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Intro to Mojo & Services

[TOC]

Overview

This document contains the minimum amount of information needed for a developer to start using Mojo effectively in Chromium, with example Mojo interface usage, service definition and hookup, and a brief overview of the Content layer's core services.

See other Mojo & Services documentation for introductory guides, API references, and more.

Mojo Terminology

A message pipe is a pair of endpoints. Each endpoint has a queue of incoming messages, and writing a message at one endpoint effectively enqueues that message on the other (peer) endpoint. Message pipes are thus bidirectional.

A mojom file describes interfaces, which are strongly-typed collections of messages. Each interface message is roughly analogous to a single proto message, for developers who are familiar with Google protobufs.

Given a mojom interface and a message pipe, one of the endpoints can be designated as a Remote and is used to send messages described by the interface. The other endpoint can be designated as a Receiver and is used to receive interface messages.

*** aside NOTE: The above generalization is a bit oversimplified. Remember that the message pipe is still bidirectional, and it's possible for a mojom message to expect a reply. Replies are sent from the Receiver endpoint and received by the Remote endpoint.

The Receiver endpoint must be associated with (i.e. bound to) an implementation of its mojom interface in order to process received messages. A received message is dispatched as a scheduled task invoking the corresponding interface method on the implementation object.

Another way to think about all this is simply that a Remote makes calls on a remote implementation of its interface associated with a corresponding remote Receiver.

Example: Defining a New Frame Interface

Let's apply this to Chrome. Suppose we want to send a "Ping" message from a render frame to its corresponding RenderFrameHostImpl instance in the browser process. We need to define a nice mojom interface for this purpose, create a pipe to use that interface, and then plumb one end of the pipe to the right place so the sent messages can be received and processed there. This section goes through that process in detail.

Defining the Interface

The first step involves creating a new .mojom file with an interface definition, like so:

// src/example/public/mojom/ping_responder.mojom
module example.mojom;

interface PingResponder {
  // Receives a "Ping" and responds with a random integer.
  Ping() => (int32 random);
};

This should have a corresponding build rule to generate C++ bindings for the definition here:

# src/example/public/mojom/BUILD.gn
import("//mojo/public/tools/bindings/mojom.gni")
mojom("mojom") {
  sources = [ "ping_responder.mojom" ]
}

Creating the Pipe

Now let's create a message pipe to use this interface.

*** aside As a general rule and as a matter of convenience when using Mojo, the client of an interface (i.e. the Remote side) is typically the party who creates a new pipe. This is convenient because the Remote may be used to start sending messages immediately without waiting for the InterfaceRequest endpoint to be transferred or bound anywhere.

This code would be placed somewhere in the renderer:

// src/third_party/blink/example/public/ping_responder.h
mojo::Remote<example::mojom::PingResponder> ping_responder;
mojo::PendingReceiver<example::mojom::PingResponder> receiver =
    ping_responder.BindNewPipeAndPassReceiver();

In this example, ping_responder is the Remote, and receiver is a PendingReceiver, which is a Receiver precursor that will eventually be turned into a Receiver. BindNewPipeAndPassReceiver is the most common way to create a message pipe: it yields the PendingReceiver as the return value.

*** aside NOTE: A PendingReceiver doesn't actually do anything. It is an inert holder of a single message pipe endpoint. It exists only to make its endpoint more strongly-typed at compile-time, indicating that the endpoint expects to be bound by a Receiver of the same interface type.

Sending a Message

Finally, we can call the Ping() method on our Remote to send a message:

// src/third_party/blink/example/public/ping_responder.h
ping_responder->Ping(base::BindOnce(&OnPong));

*** aside IMPORTANT: If we want to receive the response, we must keep the ping_responder object alive until OnPong is invoked. After all, ping_responder owns its message pipe endpoint. If it's destroyed then so is the endpoint, and there will be nothing to receive the response message.

We're almost done! Of course, if everything were this easy, this document wouldn't need to exist. We've taken the hard problem of sending a message from a renderer process to the browser process, and transformed it into a problem where we just need to take the receiver object from above and pass it to the browser process somehow where it can be turned into a Receiver that dispatches its received messages.

Sending a PendingReceiver to the Browser

It's worth noting that PendingReceivers (and message pipe endpoints in general) are just another type of object that can be freely sent over mojom messages. The most common way to get a PendingReceiver somewhere is to pass it as a method argument on some other already-connected interface.

One such interface which we always have connected between a renderer's RenderFrameImpl and its corresponding RenderFrameHostImpl in the browser is BrowserInterfaceBroker. This interface is a factory for acquiring other interfaces. Its GetInterface method takes a GenericPendingReceiver, which allows passing arbitrary interface receivers.

interface BrowserInterfaceBroker {
  GetInterface(mojo_base.mojom.GenericPendingReceiver receiver);
}

Since GenericPendingReceiver can be implicitly constructed from any specific PendingReceiver, it can call this method with the receiver object it created earlier via BindNewPipeAndPassReceiver:

RenderFrame* my_frame = GetMyFrame();
my_frame->GetBrowserInterfaceBroker().GetInterface(std::move(receiver));

This will transfer the PendingReceiver endpoint to the browser process where it will be received by the corresponding BrowserInterfaceBroker implementation. More on that below.

