gattrs

This crate provides proc macros for creating a Bluez GATT server, allowing a device to operate as a Bluetooth Low Energy (BLE) peripheral. It is a thin but opinionated wrapper around zbus crate using an object-oriented-like style to define GATT services and characteristics.

Installation

Because gattrs is a GATT-specific wrapper around zbus, both crates must be listed as dependencies.

[dependencies]
gattrs = { git = "https://github.com/benarmstrongg/gattrs.git", branch = "main" }
zbus = { version = "2.2.0", default-features = false }

An async runtime is also required to use this crate. The examples in this repo use tokio.

[dependencies]
# ...
tokio = { version = "1", features = ["full"] }

Usage

A GATT application must expose at least one service containing at least one characteristic. The proc macros gatt_characteristic and gatt_service add the methods required for D-Bus interfaces org.bluez.GattCharacteristic1 and org.bluez.GattService1. Services and characteristics require UUIDs to identify themselves to client devices.

See example directory for full code examples.

Characteristics

A GATT characteristic is an individual property on a GATT service. In addition to UUID, a characteristic declaration must include a list of flags, which can be one of read, write, and notify.

Read

Characteristics with the read flag must declare a method read returning a GattReadResult.

#[gatt_characteristic(
    uuid = "42000000-0000-0000-0000-000000000001",
    flags = ["read"]
)]
struct CountCharacteristic {
    count: u8,
}

impl CountCharacteristic {
    async fn read(&self) -> gattrs::GattReadResult {
        Ok(vec![self.count])
    }
}

Write

Characteristics with the write flag must declare a method write, taking raw bytes as input and returing a GattWriteResult.

#[gatt_characteristic(
    uuid = "42000000-0000-0000-0000-000000000001",
    flags = ["write"]
)]
struct CountCharacteristic {
    count: u8,
}

impl CountCharacteristic {
    async fn write(&mut self, val: &[u8]) -> gattrs::GattWriteResult {
        let count = val.first().unwrap_or(&self.count).to_owned();
        self.count = count;
        Ok(())
    }
}

Notify

Characteristics with the notify flag don't need to declare any special methods. Instead, they gain access to the notify method, which brodcasts the characteristics value property to subscribed clients.

#[gatt_characteristic(
    uuid = "42000000-0000-0000-0000-000000000001",
    flags = ["notify"]
)]
struct CountCharacteristic {}

impl CountCharacteristic {
    async fn hello(&mut self) -> zbus::fdo::Result<()> {
        self.value = "hello world".as_bytes().to_vec();
        self.notify()
    }
}

Characteristics with both write and notify flags will emit notifications on writes as well.

Services

A service is an interface exposing one or more characteristics to client devices. The service declaration must include a UUID.

#[gatt_service(uuid = "42000000-0000-0000-0000-000000000000")]
struct CounterService {}

A GATT service is only responsible for exposing characteristics, so only a get_characteristics method is required, returning Vec<impl CharacteristicRegister>. CharacteristicRegister trait is automatically applied to all structs with the gatt_characteristic proc macro.

impl CounterService {
    fn get_characteristics(&self) -> Vec<impl CharacteristicRegister> {
        vec![CountCharacteristic::default()]
    }
}

Application

A GATT application is a collection of GATT services registered by the D-Bus interface org.bluez.GattManager1.

The GattApplication struct provides 2 ways for creating a GATT application.

// Using the default system message bus
GattApplication::new().await

// Using a session bus
let session_bus = zbus::Connection::session().await.unwrap();
GattApplication::on_bus(session_bus)

The GattApplication struct's builder methods can then be used to register services/advertisement and set the application path.

GattApplication::new()
    // Set path
    .path("/MyApp")
    // Register services
    .service(MyService1::default())
    .await
    .service(MyService2::new("hello world"))
    .await
    // Register advertisement
    .advertise()
    // Serve application
    .serve();

In order to expose your GATT application to BLE clients, an advertisement matching D-Bus interface org.bluez.LEAdvertisement1 must be registered at the same path as your GATT application. The LEAdvertisement contains a list of GATT service UUIDs to be exposed to clients. Registering an advertisement can be done automatically using the .advertise() method on GattApplication struct.

benarmstrongg / gattrs