Write CBCharacteristic by UUID

Question

I am attempting to use CoreBluetooth to write to a specific, known characteristic. I feel this should be possible as I have used a Texas Instruments BLE utility that you can select a "write value" action on a connected peripheral and simply enter the characteristic UUID and the value you want written and it executes no problem.

By my understanding, in order to do this I have to make a call to

[peripheral writeValue:data forCharacteristic:...];

with a CBCharacteristic object configured to have the correct UUID.

I have tried making a CBMutableCharacteristic with the correct UUID, and even correct permissions for what I know is an existing characteristic in this peripheral's profile, and I get the following crash, when attempting to perform any read/write operation with that characteristic on the peripheral:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[CBMutableCharacteristic peripheral]: unrecognized selector sent to instance 0x1ed57690'

And here is the code I use to set up and write to the characteristic:

NSString *characteristicUUIDstring = @"fff1";
CBUUID *characteristicUUID = [CBUUID UUIDWithString:characteristicUUIDstring];

char dataByte = 0x10;
NSData *data = [NSData dataWithBytes:&dataByte length:1];

CBMutableCharacteristic *testCharacteristic = [[CBMutableCharacteristic alloc] initWithType:characteristicUUID properties:CBCharacteristicPropertyRead|CBCharacteristicPropertyWrite value:data permissions:CBAttributePermissionsReadable|CBAttributePermissionsWriteable];

[peripheral writeValue:data forCharacteristic:testCharacteristic type:CBCharacteristicWriteWithResponse];

Tommy Devoy · Accepted Answer · 2013-08-07T21:12:16.107

15

While a characteristic like that may present inside the peripheral, that's not the actual characteristic it knows it contains. You need to discover all your services and characteristics and then write directly from those services and chars.

//Assuming you've already discovered all your services and characteristics, do something like this:

for(CBService *service in peripheral.services)
{
      if([service.UUID isEqual:theServiceCBUUIDYouAreLookingFor])
      {
           for(CBCharacteristic *charac in service.characteristics)
           {
               if([charac.UUID isEqual:theCharCBUUIDYoureLookingFor])
               {
                   //NOW DO YOUR WRITING/READING AND YOU'LL BE GOOD TO GO
               }
           }
       }
 }

edited Aug 07 '13 at 21:12

answered Aug 06 '13 at 22:28

Tommy Devoy

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Even though this kind of behavior is possible using other BLE utilities, I have to actually discover the characteristic? What is the point of having the interfaces for `CBMutableCharacteristic` and `CBMutableService` if they can't be used this way? – Dan F Aug 07 '13 at 13:08
CBMutableCharacteristics and CBMutableServices are there so that you can emulate a peripheral on an iOS device. They exist solely for use in the peripheralManager. What ble utilities are you referring to? – Tommy Devoy Aug 07 '13 at 16:44
Texas Instruments BTool has a "Read Using Characteristic UUID" and the ability to write to a characteristic using the characteristic handle. – Dan F Aug 07 '13 at 17:36
What do you mean by characteristic handle? As in they write with the CBMutableCharacteristic? Do you have a link to this code? – Tommy Devoy Aug 07 '13 at 21:12
Characteristics have handles, and I do not have any code for it as it is a downloaded utility from Texas Instruments to run on Windows – Dan F Aug 07 '13 at 21:25
I guarantee you that under the covers the texas instruments utility is caching the characteristics the device contains, and then writing directly to them. – Tommy Devoy Aug 07 '13 at 21:32
@tdevoy is right. This is the way you should use characteristics. If you want to, you can store them in (weak) variables for faster access. You will need to take care of discovering them first and then use the values directly in other parts of the code. If they are niled out automatically (as they are weak) or you get an error, then you should reinitialize everything. – allprog Aug 08 '13 at 07:52
Alright, I was hoping I could get away with not making the services/characteristics discoverable, but if this is what we have to use, so be it – Dan F Aug 08 '13 at 13:11
@TommyDevoy do you have a source to confirm this answer. It really is atypical from how you do it on Android. – Nonlin Jun 07 '23 at 01:21

score 0 · Answer 2 · answered Jan 13 '15 at 19:04

0

be aware that if you use the "authenticated_read" or "authenticated_write" attributes in your GATT characteristics you may not be able to discover the CBCharacteristic

answered Jan 13 '15 at 19:04

Al Baldrick li suen els ous

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Write CBCharacteristic by UUID

2 Answers2