Implementing the Interface

Finally, we need a browser-side implementation of our PingResponder interface.

#include "example/public/mojom/ping_responder.mojom.h"

class PingResponderImpl : example::mojom::PingResponder {
 public:
  explicit PingResponderImpl(mojo::PendingReceiver<example::mojom::PingResponder> receiver)
      : receiver_(this, std::move(receiver)) {}
  PingResponderImpl(const PingResponderImpl&) = delete;
  PingResponderImpl& operator=(const PingResponderImpl&) = delete;

  // example::mojom::PingResponder:
  void Ping(PingCallback callback) override {
    // Respond with a random 4, chosen by fair dice roll.
    std::move(callback).Run(4);
  }

 private:
  mojo::Receiver<example::mojom::PingResponder> receiver_;
};

RenderFrameHostImpl owns an implementation of BrowserInterfaceBroker. When this implementation receives a GetInterface method call, it calls the handler previously registered for this specific interface.

// render_frame_host_impl.h
class RenderFrameHostImpl
  ...
  void GetPingResponder(mojo::PendingReceiver<example::mojom::PingResponder> receiver);
  ...
 private:
  ...
  std::unique_ptr<PingResponderImpl> ping_responder_;
  ...
};

// render_frame_host_impl.cc
void RenderFrameHostImpl::GetPingResponder(
    mojo::PendingReceiver<example::mojom::PingResponder> receiver) {
  ping_responder_ = std::make_unique<PingResponderImpl>(std::move(receiver));
}

// browser_interface_binders.cc
void PopulateFrameBinders(RenderFrameHostImpl* host,
                          mojo::BinderMap* map) {
...
  // Register the handler for PingResponder.
  map->Add<example::mojom::PingResponder>(base::BindRepeating(
    &RenderFrameHostImpl::GetPingResponder, base::Unretained(host)));
}

And we're done. This setup is sufficient to plumb a new interface connection between a renderer frame and its browser-side host object!

Assuming we kept our ping_responder object alive in the renderer long enough, we would eventually see its OnPong callback invoked with the totally random value of 4, as defined by the browser-side implementation above.

Services Overview & Terminology

The previous section only scratches the surface of how Mojo IPC is used in Chromium. While renderer-to-browser messaging is simple and possibly the most prevalent usage by sheer code volume, we are incrementally decomposing the codebase into a set of services with a bit more granularity than the traditional Content browser/renderer/gpu/utility process split.

A service is a self-contained library of code which implements one or more related features or behaviors and whose interaction with outside code is done exclusively through Mojo interface connections, typically brokered by the browser process.

Each service defines and implements a main Mojo interface which can be used by the browser to manage an instance of the service.

Example: Building a Simple Out-of-Process Service

There are multiple steps typically involved to get a new service up and running in Chromium:

  • Define the main service interface and implementation
  • Hook up the implementation in out-of-process code
  • Write some browser logic to launch a service process

This section walks through these steps with some brief explanations. For more thorough documentation of the concepts and APIs used herein, see the Mojo documentation.

Defining the Service

Typically service definitions are placed in a services directory, either at the top level of the tree or within some subdirectory. In this example, we'll define a new service for use by Chrome specifically, so we'll define it within //chrome/services.

We can create the following files. First some mojoms:

// src/chrome/services/math/public/mojom/math_service.mojom
module math.mojom;

interface MathService {
  Divide(int32 dividend, int32 divisor) => (int32 quotient);
};
# src/chrome/services/math/public/mojom/BUILD.gn
import("//mojo/public/tools/bindings/mojom.gni")

mojom("mojom") {
  sources = [
    "math_service.mojom",
  ]
}

Then the actual MathService implementation:

// src/chrome/services/math/math_service.h
#include "base/macros.h"
#include "chrome/services/math/public/mojom/math_service.mojom.h"

namespace math {

class MathService : public mojom::MathService {
 public:
  explicit MathService(mojo::PendingReceiver<mojom::MathService> receiver);
  ~MathService() override;
  MathService(const MathService&) = delete;
  MathService& operator=(const MathService&) = delete;

 private:
  // mojom::MathService:
  void Divide(int32_t dividend,
              int32_t divisor,
              DivideCallback callback) override;

  mojo::Receiver<mojom::MathService> receiver_;
};

}  // namespace math
// src/chrome/services/math/math_service.cc
#include "chrome/services/math/math_service.h"

namespace math {

MathService::MathService(mojo::PendingReceiver<mojom::MathService> receiver)
    : receiver_(this, std::move(receiver)) {}

MathService::~MathService() = default;

void MathService::Divide(int32_t dividend,
                         int32_t divisor,
                         DivideCallback callback) {
  // Respond with the quotient!
  std::move(callback).Run(dividend / divisor);
}

}  // namespace math
# src/chrome/services/math/BUILD.gn

source_set("math") {
  sources = [
    "math_service.cc",
    "math_service.h",
  ]

  deps = [
    "//base",
    "//chrome/services/math/public/mojom",
  ]
}

Now we have a fully defined MathService implementation that we can make available in- or out-of-process.

