How do I use the "Service Changed" characteristic to invalidate attributes cache?
I'm writing a small BT app on a microcontroller (using aioble on Micropython), but my question is more general on the BT spec itself.
Since I'm still developing the device, I keep adding/changing services and characteristics all the time, and found out that Chrome seems to cache the old services and chars UUID it found from previous runs. I understand this is done for performance and resources reasons, but I'd like to invalidate this cache on every boot.
I found out about the Generic Attribute Service (0x1801) and the Service Changed Characteristic (0x2A05) which suppose to do exactly this. I couldn't find official specs in the official BT site (this link for example seems to be broken), but found some other resources that basically say that the server should send a notification with a value representing the range of handles that should be invalidated.
That sounds exactly like what I want, but nothing describes how do I represent a "handle range".. I basically want to erase all.
Any help? Also - do I write value to the char once at startup or I can send notification throughout the device's lifecycle?
The Bluetooth Core Specification v5.3 describes the Service Changed Characteristic on page 1532:
The Service Changed Characteristic Value is two 16-bit Attribute Handles concatenated together indicating the beginning and ending Attribute Handles affected by an addition, removal, or modification to a GATT-based service on the server. A change to a characteristic value is not considered a modification of the service. If a change has been made to any of the GATT service definition characteristic values other than the Service Changed characteristic value and the Client Supported Features characteristic value, the range shall also include the beginning and ending Attribute Handle for the GATT service definition.
This table describes the value decleration needed for the Service Changed Characteristic:
