/*
* Copyright 2020 The WebRTC Project Authors. All rights reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef RTC_BASE_CALLBACK_LIST_H_
#define RTC_BASE_CALLBACK_LIST_H_
#include <utility>
#include <vector>
#include "api/function_view.h"
#include "rtc_base/checks.h"
#include "rtc_base/system/assume.h"
#include "rtc_base/system/inline.h"
#include "rtc_base/system/rtc_export.h"
#include "rtc_base/untyped_function.h"
namespace webrtc {
namespace callback_list_impl {
class RTC_EXPORT CallbackListReceivers {
public:
CallbackListReceivers();
CallbackListReceivers(const CallbackListReceivers&) = delete;
CallbackListReceivers& operator=(const CallbackListReceivers&) = delete;
CallbackListReceivers(CallbackListReceivers&&) = delete;
CallbackListReceivers& operator=(CallbackListReceivers&&) = delete;
~CallbackListReceivers();
template <typename UntypedFunctionArgsT>
RTC_NO_INLINE void AddReceiver(const void* removal_tag,
UntypedFunctionArgsT args) {
RTC_CHECK(!send_in_progress_);
RTC_DCHECK(removal_tag != nullptr);
receivers_.push_back({removal_tag, UntypedFunction::Create(args)});
}
template <typename UntypedFunctionArgsT>
RTC_NO_INLINE void AddReceiver(UntypedFunctionArgsT args) {
RTC_CHECK(!send_in_progress_);
receivers_.push_back({nullptr, UntypedFunction::Create(args)});
}
void RemoveReceivers(const void* removal_tag);
void Foreach(rtc::FunctionView<void(UntypedFunction&)> fv);
private:
// Special protected pointer value that's used as a removal_tag for
// receivers that want to unsubscribe from within a callback.
// Note we could use `&receivers_` too, but since it's the first member
// variable of the class, its address will be the same as the instance
// CallbackList instance, so we take an extra step to avoid collision.
const void* pending_removal_tag() const { return &send_in_progress_; }
struct Callback {
const void* removal_tag;
UntypedFunction function;
};
std::vector<Callback> receivers_;
bool send_in_progress_ = false;
};
extern template void CallbackListReceivers::AddReceiver(
const void*,
UntypedFunction::TrivialUntypedFunctionArgs<1>);
extern template void CallbackListReceivers::AddReceiver(
const void*,
UntypedFunction::TrivialUntypedFunctionArgs<2>);
extern template void CallbackListReceivers::AddReceiver(
const void*,
UntypedFunction::TrivialUntypedFunctionArgs<3>);
extern template void CallbackListReceivers::AddReceiver(
const void*,
UntypedFunction::TrivialUntypedFunctionArgs<4>);
extern template void CallbackListReceivers::AddReceiver(
const void*,
UntypedFunction::NontrivialUntypedFunctionArgs);
extern template void CallbackListReceivers::AddReceiver(
const void*,
UntypedFunction::FunctionPointerUntypedFunctionArgs);
extern template void CallbackListReceivers::AddReceiver(
UntypedFunction::TrivialUntypedFunctionArgs<1>);
extern template void CallbackListReceivers::AddReceiver(
UntypedFunction::TrivialUntypedFunctionArgs<2>);
extern template void CallbackListReceivers::AddReceiver(
UntypedFunction::TrivialUntypedFunctionArgs<3>);
extern template void CallbackListReceivers::AddReceiver(
UntypedFunction::TrivialUntypedFunctionArgs<4>);
extern template void CallbackListReceivers::AddReceiver(
UntypedFunction::NontrivialUntypedFunctionArgs);
extern template void CallbackListReceivers::AddReceiver(
UntypedFunction::FunctionPointerUntypedFunctionArgs);
} // namespace callback_list_impl
// A collection of receivers (callable objects) that can be called all at once.
// Optimized for minimal binary size. The template arguments dictate what
// signature the callbacks must have; for example, a CallbackList<int, float>
// will require callbacks with signature void(int, float).
//
// CallbackList is neither copyable nor movable (could easily be made movable if
// necessary). Callbacks must be movable, but need not be copyable.
