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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.
//
// NOTE: Header files that do not require the full definition of
// base::{Once,Repeating}Callback or base::{Once,Repeating}Closure should
// #include "base/callback_forward.h" instead of this file.
#ifndef BASE_CALLBACK_H_
#define BASE_CALLBACK_H_
#include <stddef.h>
#include "base/callback_forward.h"
#include "base/callback_internal.h"
// -----------------------------------------------------------------------------
// Usage documentation
// -----------------------------------------------------------------------------
//
// Overview:
// A callback is similar in concept to a function pointer: it wraps a runnable
// object such as a function, method, lambda, or even another callback, allowing
// the runnable object to be invoked later via the callback object.
//
// Unlike function pointers, callbacks are created with base::BindOnce() or
// base::BindRepeating() and support partial function application.
//
// A base::OnceCallback may be Run() at most once; a base::RepeatingCallback may
// be Run() any number of times. |is_null()| is guaranteed to return true for a
// moved-from callback.
//
// // The lambda takes two arguments, but the first argument |x| is bound at
// // callback creation.
// base::OnceCallback<int(int)> cb = base::BindOnce([] (int x, int y) {
// return x + y;
// }, 1);
// // Run() only needs the remaining unbound argument |y|.
// printf("1 + 2 = %d\n", std::move(cb).Run(2)); // Prints 3
// printf("cb is null? %s\n",
// cb.is_null() ? "true" : "false"); // Prints true
// std::move(cb).Run(2); // Crashes since |cb| has already run.
//
// Callbacks also support cancellation. A common use is binding the receiver
// object as a WeakPtr<T>. If that weak pointer is invalidated, calling Run()
// will be a no-op. Note that |IsCancelled()| and |is_null()| are distinct:
// simply cancelling a callback will not also make it null.
//
// base::Callback is currently a type alias for base::RepeatingCallback. In the
// future, we expect to flip this to default to base::OnceCallback.
//
// See //docs/callback.md for the full documentation.
namespace base {
template <typename R, typename... Args>
class OnceCallback<R(Args...)> : public internal::CallbackBase {
public:
using RunType = R(Args...);
using PolymorphicInvoke = R (*)(internal::BindStateBase*,
internal::PassingType<Args>...);
constexpr OnceCallback() = default;
OnceCallback(std::nullptr_t) = delete;
explicit OnceCallback(internal::BindStateBase* bind_state)
: internal::CallbackBase(bind_state) {}
OnceCallback(const OnceCallback&) = delete;
OnceCallback& operator=(const OnceCallback&) = delete;
OnceCallback(OnceCallback&&) noexcept = default;
OnceCallback& operator=(OnceCallback&&) noexcept = default;
OnceCallback(RepeatingCallback<RunType> other)
: internal::CallbackBase(std::move(other)) {}
OnceCallback& operator=(RepeatingCallback<RunType> other) {
static_cast<internal::CallbackBase&>(*this) = std::move(other);
return *this;
}
R Run(Args... args) const & {
static_assert(!sizeof(*this),
"OnceCallback::Run() may only be invoked on a non-const "
"rvalue, i.e. std::move(callback).Run().");
NOTREACHED();
}
R Run(Args... args) && {
// Move the callback instance into a local variable before the invocation,
// that ensures the internal state is cleared after the invocation.
// It's not safe to touch |this| after the invocation, since running the
// bound function may destroy |this|.
OnceCallback cb = std::move(*this);
PolymorphicInvoke f =
reinterpret_cast<PolymorphicInvoke>(cb.polymorphic_invoke());
return f(cb.bind_state_.get(), std::forward<Args>(args)...);
}
};
template <typename R, typename... Args>
class RepeatingCallback<R(Args...)> : public internal::CallbackBaseCopyable {
public:
using RunType = R(Args...);
using PolymorphicInvoke = R (*)(internal::BindStateBase*,
internal::PassingType<Args>...);
constexpr RepeatingCallback() = default;
RepeatingCallback(std::nullptr_t) = delete;
explicit RepeatingCallback(internal::BindStateBase* bind_state)
: internal::CallbackBaseCopyable(bind_state) {}
// Copyable and movable.
RepeatingCallback(const RepeatingCallback&) = default;
RepeatingCallback& operator=(const RepeatingCallback&) = default;
RepeatingCallback(RepeatingCallback&&) noexcept = default;
RepeatingCallback& operator=(RepeatingCallback&&) noexcept = default;
bool operator==(const RepeatingCallback& other) const {
return EqualsInternal(other);
}
bool operator!=(const RepeatingCallback& other) const {
return !operator==(other);
}
R Run(Args... args) const & {
PolymorphicInvoke f =
reinterpret_cast<PolymorphicInvoke>(this->polymorphic_invoke());
return f(this->bind_state_.get(), std::forward<Args>(args)...);
}
R Run(Args... args) && {
// Move the callback instance into a local variable before the invocation,
// that ensures the internal state is cleared after the invocation.
// It's not safe to touch |this| after the invocation, since running the
// bound function may destroy |this|.
RepeatingCallback cb = std::move(*this);
PolymorphicInvoke f =
reinterpret_cast<PolymorphicInvoke>(cb.polymorphic_invoke());
return f(std::move(cb).bind_state_.get(), std::forward<Args>(args)...);
}
};
} // namespace base
#endif // BASE_CALLBACK_H_
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