chromium/base/functional/callback_helpers.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This defines helpful methods for dealing with Callbacks.  Because Callbacks
// are implemented using templates, with a class per callback signature, adding
// methods to Callback<> itself is unattractive (lots of extra code gets
// generated).  Instead, consider adding methods here.

#ifndef BASE_FUNCTIONAL_CALLBACK_HELPERS_H_
#define BASE_FUNCTIONAL_CALLBACK_HELPERS_H_

#include <atomic>
#include <memory>
#include <ostream>
#include <type_traits>
#include <utility>

#include "base/atomicops.h"
#include "base/base_export.h"
#include "base/check.h"
#include "base/functional/bind.h"
#include "base/functional/callback.h"
#include "base/functional/callback_tags.h"

namespace base {

namespace internal {

template <typename T>
struct IsBaseCallbackImpl : std::false_type { … };

IsBaseCallbackImpl<OnceCallback<R (Args...)>>;

IsBaseCallbackImpl<RepeatingCallback<R (Args...)>>;

}  // namespace internal

// IsBaseCallback<T> is satisfied if and only if T is an instantiation of
// base::OnceCallback<Signature> or base::RepeatingCallback<Signature>.
IsBaseCallback;

namespace internal {

template <typename... Args>
class OnceCallbackHolder final { … };

template <typename... Args>
void ForwardRepeatingCallbacksImpl(
    std::vector<RepeatingCallback<void(Args...)>> cbs,
    Args... args) { … }

}  // namespace internal

// Wraps the given RepeatingCallbacks and return one RepeatingCallbacks with an
// identical signature. On invocation of this callback, all the given
// RepeatingCallbacks will be called with the same arguments. Unbound arguments
// must be copyable.
template <typename... Args>
RepeatingCallback<void(Args...)> ForwardRepeatingCallbacks(
    std::initializer_list<RepeatingCallback<void(Args...)>>&& cbs) { … }

// Wraps the given OnceCallback and returns two OnceCallbacks with an identical
// signature. On first invokation of either returned callbacks, the original
// callback is invoked. Invoking the remaining callback results in a crash.
template <typename... Args>
std::pair<OnceCallback<void(Args...)>, OnceCallback<void(Args...)>>
SplitOnceCallback(OnceCallback<void(Args...)> callback) { … }

// Adapts `callback` for use in a context which is expecting a callback with
// additional parameters. Returns a null callback if `callback` is null.
//
// Usage:
//   bool LogError(char* error_message) {
//     if (error_message) {
//       cout << "Log: " << error_message << endl;
//       return false;
//     }
//     return true;
//   }
//   base::RepeatingCallback<bool(int, char*)> cb =
//      base::IgnoreArgs<int>(base::BindRepeating(&LogError));
//   CHECK_EQ(true, cb.Run(42, nullptr));
//
// Note in the example above that the type(s) passed to `IgnoreArgs`
// represent the additional prepended parameters (those which will be
// "ignored").
template <typename... Preargs, typename... Args, typename R>
RepeatingCallback<R(Preargs..., Args...)> IgnoreArgs(
    RepeatingCallback<R(Args...)> callback) { … }

// As above, but for OnceCallback.
template <typename... Preargs, typename... Args, typename R>
OnceCallback<R(Preargs..., Args...)> IgnoreArgs(
    OnceCallback<R(Args...)> callback) { … }

// ScopedClosureRunner is akin to std::unique_ptr<> for Closures. It ensures
// that the Closure is executed no matter how the current scope exits.
// If you are looking for "ScopedCallback", "CallbackRunner", or
// "CallbackScoper" this is the class you want.
class BASE_EXPORT ScopedClosureRunner { … };

// Returns a placeholder type that will implicitly convert into a null callback,
// similar to how std::nullopt / std::nullptr work in conjunction with
// std::optional and various smart pointer types.
constexpr auto NullCallback() { … }

// Returns a placeholder type that will implicitly convert into a callback that
// does nothing, similar to how std::nullopt / std::nullptr work in conjunction
// with std::optional and various smart pointer types.
constexpr auto DoNothing() { … }

// Similar to the above, but with a type hint. Useful for disambiguating
// among multiple function overloads that take callbacks with different
// signatures:
//
// void F(base::OnceCallback<void()> callback);     // 1
// void F(base::OnceCallback<void(int)> callback);  // 2
//
// F(base::NullCallbackAs<void()>());               // calls 1
// F(base::DoNothingAs<void(int)>());               // calls 2
template <typename Signature>
constexpr auto NullCallbackAs() { … }

template <typename Signature>
constexpr auto DoNothingAs() { … }

// Similar to DoNothing above, but with bound arguments. This helper is useful
// for keeping objects alive until the callback runs.
// Example:
//
// void F(base::OnceCallback<void(int)> result_callback);
//
// std::unique_ptr<MyClass> ptr;
// F(base::DoNothingWithBoundArgs(std::move(ptr)));
template <typename... Args>
constexpr auto DoNothingWithBoundArgs(Args&&... args) { … }

// Creates a callback that returns `value` when invoked. This helper is useful
// for implementing factories that return a constant value.
// Example:
//
// void F(base::OnceCallback<Widget()> factory);
//
// Widget widget = ...;
// F(base::ReturnValueOnce(std::move(widget)));
template <typename T>
constexpr OnceCallback<T(void)> ReturnValueOnce(T value) { … }

// Useful for creating a Closure that will delete a pointer when invoked. Only
// use this when necessary. In most cases MessageLoop::DeleteSoon() is a better
// fit.
template <typename T>
void DeletePointer(T* obj) { … }

#if __OBJC__

// Creates an Objective-C block with the same signature as the corresponding
// callback. Can be used to implement a callback based API internally based
// on a block based Objective-C API.
//
// Overloaded to work with both repeating and one shot callbacks. Calling the
// block wrapping a base::OnceCallback<...> multiple times will crash (there
// is no way to mark the block as callable only once). Only use that when you
// know that Objective-C API will only invoke the block once.
template <typename R, typename... Args>
auto CallbackToBlock(base::OnceCallback<R(Args...)> callback) {
  __block base::OnceCallback<R(Args...)> block_callback = std::move(callback);
  return ^(Args... args) {
    return std::move(block_callback).Run(std::forward<Args>(args)...);
  };
}

template <typename R, typename... Args>
auto CallbackToBlock(base::RepeatingCallback<R(Args...)> callback) {
  return ^(Args... args) {
    return callback.Run(std::forward<Args>(args)...);
  };
}

#endif  // __OBJC__

}  // namespace base

#endif  // BASE_FUNCTIONAL_CALLBACK_HELPERS_H_