Mojo Basics

This document aims to provide a brief overview of the different concepts in Mojo and how they work together. For more details about more complex and/or Chrome-specific Mojo use cases, please consult Intro to Mojo & Services.

Interfaces

Mojo provides a C++-like interface definition language for defining interfaces for making interprocess calls (IPCs):

module math.mojom;

interface Math {
  // Adds two int32s and returns the result as an int64 (to avoid
  // overflow issues).
  Add(int32 x, int32 y) => (int64 sum);
};

Interfaces are built using the mojom (or mojom_component) GN template:

mojom("mojom") {
  sources = ["math.mojom"]
}

This will generate C++ (and optionally, Java and JavaScript) interfaces. Writing code to handle IPCs is a simple matter of implementing the generated interface:

class MathImpl : public math::mojom::Math {
 public:
  explicit MathImpl(mojo::PendingReceiver<math::mojom::Math> receiver)
      : receiver_(this, std::move(receiver)) {}

  // math::mojom::Math overrides:
  // Note: AddCallback is a type alias for base::OnceCallback<void(int64_t)>.
  // The parameters to the callback are the reply parameters specified in the
  // Mojo IDL method definition. This is part of the boilerplate generated by
  // Mojo: invoking |reply| will send a reply to the caller.
  void Add(int32_t x, int32_t y, AddCallback reply) override {
    // Note: Mojo always returns results via callback. While it is possible to
    // make a sync IPC which blocks on the reply, the handler will always return
    // the result via callback.
    std::move(reply).Run(static_cast<int64_t>(x) + y);
  }

 private:
  // Wraps a message pipe endpoint that receives incoming messages. See the
  // message pipes section below for more information.
  mojo::Receiver<math::mojom::Math> receiver_;
};

Note: the build process also generates proxy classes (e.g. MathProxy) which encapsulate the details of making the actual cross-process call. These are used internally and are an implementation detail that can typically be ignored.

Message Pipes

Interfaces are layered on top of low-level message pipes. Each message pipe has two bidirectional endpoints. The Mojo bindings enforce additional conventions on top of message pipes, where one endpoint is the sender/caller, represented as:

// Wraps a message pipe endpoint for making remote calls. May only be used on
// the sequence where the mojo::Remote was bound.
mojo::Remote<math::mojom::Math> remote_math = ...;

And the other endpoint is the receiving/callee, represented as:

// Usually a class member. Wraps a message pipe endpoint that receives incoming
// messages. Routes and dispatches IPCs to the handler—typically |this|—on the
// sequence where the mojo::Receiver was bound.
mojo::Receiver<math::mojom::Math> receiver_;

This allows limited bidirectional communication. For one interface, the sender (A) may make any number of calls to the receiver (B). (B) may send a single reply for each call from (A). More expressive APIs are often implemented as a pair of interfaces (with two underlying message pipes), allowing calls to be made in either direction between (A) and (B).

Message pipe endpoints are typically created using one of:

mojo::Remote::BindNewPipeAndPassReceiver

Used when the sender/caller creates the endpoints. One endpoint is retained for itself to send IPCs, and the other endpoint is returned as an unbound mojo::PendingReceiver<T> for the receiver/callee to bind to a mojo::Receiver<T>.

mojo::Remote<math::mojom::Math> remote_math;

// BindNewPipeAndPassReceiver() returns a
// mojo::PendingReceiver<math::mojom::Math>. This may be bound to a
// mojo::Receiver<math::mojom::Math> to handle calls received from
// |remote_math|.
LaunchAndBindRemoteMath(remote_math.BindNewPipeAndPassReceiver());

// |remote_math| may be immediately used. The Add() call will be buffered by the
// receiving end and dispatched when mojo::PendingReceiver<math::mojom::Math> is
// bound to a mojo::Receiver<math::mojom::Math>.
remote_math->Add(2, 2, base::BindOnce(...));

mojo::Receiver::BindNewPipeAndPassRemote

Used when the receiver/callee creates the endpoints. One endpoint is retained for itself to receive IPCs, and the other endpoint is returned as an unbound mojo::PendingRemote<T> for the sender/callee to bind to a mojo::Remote<T>.

class MathImpl : public math::mojom::MathImpl {
  // ...addition to the previous MathImpl definition...

  mojo::PendingRemote<math::mojom::Math> GetRemoteMath() {
    // BindNewPipeAndPassRemote() returns a
    // `mojo::PendingRemote<math::mojom::Math>`. This may be bound to a
    // `mojo::Remote<math::mojom::Math> which can be used to send IPCs that will
    // be handled by |this|.
    return receiver_.BindNewPipeAndPassRemote();
  }
};

mojo::PendingRemote::InitWithNewPipeAndPassReceiver

Less common, but similar to mojo::Remote<T>::BindNewPipeAndPassReceiver(). Typically used by broker code that needs to hand off a mojo::PendingRemote<T> to the sender/caller side and hand off a mojo::PendingReceiver<T> to the receiver/callee side.

mojo::Remote/mojo::Receiver and mojo::PendingRemote/mojo::PendingReceiver

Both mojo::Remote<T> and mojo::Receiver<T> have a corresponding unbound version: this allows either endpoint to be passed between sequences in the same process or even between processes over IPC.

mojo::Remote<math::mojom::MathImpl> remote = ...;
// |pending_remote| is movable and may be passed around. While unbound, the
// endpoint cannot be used to send IPCs. The pending remote may be passed to
// the mojo::Remote<T> constructor or mojo::Remote<T>::Bind() to rebind the
// endpoint.
mojo::PendingRemote<math::mojom::MathImpl> pending_remote = remote.Unbind();

mojo::Receiver<math::mojom::MathImpl> receiver = ...;
// |pending_receiver| is movable and may be passed around. While unbound,
// received IPCs are buffered and not processed. The pending receiver may be
// passed to the mojo::Receiver<T> constructor or mojo::Receiver<T>::Bind() to
// rebind the endpoint.
mojo::PendingReceiver<math::mojom::MathImpl> pending_receiver = receiver.Unbind();