//
// Usage example:
//
// // Declaration (usually a member variable).
// CallbackList<int, float> foo_;
//
// // Register callbacks. This can be done zero or more times. The
// // callbacks must accept the arguments types listed in the CallbackList's
// // template argument list, and must return void.
// foo_.AddReceiver([...](int a, float b) {...}); // Lambda.
// foo_.AddReceiver(SomeFunction); // Function pointer.
//
// // Call the zero or more receivers, one after the other.
// foo_.Send(17, 3.14);
//
// Callback lifetime considerations
// --------------------------------
//
// CallbackList::AddReceiver() takes ownership of the given callback by moving
// it in place. The callback can be any callable object; in particular, it may
// have a nontrivial destructor, which will be run when the CallbackList is
// destroyed. The callback may thus access data via any type of smart pointer,
// expressing e.g. unique, shared, or weak ownership. Of course, if the data is
// guaranteed to outlive the callback, a plain raw pointer can be used.
//
// Take care when trying to have the callback own reference-counted data. The
// CallbackList will keep the callback alive, and the callback will keep its
// data alive, so as usual with reference-counted ownership, keep an eye out for
// cycles!
//
// Thread safety
// -------------
//
// Like most C++ types, CallbackList is thread compatible: it's not safe to
// access it concurrently from multiple threads, but it can be made safe if it
// is protected by a mutex, for example.
//
// Excercise some care when deciding what mutexes to hold when you call
// CallbackList::Send(). In particular, do not hold mutexes that callbacks may
// need to grab. If a larger object has a CallbackList member and a single mutex
// that protects all of its data members, this may e.g. make it necessary to
// protect its CallbackList with a separate mutex; otherwise, there will be a
// deadlock if the callbacks try to access the object.
//
// CallbackList as a class data member
// -----------------------------------
//
// CallbackList is a normal C++ data type, and should be private when it is a
// data member of a class. For thread safety reasons (see above), it is likely
// best to not have an accessor for the entire CallbackList, and instead only
// allow callers to add callbacks:
//
// template <typename F>
// void AddFooCallback(F&& callback) {
// // Maybe grab a mutex here?
// foo_callbacks_.AddReceiver(std::forward<F>(callback));
// }
//
template <typename... ArgT>
class CallbackList {
public:
CallbackList() = default;
CallbackList(const CallbackList&) = delete;
CallbackList& operator=(const CallbackList&) = delete;
CallbackList(CallbackList&&) = delete;
CallbackList& operator=(CallbackList&&) = delete;
// Adds a new receiver. The receiver (a callable object or a function pointer)
// must be movable, but need not be copyable. Its call signature should be
// `void(ArgT...)`. The removal tag is a pointer to an arbitrary object that
// you own, and that will stay alive until the CallbackList is gone, or until
// all receivers using it as a removal tag have been removed; you can use it
// to remove the receiver.
template <typename F>
void AddReceiver(const void* removal_tag, F&& f) {
receivers_.AddReceiver(
removal_tag,
UntypedFunction::PrepareArgs<void(ArgT...)>(std::forward<F>(f)));
}
// Adds a new receiver with no removal tag.
template <typename F>
void AddReceiver(F&& f) {
receivers_.AddReceiver(
UntypedFunction::PrepareArgs<void(ArgT...)>(std::forward<F>(f)));
}
// Removes all receivers that were added with the given removal tag.
void RemoveReceivers(const void* removal_tag) {
receivers_.RemoveReceivers(removal_tag);
}
// Calls all receivers with the given arguments. While the Send is in
// progress, no method calls are allowed; specifically, this means that the
// callbacks may not do anything with this CallbackList instance.
//
// Note: Receivers are called serially, but not necessarily in the same order
// they were added.
template <typename... ArgU>
void Send(ArgU&&... args) {
receivers_.Foreach([&](UntypedFunction& f) {
f.Call<void(ArgT...)>(std::forward<ArgU>(args)...);
});
}
private:
callback_list_impl::CallbackListReceivers receivers_;
};
} // namespace webrtc
#endif // RTC_BASE_CALLBACK_LIST_H_