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.
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.
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:
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(...));
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/caller 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(); } };
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.
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();