DBus

Utilities for declaring DBus interfaces and using them as services or client proxies in a fully typed manner.

Read more about using DBus in GJS on gjs.guide.

What is DBus

DBus is a message bus for interprocess communication: processes connect to a shared bus, usually the session bus, or the system bus for system-wide services, and exchange messages.

On connection, each process is assigned a unique name such as :1.42. Since unique names are unpredictable, a process can additionally request a well-known name such as com.example.MyApplication, which is what makes IPC practical: clients address the service by its well-known name without having to know which process currently provides it. A Gio.Application instance automatically owns its application ID as a well-known name on the session bus.

Under a name, a process exports objects identified by object paths such as /com/example/MyApplication/window1, and each object can export multiple interfaces that they implement. Addressing a remote property, method or signal therefore takes the triple of name, object path and interface name.

Example usage

Declare an interface

import { createDBusInterface, property, method, signal } from "gnim/dbus"

const MyInterface = createDBusInterface("example.gjs.MyInterface", {
  MyProperty: property("s"),
  MyMethod: method(["s"], ["s"]),
  MySignal: signal("s"),
})

Use it as a service

const service = await MyInterface.serve({
  name: "example.gjs.MyInterface",
  objectPath: "/example/gjs/MyInterface",
  implementation: (emitter) => ({
    MyProperty: "initial value",
    MyMethod(str): [string] {
      emitter.MySignal(str)
      return [str]
    },
  }),
})

service.connect("my-signal", (_, str: string) => {
  console.log(`MySignal emitted with argument: "${str}"`)
})

service.connect("notify::my-property", () => {
  console.log(`MyProperty set to ${service.MyProperty}`)
})

Use it as a proxy

const proxy = await MyInterface.proxy({
  name: "example.gjs.MyInterface",
  objectPath: "/example/gjs/MyInterface",
})

proxy.connect("my-signal", (_, str: string) => {
  console.log(`MySignal emitted with argument: "${str}"`)
})

proxy.connect("notify::my-property", () => {
  console.log(`MyProperty set to ${service.MyProperty}`)
})

proxy.MyProperty = "new value"

const [value] = await proxy.MyMethod("hello")
console.log(value) // "hello"

Both services and proxies are GObject.Object instances: DBus signals are also GObject signals and DBus properties are also GObject properties, using kebab-cased names.

Synchronous instantiation

You can instead use the new operator, export services and init proxies.

const service = new MyInterface.Service((emitter) => ({
  // impl
}))

service.export(Gio.DBus.session, "/example/gjs/MyInterface")

NOTE

To own a name other than the application ID of the main Gio.Application instance, you can use Gio.bus_own_name.

const proxy = new MyInterface.Proxy({
  name: "example.gjs.MyInterface",
  objectPath: "/example/gjs/MyInterface",
})

proxy.init() // blocks IO
print(proxy.MyProperty)

// non blocking
proxy.initAsync().then(() => {
  print(proxy.MyProperty)
})

NOTE

The proxy function on MyInterface is just a wrapper over asynchronous initialization.

createDBusInterface

Declares a DBus interface from which both services and proxies can be created.

function createDBusInterface<T extends InterfaceDeclaration>(
  name: string,
  interfaceDeclaration: T,
): DBusInterface<T>

interface DBusInterface<T extends InterfaceDeclaration> {
  Proxy: ProxyClass<T>
  Service: ServiceClass<T>
  serve(props: ServeProps<T>): Promise<ServiceInstance<T>>
  proxy(props: ProxyProps): Promise<ProxyInstance<T>>
}

Primitives it is composed of

If you only need the proxy part or the service part, you can use the underlying primitives instead.

import { createInterfaceInfo } from "gnim/dbus"

const ExampleServiceInterface = createInterfaceInfo("example.Service", {
  Method: method(),
  Signal: signal(),
})

Services only:

import { createServiceClass, serve } from "gnim/dbus"

const ExampleService = createServiceClass(ExampleServiceInterface)

const exampleService = await serve(ExampleService, {
  implementation: (emitter) => ({
    Method: () => emitter.Signal(),
  }),
})

Proxies only:

import { createProxyClass, proxy } from "gnim/dbus"

const ExampleProxy = createProxyClass(ExampleServiceInterface)

const exampleProxy = await proxy(ExampleProxy, {
  name: "example.Service",
  objectPath: "/example/Service",
})

property

Declares a DBus property with a variant type string.

function property(type: string, access?: "r" | "w" | "rw")

Properties are read-write by default. Pass "r" to declare a read-only or "w" to declare a write-only property, which is reflected in the types of the service implementation and the proxy.

method

Declares a DBus method.

type Arg = string | { name: string; type: string }

function method(...inArgs: Arg[])
function method(inArgs: Arg[], outArgs: Arg[])

Return values are declared as a list of out arguments, which implementations return and proxies resolve as a tuple.

