blob: c107a93e42e8ea162e285c5d823dfbca05831651 [file] [log] [blame]
// 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.
#include <unordered_map>
#include "base/bind.h"
#include "base/location.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/observer_list.h"
#include "base/sequenced_task_runner.h"
#include "base/stl_util.h"
#include "base/synchronization/lock.h"
#include "base/threading/sequenced_task_runner_handle.h"
#include "base/threading/thread_local.h"
#include "build/build_config.h"
// TODO(fdoray): Removing these includes causes IWYU failures in other headers,
// remove them in a follow- up CL.
#include "base/memory/ptr_util.h"
#include "base/single_thread_task_runner.h"
#include "base/threading/thread_task_runner_handle.h"
// A thread-safe container for a list of observers. This is similar to the
// observer_list (see observer_list.h), but it is more robust for multi-
// threaded situations.
// The following use cases are supported:
// * Observers can register for notifications from any sequence. They are
// always notified on the sequence from which they were registered.
// * Any sequence may trigger a notification via Notify().
// * Observers can remove themselves from the observer list inside of a
// callback.
// * If one sequence is notifying observers concurrently with an observer
// removing itself from the observer list, the notifications will be
// silently dropped.
// The drawback of the threadsafe observer list is that notifications are not
// as real-time as the non-threadsafe version of this class. Notifications
// will always be done via PostTask() to another sequence, whereas with the
// non-thread-safe observer_list, notifications happen synchronously.
namespace base {
namespace internal {
template <typename ObserverType, typename Method>
struct Dispatcher;
template <typename ObserverType, typename ReceiverType, typename... Params>
struct Dispatcher<ObserverType, void(ReceiverType::*)(Params...)> {
static void Run(void(ReceiverType::* m)(Params...),
Params... params, ObserverType* obj) {
} // namespace internal
template <class ObserverType>
class ObserverListThreadSafe
: public RefCountedThreadSafe<ObserverListThreadSafe<ObserverType>> {
using NotificationType =
typename ObserverList<ObserverType>::NotificationType;
ObserverListThreadSafe() = default;
explicit ObserverListThreadSafe(NotificationType type) : type_(type) {}
// Adds |observer| to the list. |observer| must not already be in the list.
void AddObserver(ObserverType* observer) {
// TODO(fdoray): Change this to a DCHECK once all call sites have a
// SequencedTaskRunnerHandle.
if (!SequencedTaskRunnerHandle::IsSet())
AutoLock auto_lock(lock_);
// Add |observer| to the list of observers.
DCHECK(!ContainsKey(observers_, observer));
const scoped_refptr<SequencedTaskRunner> task_runner =
observers_[observer] = task_runner;
// If this is called while a notification is being dispatched on this thread
// and |type_| is NOTIFY_ALL, |observer| must be notified (if a notification
// is being dispatched on another thread in parallel, the notification may
// or may not make it to |observer| depending on the outcome of the race to
// |lock_|).
if (type_ == NotificationType::NOTIFY_ALL) {
const NotificationData* current_notification =
if (current_notification) {
BindOnce(&ObserverListThreadSafe<ObserverType>::NotifyWrapper, this,
observer, *current_notification));
// Remove an observer from the list if it is in the list.
// If a notification was sent to the observer but hasn't started to run yet,
// it will be aborted. If a notification has started to run, removing the
// observer won't stop it.
void RemoveObserver(ObserverType* observer) {
AutoLock auto_lock(lock_);
// Verifies that the list is currently empty (i.e. there are no observers).
void AssertEmpty() const {
AutoLock auto_lock(lock_);
// Asynchronously invokes a callback on all observers, on their registration
// sequence. You cannot assume that at the completion of the Notify call that
// all Observers have been Notified. The notification may still be pending
// delivery.
template <typename Method, typename... Params>
void Notify(const tracked_objects::Location& from_here,
Method m, Params&&... params) {
Callback<void(ObserverType*)> method =
Bind(&internal::Dispatcher<ObserverType, Method>::Run,
m, std::forward<Params>(params)...);
AutoLock lock(lock_);
for (const auto& observer : observers_) {
BindOnce(&ObserverListThreadSafe<ObserverType>::NotifyWrapper, this,
observer.first, NotificationData(from_here, method)));
friend class RefCountedThreadSafe<ObserverListThreadSafe<ObserverType>>;
struct NotificationData {
NotificationData(const tracked_objects::Location& from_here_in,
const Callback<void(ObserverType*)>& method_in)
: from_here(from_here_in), method(method_in) {}
tracked_objects::Location from_here;
Callback<void(ObserverType*)> method;
~ObserverListThreadSafe() = default;
void NotifyWrapper(ObserverType* observer,
const NotificationData& notification) {
AutoLock auto_lock(lock_);
// Check whether the observer still needs a notification.
auto it = observers_.find(observer);
if (it == observers_.end())
// Keep track of the notification being dispatched on the current thread.
// This will be used if the callback below calls AddObserver().
// Note: |tls_current_notification_| may not be nullptr if this runs in a
// nested loop started by a notification callback. In that case, it is
// important to save the previous value to restore it later.
const NotificationData* const previous_notification =
// Invoke the callback.
// Reset the notification being dispatched on the current thread to its
// previous value.
const NotificationType type_ = NotificationType::NOTIFY_ALL;
// Synchronizes access to |observers_|.
mutable Lock lock_;
// Keys are observers. Values are the SequencedTaskRunners on which they must
// be notified.
std::unordered_map<ObserverType*, scoped_refptr<SequencedTaskRunner>>
// Notification being dispatched on the current thread.
ThreadLocalPointer<const NotificationData> tls_current_notification_;
} // namespace base