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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.
#include <algorithm>
#include <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/message_loop/message_loop.h"
#include "base/observer_list.h"
#include "base/single_thread_task_runner.h"
#include "base/stl_util.h"
#include "base/thread_task_runner_handle.h"
#include "base/threading/platform_thread.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 thread.
// Callbacks to the observer will occur on the same thread where
// the observer initially called AddObserver() from.
// * Any thread may trigger a notification via Notify().
// * Observers can remove themselves from the observer list inside
// of a callback.
// * If one thread 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 thread,
// whereas with the non-thread-safe observer_list, notifications happen
// synchronously and immediately.
// The ObserverListThreadSafe maintains an ObserverList for each thread
// which uses the ThreadSafeObserver. When Notifying the observers,
// we simply call PostTask to each registered thread, and then each thread
// will notify its regular ObserverList.
namespace base {
// Forward declaration for ObserverListThreadSafeTraits.
template <class ObserverType>
class ObserverListThreadSafe;
namespace internal {
// An UnboundMethod is a wrapper for a method where the actual object is
// provided at Run dispatch time.
template <class T, class Method, class Params>
class UnboundMethod {
UnboundMethod(Method m, const Params& p) : m_(m), p_(p) {
"bad unbound method params");
void Run(T* obj) const {
DispatchToMethod(obj, m_, p_);
Method m_;
Params p_;
} // namespace internal
// This class is used to work around VS2005 not accepting:
// friend class
// base::RefCountedThreadSafe<ObserverListThreadSafe<ObserverType>>;
// Instead of friending the class, we could friend the actual function
// which calls delete. However, this ends up being
// RefCountedThreadSafe::DeleteInternal(), which is private. So we
// define our own templated traits class so we can friend it.
template <class T>
struct ObserverListThreadSafeTraits {
static void Destruct(const ObserverListThreadSafe<T>* x) {
delete x;
template <class ObserverType>
class ObserverListThreadSafe
: public RefCountedThreadSafe<
ObserverListThreadSafeTraits<ObserverType>> {
typedef typename ObserverList<ObserverType>::NotificationType
: type_(ObserverListBase<ObserverType>::NOTIFY_ALL) {}
explicit ObserverListThreadSafe(NotificationType type) : type_(type) {}
// Add an observer to the list. An observer should not be added to
// the same list more than once.
void AddObserver(ObserverType* obs) {
// If there is not a current MessageLoop, it is impossible to notify on it,
// so do not add the observer.
if (!MessageLoop::current())
ObserverList<ObserverType>* list = nullptr;
PlatformThreadId thread_id = PlatformThread::CurrentId();
AutoLock lock(list_lock_);
if (observer_lists_.find(thread_id) == observer_lists_.end())
observer_lists_[thread_id] = new ObserverListContext(type_);
list = &(observer_lists_[thread_id]->list);
// Remove an observer from the list if it is in the list.
// If there are pending notifications in-transit to the observer, they will
// be aborted.
// If the observer to be removed is in the list, RemoveObserver MUST
// be called from the same thread which called AddObserver.
void RemoveObserver(ObserverType* obs) {
ObserverListContext* context = nullptr;
ObserverList<ObserverType>* list = nullptr;
PlatformThreadId thread_id = PlatformThread::CurrentId();
AutoLock lock(list_lock_);
typename ObserversListMap::iterator it = observer_lists_.find(thread_id);
if (it == observer_lists_.end()) {
// This will happen if we try to remove an observer on a thread
// we never added an observer for.
context = it->second;
list = &context->list;
// If we're about to remove the last observer from the list,
// then we can remove this observer_list entirely.
if (list->HasObserver(obs) && list->size() == 1)
// If RemoveObserver is called from a notification, the size will be
// nonzero. Instead of deleting here, the NotifyWrapper will delete
// when it finishes iterating.
if (list->size() == 0)
delete context;
// Verifies that the list is currently empty (i.e. there are no observers).
void AssertEmpty() const {
AutoLock lock(list_lock_);
// Notify methods.
// Make a thread-safe callback to each Observer in the list.
// Note, these calls are effectively asynchronous. 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 <class Method, class... Params>
void Notify(const tracked_objects::Location& from_here,
Method m,
const Params&... params) {
internal::UnboundMethod<ObserverType, Method, Tuple<Params...>> method(
m, MakeTuple(params...));
AutoLock lock(list_lock_);
for (const auto& entry : observer_lists_) {
ObserverListContext* context = entry.second;
Bind(&ObserverListThreadSafe<ObserverType>::template NotifyWrapper<
Method, Tuple<Params...>>,
this, context, method));
// See comment above ObserverListThreadSafeTraits' definition.
friend struct ObserverListThreadSafeTraits<ObserverType>;
struct ObserverListContext {
explicit ObserverListContext(NotificationType type)
: task_runner(ThreadTaskRunnerHandle::Get()), list(type) {}
scoped_refptr<SingleThreadTaskRunner> task_runner;
ObserverList<ObserverType> list;
~ObserverListThreadSafe() {
// Wrapper which is called to fire the notifications for each thread's
// ObserverList. This function MUST be called on the thread which owns
// the unsafe ObserverList.
template <class Method, class Params>
void NotifyWrapper(
ObserverListContext* context,
const internal::UnboundMethod<ObserverType, Method, Params>& method) {
// Check that this list still needs notifications.
AutoLock lock(list_lock_);
typename ObserversListMap::iterator it =
// The ObserverList could have been removed already. In fact, it could
// have been removed and then re-added! If the master list's loop
// does not match this one, then we do not need to finish this
// notification.
if (it == observer_lists_.end() || it->second != context)
typename ObserverList<ObserverType>::Iterator it(&context->list);
ObserverType* obs;
while ((obs = it.GetNext()) != nullptr)
// If there are no more observers on the list, we can now delete it.
if (context->list.size() == 0) {
AutoLock lock(list_lock_);
// Remove |list| if it's not already removed.
// This can happen if multiple observers got removed in a notification.
// See
typename ObserversListMap::iterator it =
if (it != observer_lists_.end() && it->second == context)
delete context;
// Key by PlatformThreadId because in tests, clients can attempt to remove
// observers without a MessageLoop. If this were keyed by MessageLoop, that
// operation would be silently ignored, leaving garbage in the ObserverList.
typedef std::map<PlatformThreadId, ObserverListContext*>
mutable Lock list_lock_; // Protects the observer_lists_.
ObserversListMap observer_lists_;
const NotificationType type_;
} // namespace base