callback_internal.h

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// Copyright (c) 2012 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.

// This file contains utility functions and classes that help the

// implementation, and management of the Callback objects.

#ifndef BASE_CALLBACK_INTERNAL_H_

#define BASE_CALLBACK_INTERNAL_H_

#include "base/base_export.h"

#include "base/callback_forward.h"

#include "base/macros.h"

#include "base/memory/ref_counted.h"

namespace base {

struct FakeBindState;

namespace internal {

class BindStateBase;

class FinallyExecutorCommon;

class ThenAndCatchExecutorCommon;

template <typename ReturnType>

class PostTaskExecutor;

template <typename Functor, typename... BoundArgs>

struct BindState;

class CallbackBase;

class CallbackBaseCopyable;

struct BindStateBaseRefCountTraits {

  static void Destruct(const BindStateBase*);

};

template <typename T>

using PassingType = std::conditional_t<std::is_scalar<T>::value, T, T&&>;

// BindStateBase is used to provide an opaque handle that the Callback

// class can use to represent a function object with bound arguments.  It

// behaves as an existential type that is used by a corresponding

// DoInvoke function to perform the function execution.  This allows

// us to shield the Callback class from the types of the bound argument via

// "type erasure."

// At the base level, the only task is to add reference counting data. Avoid

// using or inheriting any virtual functions. Creating a vtable for every

// BindState template instantiation results in a lot of bloat. Its only task is

// to call the destructor which can be done with a function pointer.

class BASE_EXPORT BindStateBase

    : public RefCountedThreadSafe<BindStateBase, BindStateBaseRefCountTraits> {

 public:

  REQUIRE_ADOPTION_FOR_REFCOUNTED_TYPE();

  enum CancellationQueryMode {

    IS_CANCELLED,

    MAYBE_VALID,

};

  using InvokeFuncStorage = void(*)();

 private:

  BindStateBase(InvokeFuncStorage polymorphic_invoke,

                void (*destructor)(const BindStateBase*));

  BindStateBase(InvokeFuncStorage polymorphic_invoke,

                void (*destructor)(const BindStateBase*),

                bool (*query_cancellation_traits)(const BindStateBase*,

                                                  CancellationQueryMode mode));

  ~BindStateBase() = default;

  friend struct BindStateBaseRefCountTraits;

  friend class RefCountedThreadSafe<BindStateBase, BindStateBaseRefCountTraits>;

  friend class CallbackBase;

  friend class CallbackBaseCopyable;

  // Whitelist subclasses that access the destructor of BindStateBase.

  template <typename Functor, typename... BoundArgs>

  friend struct BindState;

  friend struct ::base::FakeBindState;

  bool IsCancelled() const {

    return query_cancellation_traits_(this, IS_CANCELLED);

  bool MaybeValid() const {

    return query_cancellation_traits_(this, MAYBE_VALID);

  // In C++, it is safe to cast function pointers to function pointers of

  // another type. It is not okay to use void*. We create a InvokeFuncStorage

  // that that can store our function pointer, and then cast it back to

  // the original type on usage.

  InvokeFuncStorage polymorphic_invoke_;

  // Pointer to a function that will properly destroy |this|.

  void (*destructor_)(const BindStateBase*);

  bool (*query_cancellation_traits_)(const BindStateBase*,

                                     CancellationQueryMode mode);

  DISALLOW_COPY_AND_ASSIGN(BindStateBase);

};

// Holds the Callback methods that don't require specialization to reduce

// template bloat.

// CallbackBase<MoveOnly> is a direct base class of MoveOnly callbacks, and

// CallbackBase<Copyable> uses CallbackBase<MoveOnly> for its implementation.

class BASE_EXPORT CallbackBase {

 public:

  inline CallbackBase(CallbackBase&& c) noexcept;

  CallbackBase& operator=(CallbackBase&& c) noexcept;

  explicit CallbackBase(const CallbackBaseCopyable& c);

  CallbackBase& operator=(const CallbackBaseCopyable& c);

  explicit CallbackBase(CallbackBaseCopyable&& c) noexcept;

  CallbackBase& operator=(CallbackBaseCopyable&& c) noexcept;

  // Returns true if Callback is null (doesn't refer to anything).

  bool is_null() const { return !bind_state_; }

  explicit operator bool() const { return !is_null(); }

  // Returns true if the callback invocation will be nop due to an cancellation.

  // It's invalid to call this on uninitialized callback.

//

  // Must be called on the Callback's destination sequence.

  bool IsCancelled() const;

  // If this returns false, the callback invocation will be a nop due to a

  // cancellation. This may(!) still return true, even on a cancelled callback.

//

  // This function is thread-safe.

  bool MaybeValid() const;

  // Returns the Callback into an uninitialized state.

  void Reset();

 protected:

  friend class FinallyExecutorCommon;

  friend class ThenAndCatchExecutorCommon;

  template <typename ReturnType>

  friend class PostTaskExecutor;

  using InvokeFuncStorage = BindStateBase::InvokeFuncStorage;

  // Returns true if this callback equals |other|. |other| may be null.

  bool EqualsInternal(const CallbackBase& other) const;

  constexpr inline CallbackBase();

  // Allow initializing of |bind_state_| via the constructor to avoid default

  // initialization of the scoped_refptr.

  explicit inline CallbackBase(BindStateBase* bind_state);

  InvokeFuncStorage polymorphic_invoke() const {

    return bind_state_->polymorphic_invoke_;

  // Force the destructor to be instantiated inside this translation unit so

  // that our subclasses will not get inlined versions.  Avoids more template

  // bloat.

  ~CallbackBase();

  scoped_refptr<BindStateBase> bind_state_;

};

constexpr CallbackBase::CallbackBase() = default;

CallbackBase::CallbackBase(CallbackBase&&) noexcept = default;

CallbackBase::CallbackBase(BindStateBase* bind_state)

    : bind_state_(AdoptRef(bind_state)) {}

// CallbackBase<Copyable> is a direct base class of Copyable Callbacks.

class BASE_EXPORT CallbackBaseCopyable : public CallbackBase {

 public:

  CallbackBaseCopyable(const CallbackBaseCopyable& c);

  CallbackBaseCopyable(CallbackBaseCopyable&& c) noexcept = default;

  CallbackBaseCopyable& operator=(const CallbackBaseCopyable& c);

  CallbackBaseCopyable& operator=(CallbackBaseCopyable&& c) noexcept;

 protected:

  constexpr CallbackBaseCopyable() = default;

  explicit CallbackBaseCopyable(BindStateBase* bind_state)

      : CallbackBase(bind_state) {}

  ~CallbackBaseCopyable() = default;

};

}  // namespace internal

}  // namespace base

#endif  // BASE_CALLBACK_INTERNAL_H_