CoreBluetooth writeValue:forDescriptor: issue

Question

My CoreBluetooth application need to enable the "indication bit" in Client Characteristic Configuration descriptors. Here is what I did:

1. Start to scan
2. Start to connect to the device
3. Call discoverServices

4. Call discoverCharacteristics inside the callback

   -(void)peripheral:(CBPeripheral *)peripheral didDiscoverServices:(NSError *)error

5. Call discoverDescriptorsForCharacteristic inside callback

   -(void)peripheral:(CBPeripheral *)peripheral didDiscoverCharacteristicsForService:(CBService *)service error:(NSError *)error

6. Inside callback

   -(void)peripheral:(CBPeripheral *)peripheral didDiscoverDescriptorsForCharacteristic:(CBCharacteristic *)characteristic error:(NSError *)error

I called:

        if ( [[descriptor.UUID representativeString] isEqualToString:@"2902" ] )
        {
            const unsigned char raw_data[] = {0x02};
            NSData *myData = [NSData dataWithBytes: raw_data length: 2];
            [self.cBCP writeValue:myData forDescriptor:descriptor];
        }

But My app crashes in writeVale: . The error message in console is :

Cannot write Client Characteristic Configuration descriptors using this method!

Any idea? Thanks

Answer 1

Pretty old question, but since it wasn't answered, seems like if the method setNotifyValue(_:for:) will handle that for you, it depends on the charcateristics properties below:

• Notify only: Notifications will be enabled.
• Indicate only: Indications will be enabled.
• Indicate & Notify: ONLY Notifications will be enabled.

So if you need to enable indications, the characteristic must only have the indicate property, and notify should be removed.

I guess the main reasoning behind that is indications are much slower, so iOS will always prefer the fastest possible option unless it's a requirement to indicate.

Read more on the Apple docs

Answer 2

The problem is that you cannot use the writeValue:forDescriptor: method to write the value of a Client Configuration Descriptor (UUID = 2902) as you did.

Instead, you should use the setNotifyValue:forCharacteristic: method of the CBPeripheral class to configure client indications or notifications of a characteristic’s value on a server.