cpp/bindings/pending_receiver.h

// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef MOJO_PUBLIC_CPP_BINDINGS_PENDING_RECEIVER_H_
#define MOJO_PUBLIC_CPP_BINDINGS_PENDING_RECEIVER_H_

#include <type_traits>
#include <utility>

#include "base/macros.h"
#include "build/build_config.h"
#include "mojo/public/cpp/bindings/connection_group.h"
#include "mojo/public/cpp/bindings/interface_request.h"
#include "mojo/public/cpp/bindings/lib/bindings_internal.h"
#include "mojo/public/cpp/bindings/lib/pending_receiver_state.h"
#include "mojo/public/cpp/bindings/lib/serialization_context.h"
#include "mojo/public/cpp/system/message_pipe.h"

namespace mojo {

template <typename T>
struct PendingReceiverConverter;

// A PendingReceiver receives and accumulates a queue of incoming Interface
// method calls made by a single corresponding Remote. PendingReceiver instances
// may be freely moved to another thread/sequence, or even transferred to
// another process via a Mojo interface call (see pending_receiver<T> syntax in
// mojom IDL).
//
// This object should eventually be consumed to bind a Receiver, which will then
// begin dispatching any queued and future incoming method calls to a local
// implementation of Interface. See Receiver documentation for more details.
//
// NOTE: This object is essentially semantic sugar wrapping a message pipe
// handle that is expected to receive Interface messages from a Remote. As such,
// consumers who know what they're doing (i.e. who are confident about what lies
// on the other end of a pipe) may freely convert between a PendingReceiver and
// a raw message pipe handle.
template <typename Interface>
class PendingReceiver {
 public:
  // Constructs an invalid PendingReceiver. This object is not entangled with
  // any Remote and cannot be used to bind a Receiver.
  //
  // A valid PendingReceiver is commonly obtained by calling
  // |Remote::BindNewPipeAndPassReceiver()| on an existing unbound Remote
  // instance or less commonly by calling calling
  // |PendingRemote::InitWithNewPipeAndPassReceiver()| on an existing but
  // invalid PendingRemote instance.
  PendingReceiver() = default;
  PendingReceiver(PendingReceiver&&) noexcept = default;

  // Temporary implicit move constructor to aid in converting from use of
  // InterfaceRequest<Interface> to PendingReceiver.
  PendingReceiver(InterfaceRequest<Interface>&& request)
      : PendingReceiver(request.PassMessagePipe()) {
    set_connection_group(request.PassConnectionGroupRef());
  }

  // Constructs a valid PendingReceiver from a valid raw message pipe handle.
  explicit PendingReceiver(ScopedMessagePipeHandle pipe)
      : state_(std::move(pipe)) {}

  // Disabled on NaCl since it crashes old version of clang.
#if !defined(OS_NACL)
  // Move conversion operator for custom receiver types. Only participates in
  // overload resolution if a typesafe conversion is supported.
  template <
      typename T,
      std::enable_if_t<std::is_same<
          PendingReceiver<Interface>,
          std::result_of_t<decltype (&PendingReceiverConverter<T>::template To<
                                     Interface>)(T&&)>>::value>* = nullptr>
  PendingReceiver(T&& other)
      : PendingReceiver(PendingReceiverConverter<T>::template To<Interface>(
            std::forward<T>(other))) {}
#endif  // !defined(OS_NACL)

  ~PendingReceiver() = default;

  PendingReceiver& operator=(PendingReceiver&&) noexcept = default;

  // Temporary implicit conversion operator to InterfaceRequest<Interface> to
  // aid in converting usage to PendingReceiver.
  operator InterfaceRequest<Interface>() && {
    InterfaceRequest<Interface> request(PassPipe());
    request.set_connection_group(PassConnectionGroupRef());
    return request;
  }

  // Indicates whether the PendingReceiver is valid, meaning it can ne used to
  // bind a Receiver that wants to begin dispatching method calls made by the
  // entangled Remote.
  bool is_valid() const { return state_.pipe.is_valid(); }
  explicit operator bool() const { return is_valid(); }

  // Resets this PendingReceiver to an invalid state. If it was entangled with a
  // Remote or PendingRemote, that object remains in a valid state and will
  // eventually detect that its receiver is gone. Any calls it makes will
  // effectively be dropped.
  void reset() { state_.reset(); }

  // Like above but provides a reason for the disconnection.
  void ResetWithReason(uint32_t reason, const std::string& description) {
    InterfaceRequest<Interface>(PassPipe())
        .ResetWithReason(reason, description);
  }

  // Passes ownership of this PendingReceiver's message pipe handle. After this
  // call, the PendingReceiver is no longer in a valid state and can no longer
  // be used to bind a Receiver.
  ScopedMessagePipeHandle PassPipe() WARN_UNUSED_RESULT {
    return std::move(state_.pipe);
  }

  // Assigns this PendingReceiver to the ConnectionGroup referenced by |ref|.
  // Any Receiver which binds this PendingReceiver will inherit the Ref.
  void set_connection_group(ConnectionGroup::Ref ref) {
    state_.connection_group = std::move(ref);
  }

  const ConnectionGroup::Ref& connection_group() const {
    return state_.connection_group;
  }

  // Passes ownership of this PendingReceiver's ConnectionGroup Ref, removing it
  // from its group.
  ConnectionGroup::Ref PassConnectionGroupRef() {
    return std::move(state_.connection_group);
  }

  // For internal Mojo use only.
  internal::PendingReceiverState* internal_state() { return &state_; }

 private:
  internal::PendingReceiverState state_;

  DISALLOW_COPY_AND_ASSIGN(PendingReceiver);
};

class COMPONENT_EXPORT(MOJO_CPP_BINDINGS) NullReceiver {
 public:
  template <typename Interface>
  operator PendingReceiver<Interface>() const {
    return PendingReceiver<Interface>();
  }
};

}  // namespace mojo

#endif  // MOJO_PUBLIC_CPP_BINDINGS_PENDING_RECEIVER_H_