

Based on example I found online I did

let u16 = (characteristic.value! as NSData).bytes.bindMemory(to: Int.self, capacity: characteristic.value!.count).pointee

but my u16 is null, even though characteristic.value contains

(lldb) dp characteristic.value! as NSData
<02ac08e1 07010a14 0029>

Can anyone point me in the right direction?

Answer 1

It's not super easy.

Weight Measurement

You may need to write something like this:

let data = Data([0x02,0xac,0x08,0xe1,0x07,0x01,0x0a,0x14,0x00,0x29])

struct DataReader {
    var data: Data
    var offset: Int = 0

    mutating func read() -> UInt8 {
        let result = data[offset]
        offset += MemoryLayout<UInt8>.size
        return result
    }
    mutating func read() -> UInt16 {
        let subdata = data.subdata(in: offset..<offset+MemoryLayout<UInt16>.size)
        let result: UInt16 = subdata.withUnsafeBytes {(bytes: UnsafePointer<UInt16>) in
            bytes.pointee.littleEndian
        }
        offset += MemoryLayout<UInt16>.size
        return result
    }
}

typealias  BLEUserID = UInt8

struct BLEDateTime {
    var year: UInt16
    var month: UInt8
    var day: UInt8
    var hours: UInt8
    var minutes: UInt8
    var seconds: UInt8
}
extension DataReader {
    mutating func read() -> BLEDateTime {
        let year: UInt16 = read()
        let month: UInt8 = read()
        let day: UInt8 = read()
        let hours: UInt8 = read()
        let minutes: UInt8 = read()
        let seconds: UInt8 = read()
        return BLEDateTime(year: year, month: month, day: day, hours: hours, minutes: minutes, seconds: seconds)
    }
}
struct WeightMeasurement: CustomStringConvertible {
    struct Flags: OptionSet {
        var rawValue: UInt8
        init(rawValue: UInt8) {
            self.rawValue = rawValue
        }
        static let measurementImperial = Flags(rawValue: 1<<0)
        static let timeStampPresent = Flags(rawValue: 1<<1)
        static let userIDPresent = Flags(rawValue: 1<<2)
        static let bmiAndHeightPresent = Flags(rawValue: 1<<3)
    }
    var isMeasuremntImperial: Bool
    var weight: Decimal
    var timeStamp: BLEDateTime?
    var userID: BLEUserID?
    var bmi: Decimal?
    var height: Decimal?

    var description: String {
        return weight.description
    }
}
extension DataReader {
    mutating func read() -> WeightMeasurement {
        let flags = WeightMeasurement.Flags(rawValue: read())
        let isMeasuremntImperial = flags.contains(.measurementImperial)
        let weight: Decimal
        if isMeasuremntImperial {
            weight = Decimal(read() as UInt16) * Decimal(string: "0.01")!
        } else {
            //SI
            weight = Decimal(read() as UInt16) * Decimal(string: "0.005")!
        }
        var timeStamp: BLEDateTime?
        if flags.contains(.timeStampPresent) {
            timeStamp = read()
        }
        var userID: BLEUserID?
        if flags.contains(.userIDPresent) {
            userID = read()
        }
        var bmi: Decimal?
        var height: Decimal?
        if flags.contains(.bmiAndHeightPresent) {
            bmi = Decimal(read() as UInt16) * Decimal(string: "0.1")!
            if isMeasuremntImperial {
                height = Decimal(read() as UInt16) * Decimal(string: "0.1")!
            } else {
                //SI
                height = Decimal(read() as UInt16) * Decimal(string: "0.001")!
            }
        }
        return WeightMeasurement(isMeasuremntImperial: isMeasuremntImperial, weight: weight, timeStamp: timeStamp, userID: userID, bmi: bmi, height: height)
    }
}

var reader = DataReader(data: data, offset: 0)
let weightMeasurement: WeightMeasurement = reader.read()
print(weightMeasurement) //->11.1
print(weightMeasurement.timeStamp!) //->BLEDateTime(year: 2017, month: 1, day: 10, hours: 20, minutes: 0, seconds: 41)

(Sorry, not fully tested and you may need some modification.)