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// Copyright 2024 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_FUNCTIONAL_CONCURRENT_CALLBACKS_H_
#define BASE_FUNCTIONAL_CONCURRENT_CALLBACKS_H_
#include <memory>
#include <type_traits>
#include <vector>
#include "base/functional/bind.h"
#include "base/functional/callback.h"
#include "base/location.h"
#include "base/memory/raw_ptr.h"
#include "base/task/bind_post_task.h"
#include "base/task/sequenced_task_runner.h"
// OVERVIEW:
//
// ConcurrentCallbacks<T> is an alternative to BarrierCallback<T>, it dispenses
// OnceCallbacks via CreateCallback() and invokes the callback passed to Done()
// after all prior callbacks have been run.
//
// ConcurrentCallbacks<T> is intended to be used over BarrierCallback<T> in
// cases where the count is unknown prior to requiring a callback to start a
// task, and for cases where the count is manually derived from the code and
// subject to human error.
//
// IMPORTANT NOTES:
//
// - ConcurrentCallbacks<T> is NOT thread safe.
// - The done callback will NOT be run synchronously, it will be PostTask() to
// the sequence that Done() was invoked on.
// - ConcurrentCallbacks<T> cannot be used after Done() is called, a CHECK
// verifies this.
//
// TYPICAL USAGE:
//
// class Example {
// void OnRequestsReceived(std::vector<Request> requests) {
// base::ConcurrentCallbacks<Result> concurrent;
//
// for (Request& request : requests) {
// if (IsValidRequest(request)) {
// StartRequest(std::move(request), concurrent.CreateCallback());
// }
// }
//
// std::move(concurrent).Done(
// base::BindOnce(&Example::OnRequestsComplete, GetWeakPtr()));
// }
//
// void StartRequest(Request request,
// base::OnceCallback<void(Result)> callback) {
// // Process the request asynchronously and call callback with a Result.
// }
//
// void OnRequestsComplete(std::vector<Result> results) {
// // Invoked after all requests are completed and receives the results of
// // all of them.
// }
// };
namespace base {
template <typename T>
class ConcurrentCallbacks {
public:
using Results = std::vector<std::remove_cvref_t<T>>;
ConcurrentCallbacks() {
auto info_owner = std::make_unique<Info>();
info_ = info_owner.get();
info_run_callback_ = BindRepeating(&Info::Run, std::move(info_owner));
}
// Create a callback for the done callback to wait for.
[[nodiscard]] OnceCallback<void(T)> CreateCallback() {
CHECK(info_);
++info_->pending_;
return info_run_callback_;
}
// Finish creating concurrent callbacks and provide done callback to run once
// all prior callbacks have executed.
// `this` is no longer usable after calling Done(), must be called with
// std::move().
void Done(OnceCallback<void(Results)> done_callback,
const Location& location = FROM_HERE) && {
CHECK(info_);
info_->done_callback_ =
BindPostTask(SequencedTaskRunner::GetCurrentDefault(),
std::move(done_callback), location);
if (info_->pending_ == 0u) {
std::move(info_->done_callback_).Run(std::move(info_->results_));
}
info_ = nullptr;
}
private:
class Info {
public:
Info() = default;
void Run(T value) {
CHECK_GT(pending_, 0u);
--pending_;
results_.push_back(std::move(value));
if (done_callback_ && pending_ == 0u) {
std::move(done_callback_).Run(std::move(results_));
}
}
size_t pending_ = 0u;
Results results_;
OnceCallback<void(Results)> done_callback_;
};
RepeatingCallback<void(T)> info_run_callback_;
// info_ is owned by info_run_callback_.
raw_ptr<Info> info_;
};
} // namespace base
#endif // BASE_FUNCTIONAL_CONCURRENT_CALLBACKS_H_
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