NSString returns null

Question

I am using CoreBluetooth library in Swift and I want to read what am I receiving from peripheral in Characteristic. When I convert characteristic's value to NSString it returns nil always. Do you know why ? I think is because of encoding, because I have a Characteristic where I can read and write and when I write something to it I can Read what I wrote. When I write the value of Characteristic is 36 if I write 6 and this code works. If I only want to Read a characteristic the code does not work.

This is what my characteristic contains

<CBCharacteristic: 0x1700a2a60, UUID = FFF2, properties = 0x2, value = <02>, notifying = NO>

And this is the code

    func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {

        if error != nil {
            print("Error on updating value on characteristic: \(characteristic) - \(String(describing: error?.localizedDescription))")
            return
        }


        print(characteristic.value!)
        print(NSString(data: characteristic.value!, encoding: String.Encoding.utf8.rawValue)!).   //HERE IS NULL

        guard let stringFromData = NSString(data: characteristic.value!, encoding: String.Encoding.utf8.rawValue) else
        {
            print("Invalid data")
            return
        }

     //ALSO HERE stringFromData IS NULL

}

You could try ascii encoding, but if your data is being sent via a "virtual serial port" from your peripheral you may need to gather multiple packets to get your string — Paulw11, Jul 24 '17 at 10:31
What's that's supposed to be the value? Could it be an Int instead? — Larme, Jul 24 '17 at 11:23

score 3 · Accepted Answer · answered Jul 24 '17 at 12:55

The value property of CBCharacteristic may contain binary data encoded in thousands of formats defined by its uuid. For example, it may be one of the signed/unsigned integers of size 8-bit, 16-bit, 32-bit or 64-bit. Or 7-byte date and time, or length-preceded UTF-16(LE) String, or... And their combination may be contained.

(A complex example, decoding Weight Measurement (UUID= 2A9D) here.)

It may not be a simple string, so NSString.init(data:encoding:) (or String.init(data:encoding:) may not work.

You need to get your String representation of value for each uuid. You may need to write something like this:

func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {

    if error != nil {
        print("Error on updating value on characteristic: \(characteristic) - \(error!.localizedDescription))")
        return
    }
    guard let data = characteristic.value else {
        print("characteristic.value is nil")
        return
    }
    let uuidStr = characteristic.uuid.uuidString
    let stringFromData: String
    switch uuidStr {
    case "FFF2":
        //I do not know the UUID: FFF2, but seems you prefer it in hexadecimal
        stringFromData = String(format: "%02X", data[0])
    //Add other `case`s for other `uuid`s you want to show...
    //case "XXXX":
    //    stringFromData = ...
    //...
    default:
        stringFromData = "Unsupported UUID: \(uuidStr), data:\(data as NSData)"
    }
    print(stringFromData)
}

Thank you. I used your answer – mihral Jul 25 '17 at 10:54 — mihral, Jul 25 '17 at 10:54

score 2 · Answer 2 · answered Jul 24 '17 at 11:24

You should read it by bytes:

var out: UInt16 = 0
let formattedData = NSMutableData(length: yourBufferCapacity)!

(characteristic.value as NSData).getBytes(&out, range: NSRange(location: 0,length: MemoryLayout<UInt16>.size))
out = out.bigEndian //optional, check if you really need it
formattedData.replaceBytes(in: NSRange(location: 0,length: 2), withBytes: &out)

then you can convert formattedData to String.

NSString returns null

2 Answers2