How do you create a descriptor for a mutable characteristic? - ios

Question

The documentation for CBMutableDescriptor:initWithType:value: says to pass a "128-bit UUID that identifies the characteristic" for the type parameter. It then goes on to say you should only use one of CBUUIDCharacteristicUserDescriptionString or CBUUIDCharacteristicFormatString for the type parameter. Finally, there is no method to add a descriptor to a mutable characteristic.
It appears that the parameter is doing two mutually exclusive things. On the one hand, it is being used to tell the O/S which characteristic the descriptor applies to, and on the other, it is being used to set the type of descriptor. The second makes more sense, but then how do you add the descriptor to the characteristic?
Whether you pass the UUID for the characteristic or CBUUIDCharacteristicUserDescriptionString, iOS crashes with
Assertion failure in -[CBMutableDescriptor initWithType:value:], /SourceCache/CoreBluetooth_Sim/CoreBluetooth-59.3/CBDescriptor.m:25
What's the correct way to create the CBMutableDescriptor and add it to a CBMutableCharacteristic?

Answer 1

You're correct about the docs. But just to be clear for everyone, here is a citation found in CBDescriptor.h:
...Only the Characteristic User Description and Characteristic
Presentation Format descriptors are currently supported. The
Characteristic Extended Properties and Client Characteristic
Configuration descriptors will be created automatically upon
publication of the parent service, depending on the properties of the
characteristic itself.
So in other words, unless you are setting those descriptors, the system will block you (thus why you got the assertion failure).
So say you want to use the Characteristic User Description descriptor, you would do:
CBUUID *yourCharUUID = [CBUUID UUIDWithString:#"c07c5050-15a0-11e3-8ffd-0800200c9a66"];//whatever UUID your using
CBMutableCharacteristic *yourCharacteristic = [[CBMutableCharacteristic alloc]initWithType:yourCharUUID properties:CBCharacteristicPropertyWriteWithoutResponse value:nil permissions:perms];
CBUUID *userDescriptionUUID = [CBUUID UUIDWithString:CBUUIDCharacteristicUserDescriptionString];//or set it to the actual UUID->2901
CBMutableDescriptor *yourDescriptor = [[CBMutableDescriptor alloc]initWithType:userDescriptionUUID value:#"myDescriptorValue"];
yourCharacteristic.descriptors = #[yourDescriptor];
Let me know if you have any questions.