ios ble "Characteristic User Description" - ios

Question

Trying to retrieve readable information from an characteristics by using the function:
peripheral.discoverDescriptors(for: characteristic)
Later the delegate method:
func peripheral(_ peripheral: CBPeripheral, didDiscoverDescriptorsFor characteristic: CBCharacteristic, error: Error?)
is called but how can I get the string description? When I read the value from the descriptors it's always nil.
let descriptors = characteristic.descriptors! as [CBDescriptor]
for descriptor in descriptors {
print("\(#function): descriptor = \(descriptor) UUID = \(descriptor.uuid) value = \(descriptor.value)")
}
However, if I'm browsing and connecting with an BLE scanner it can read the characteristic human readable descriptors.

Answer 1

Reading descriptors is a two-step process, much like discovering and then reading characteristics.
Try:
public func peripheral(_ peripheral: CBPeripheral, didDiscoverDescriptorsFor characteristic: CBCharacteristic, error: Error?) {
guard let descriptors = characteristic.descriptors else { return }
for descr in descriptors {
peripheral.readValue(for: descr)
}
}
And then fill in the blanks of:
public func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor descriptor: CBDescriptor, error: Error?) {
switch descriptor.uuid.uuidString {
case CBUUIDCharacteristicExtendedPropertiesString:
guard let properties = descriptor.value as? NSNumber else {
break
}
print(" Extended properties: \(properties)")
case CBUUIDCharacteristicUserDescriptionString:
guard let description = descriptor.value as? NSString else {
break
}
print(" User description: \(description)")
case CBUUIDClientCharacteristicConfigurationString:
guard let clientConfig = descriptor.value as? NSNumber else {
break
}
print(" Client configuration: \(clientConfig)")
case CBUUIDServerCharacteristicConfigurationString:
guard let serverConfig = descriptor.value as? NSNumber else {
break
}
print(" Server configuration: \(serverConfig)")
case CBUUIDCharacteristicFormatString:
guard let format = descriptor.value as? NSData else {
break
}
print(" Format: \(format)")
case CBUUIDCharacteristicAggregateFormatString:
print(" Aggregate Format: (is not documented)")
default:
break
}
}
The string constants and the associated data types came from the overview table here.
In my (limited) experience, descriptors don't reveal anything particularly interesting.