Hooking Up the Service Implementation

For an out-of-process Chrome service, we simply register a factory function in //chrome/utility/services.cc.

auto RunMathService(mojo::PendingReceiver<math::mojom::MathService> receiver) {
  return std::make_unique<math::MathService>(std::move(receiver));
}

void RegisterMainThreadServices(mojo::ServiceFactory& services) {
  // Existing services...
  services.Add(RunFilePatcher);
  services.Add(RunUnzipper);

  // We add our own factory to this list
  services.Add(RunMathService);
  //...

With this done, it is now possible for the browser process to launch new out-of-process instances of MathService.

Launching the Service

If you're running your service in-process, there's really nothing interesting left to do. You can instantiate the service implementation just like any other object, yet you can also talk to it via a Mojo Remote as if it were out-of-process.

To launch an out-of-process service instance after the hookup performed in the previous section, use Content's ServiceProcessHost API:

mojo::Remote<math::mojom::MathService> math_service =
    content::ServiceProcessHost::Launch<math::mojom::MathService>(
        content::ServiceProcessHost::LaunchOptions()
            .WithDisplayName("Math!")
            .Pass());

Except in the case of crashes, the launched process will live as long as math_service lives. As a corollary, you can force the process to be torn down by destroying (or resetting) math_service.

We can now perform an out-of-process division:

// NOTE: As a client, we do not have to wait for any acknowledgement or
// confirmation of a connection. We can start queueing messages immediately and
// they will be delivered as soon as the service is up and running.
math_service->Divide(
    42, 6, base::BindOnce([](int32_t quotient) { LOG(INFO) << quotient; }));

*** aside NOTE: To ensure the execution of the response callback, the mojo::Remote<math::mojom::MathService> object must be kept alive (see this section and this note from an earlier section).

Specifying a sandbox

All services must specify a sandbox. Ideally services will run inside the kService process sandbox unless they need access to operating system resources. For services that need a custom sandbox, a new sandbox type must be defined in consultation with [email protected].

The preferred way to define the sandbox for your interface is by specifying a [ServiceSandbox=type] attribute on your interface {} in its .mojom file:

import "sandbox/policy/mojom/sandbox.mojom"
[ServiceSandbox=sandbox.mojom.Sandbox.kService]
interface FakeService {
  ...
};

Valid values are those in //sandbox/policy/mojom/sandbox.mojom. Note that the sandbox is only applied if the interface is launched out-of-process using content::ServiceProcessHost::Launch().

As a last resort, dynamic or feature based mapping to an underlying platform sandbox can be achieved but requires plumbing through ContentBrowserClient (e.g. ShouldEnableNetworkServiceSandbox()).

Content-Layer Services Overview

Interface Brokers

We define an explicit mojom interface with a persistent connection between a renderer's frame object and the corresponding RenderFrameHostImpl in the browser process. This interface is called BrowserInterfaceBroker and is fairly easy to work with: you add a new method on RenderFrameHostImpl:

void RenderFrameHostImpl::GetGoatTeleporter(
    mojo::PendingReceiver<magic::mojom::GoatTeleporter> receiver) {
  goat_teleporter_receiver_.Bind(std::move(receiver));
}

and register this method in PopulateFrameBinders function in browser_interface_binders.cc, which maps specific interfaces to their handlers in respective hosts:

// //content/browser/browser_interface_binders.cc
void PopulateFrameBinders(RenderFrameHostImpl* host,
                          mojo::BinderMap* map) {
...
  map->Add<magic::mojom::GoatTeleporter>(base::BindRepeating(
      &RenderFrameHostImpl::GetGoatTeleporter, base::Unretained(host)));
}

It's also possible to bind an interface on a different sequence by specifying a task runner:

// //content/browser/browser_interface_binders.cc
void PopulateFrameBinders(RenderFrameHostImpl* host,
                          mojo::BinderMap* map) {
...
  map->Add<magic::mojom::GoatTeleporter>(base::BindRepeating(
      &RenderFrameHostImpl::GetGoatTeleporter, base::Unretained(host)),
      GetIOThreadTaskRunner({}));
}

For binding an embedder-specific document-scoped interface, override ContentBrowserClient::RegisterBrowserInterfaceBindersForFrame() and add the binders to the provided map.

*** aside NOTE: if BrowserInterfaceBroker cannot find a binder for the requested interface, it will call ReportNoBinderForInterface() on the relevant context host, which results in a ReportBadMessage() call on the host's receiver (one of the consequences is a termination of the renderer). To avoid this crash in tests (when content_shell doesn't bind some Chrome-specific interfaces, but the renderer requests them anyway), use the EmptyBinderForFrame helper in browser_interface_binders.cc. However, it is recommended to have the renderer and browser sides consistent if possible.

TODO: add information about workers.

Additional Support

If this document was not helpful in some way, please post a message to your friendly [email protected] or [email protected] mailing list.