0Write major value

• You need discover the services and characteristic
• You don’t need read value from the characteristic before writing over it.
• will continue for, for this example, after read value. Modify func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?)
• Add a variable new_major and reset_characteristic

var reset_characteristic : CBCharacteristic!
func peripheral(_ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
    for characteristic in service.characteristics! {
        print("Characteristic: \(characteristic)\n error: \(error)")
        if(characteristic.uuid.uuidString == "FFF2"){
            peripheral.readValue(for: characteristic)
        }
        if(characteristic.uuid.uuidString == "FFFF"){
            reset_characteristic = characteristic
        }
    }
}
let new_major : UInt16 = 100
func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic read: \(characteristic)\n error: \(error)")
    let major = UInt16.init(bigEndian: UInt16(data: characteristic.value!)!)
    print("major: \(major)")
    peripheral.writeValue(new_major.data, for: characteristic, type: CBCharacteristicWriteType.withResponse)
}

• iPhone by deafult will send and receive bytes in Little Endian format, but my device MINEW witch chipset NRF51822 have ARM archteture and need bytes in Big Endian format, so I have to swap it.
• BLE Device documentation will say what type of input and output each characteristic will have and if you can read it like above (CBCharacteristicWriteType.withResponse).

func peripheral(_ peripheral: CBPeripheral, didWriteValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic write: \(characteristic)\n error: \(error)")
    if(characteristic.uuid.uuidString == "FFF2"){
            print("Resetting")
            peripheral.writeValue("minew123".data(using: String.Encoding.utf8)!, for: reset_characteristic, type: CBCharacteristicWriteType.withResponse)
        }
    if(characteristic.uuid.uuidString == "FFFF"){
        print("Reboot finish")
        cb_manager.cancelPeripheralConnection(peripheral)
    }
}

• To update a gatt server information you have to reboot it programmatically or save data to it and turn off and turn on manually.
• FFFF is characteristic that do it in this device.
• ‘minew123’ is the default password for reboot o save information in this case.
• run your app and watch you console for any error, I hope none, but you will not see the new value yet.

func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic read: \(characteristic)\n error: \(error)")
    let major = UInt16.init(bigEndian: UInt16(data: characteristic.value!)!)
    print("major: \(major)")
    //peripheral.writeValue(new_major.data, for: characteristic, type: CBCharacteristicWriteType.withResponse)

}

• Last step is to comment last line in method didUpdateValueFor and rerun the app, now you will the new value.