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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_OBSERVER_LIST_H__
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#define BASE_OBSERVER_LIST_H__
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#include "base/basictypes.h"
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#include "base/logging.h"
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#include "base/memory/weak_ptr.h"
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///////////////////////////////////////////////////////////////////////////////
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// A container for a list of observers. Unlike a normal STL vector or list,
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// this container can be modified during iteration without invalidating the
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// iterator. So, it safely handles the case of an observer removing itself
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// or other observers from the list while observers are being notified.
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// virtual void OnFoo(MyWidget* w) = 0;
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// virtual void OnBar(MyWidget* w, int x, int y) = 0;
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// void AddObserver(Observer* obs) {
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// observer_list_.AddObserver(obs);
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// void RemoveObserver(Observer* obs) {
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// observer_list_.RemoveObserver(obs);
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// FOR_EACH_OBSERVER(Observer, observer_list_, OnFoo(this));
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// void NotifyBar(int x, int y) {
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// FOR_EACH_OBSERVER(Observer, observer_list_, OnBar(this, x, y));
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// ObserverList<Observer> observer_list_;
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///////////////////////////////////////////////////////////////////////////////
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template <typename ObserverType>
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class ObserverListThreadSafe;
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template <class ObserverType>
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class ObserverListBase
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: public base::SupportsWeakPtr<ObserverListBase<ObserverType> > {
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// Enumeration of which observers are notified.
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enum NotificationType {
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// Specifies that any observers added during notification are notified.
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// This is the default type if non type is provided to the constructor.
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// Specifies that observers added while sending out notification are not
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// An iterator class that can be used to access the list of observers. See
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// also the FOR_EACH_OBSERVER macro defined below.
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Iterator(ObserverListBase<ObserverType>& list)
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: list_(list.AsWeakPtr()),
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max_index_(list.type_ == NOTIFY_ALL ?
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std::numeric_limits<size_t>::max() :
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list.observers_.size()) {
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++list_->notify_depth_;
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if (list_.get() && --list_->notify_depth_ == 0)
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ObserverType* GetNext() {
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ListType& observers = list_->observers_;
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// Advance if the current element is null
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size_t max_index = std::min(max_index_, observers.size());
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while (index_ < max_index && !observers[index_])
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return index_ < max_index ? observers[index_++] : NULL;
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base::WeakPtr<ObserverListBase<ObserverType> > list_;
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ObserverListBase() : notify_depth_(0), type_(NOTIFY_ALL) {}
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explicit ObserverListBase(NotificationType type)
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: notify_depth_(0), type_(type) {}
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// Add an observer to the list. An observer should not be added to
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// the same list more than once.
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void AddObserver(ObserverType* obs) {
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if (std::find(observers_.begin(), observers_.end(), obs)
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!= observers_.end()) {
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NOTREACHED() << "Observers can only be added once!";
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observers_.push_back(obs);
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// Remove an observer from the list if it is in the list.
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void RemoveObserver(ObserverType* obs) {
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typename ListType::iterator it =
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std::find(observers_.begin(), observers_.end(), obs);
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if (it != observers_.end()) {
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observers_.erase(it);
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bool HasObserver(ObserverType* observer) const {
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for (size_t i = 0; i < observers_.size(); ++i) {
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if (observers_[i] == observer)
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for (typename ListType::iterator it = observers_.begin();
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it != observers_.end(); ++it) {
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size_t size() const { return observers_.size(); }
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std::remove(observers_.begin(), observers_.end(),
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static_cast<ObserverType*>(NULL)), observers_.end());
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friend class ObserverListThreadSafe<ObserverType>;
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typedef std::vector<ObserverType*> ListType;
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NotificationType type_;
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friend class ObserverListBase::Iterator;
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DISALLOW_COPY_AND_ASSIGN(ObserverListBase);
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template <class ObserverType, bool check_empty = false>
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class ObserverList : public ObserverListBase<ObserverType> {
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typedef typename ObserverListBase<ObserverType>::NotificationType
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explicit ObserverList(NotificationType type)
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: ObserverListBase<ObserverType>(type) {}
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// When check_empty is true, assert that the list is empty on destruction.
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ObserverListBase<ObserverType>::Compact();
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DCHECK_EQ(ObserverListBase<ObserverType>::size(), 0U);
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bool might_have_observers() const {
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return ObserverListBase<ObserverType>::size() != 0;
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#define FOR_EACH_OBSERVER(ObserverType, observer_list, func) \
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if ((observer_list).might_have_observers()) { \
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ObserverListBase<ObserverType>::Iterator \
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it_inside_observer_macro(observer_list); \
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while ((obs = it_inside_observer_macro.GetNext()) != NULL) \
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#endif // BASE_OBSERVER_LIST_H__