NOTE

Optionally, you can declare the name of the arguments for DBus inspection by passing a { name: string, type: string } object instead of just the type string.

method([{ name: "str", type: "s" }], ["s"])

signal

Declares a DBus signal.

type Arg = string | { name: string; type: string }

function signal(...args: Arg[])

Unlike GObject signals, DBus signals do not have return types.

Services

serve

Instantiates an instance of service with the given implementation and attempts to own name and export it at objectPath on busType.

interface DBusInterface<T extends InterfaceDeclaration> {
  serve(props: {
    busType?: Gio.BusType // default: Gio.BusType.SESSION
    name?: string // default: the interface name
    objectPath?: string // default: the interface name as a path
    flags?: Gio.BusNameOwnerFlags // default: Gio.BusNameOwnerFlags.NONE
    cancellable?: Gio.Cancellable
    implementation: (emitter: ServiceEmitter<T>) => ServiceImplementation<T>
  }): Promise<ServiceInstance<T>>
}

NOTE

It is mostly a wrapper over Gio.bus_own_name.

Implementing properties

Read-write properties can be implemented either as plain data properties or as getter/setter pairs. Changes to data properties are detected automatically, while getters/setters have to notify through the emitter. Read-only properties are implemented with a getter and write-only properties with a setter.

const service = await MyInterface.serve({
  implementation: (emitter) => ({
    // data property: assignments automatically emit
    // PropertiesChanged and notify:: when the value changes
    MyProperty: "initial value",

    // getter/setter property: has to emit manually
    get MyOtherProperty() {
      return internalValue
    },
    set MyOtherProperty(value: string) {
      internalValue = value
      emitter.MyOtherProperty()
      emitter.ReadOnlyProperty()
    },

    // read-only property
    get ReadOnlyProperty() {
      return this.MyOtherProperty + "ReadOnly"
    },

    // write-only property
    set WriteOnlyProperty(value: number) {
      writeOnlyValue = value
    },
  }),
})

Implementing methods

Methods take their in arguments as parameters and return their out arguments as a tuple, either synchronously or as a Promise. Methods with no out arguments return nothing.

const service = await MyInterface.serve({
  implementation: (emitter) => ({
    MySyncMethod(str): [string] {
      return [str]
    },
    async MyAsyncMethod(str): Promise<[string]> {
      await somethingAsync()
      return [str]
    },
  }),
})

Thrown errors are returned to the caller as DBus errors.

Implementation classes

InferImplementation and InferEmitter extract the implementation and emitter types of a declared interface, which is useful for implementing services as classes.

import type { InferEmitter, InferImplementation } from "gnim/dbus"

type Emitter = InferEmitter<typeof MyInterface>
type Impl = InferImplementation<typeof MyInterface>

class Implementation implements Impl {
  private emitter: Emitter

  constructor(emitter: Emitter) {
    this.emitter = emitter
  }

  MyProperty = "initial value"

  MyMethod(str: string): [string] {
    this.emitter.MySignal(str)
    return [str]
  }
}

const service = await MyInterface.serve({
  implementation: (emitter) => new Implementation(emitter),
})

The service object

It is an instance of GObject.Object exposing the interface.

• Its properties read and write through the implementation and enforce access flags: reading a write-only or assigning a read-only property throws. Assigning a data property automatically emits PropertiesChanged and notify:: just like a remote write does.
• Its methods invoke the implementation directly.
• DBus signals and property changes are also emitted as GObject signals, which can be connected to with their kebab-cased names, such as my-signal or notify::my-property.

class ServiceInstance extends GObject.Object {
  implementation: ServiceImplementation<T>
  unexport(): void
}

Serving stops with unexport, which also releases the owned name when the serve function was used to instantiate the service.

Proxies

proxy

Attempts to proxy name's object at objectPath on the bus connection.

interface DBusInterface<T extends InterfaceDeclaration> {
  proxy(props?: {
    bus?: Gio.DBusConnection // default: Gio.DBus.session
    name?: string // default: the interface name
    objectPath?: string // default: the interface name as a path
    flags?: Gio.DBusProxyFlags // default: Gio.DBusProxyFlags.NONE
    timeout?: number // default: 10_000
  }): Promise<ProxyInstance<T>>
}

NOTE

It is a wrapper over the initAsync method of the Proxy class produced by createDBusInterface.

The proxy object

• Property reads are served from a cache which is kept in sync through PropertiesChanged signals. If a value is missing from the cache, it is fetched with a blocking call.
• Property writes update the cache optimistically and set the remote property asynchronously. If the remote write fails, the cached value is rolled back.
• Methods are invoked asynchronously and resolve their out arguments as a tuple, or an empty tuple for methods without out arguments. They reject with a GLib.Error if the remote implementation throws.
• DBus signals and property changes are emitted as GObject signals, which can be connected to with their kebab-cased names, such as my-signal or notify::my-property.