base/observer_list.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_OBSERVER_LIST_H_
   6 #define BASE_OBSERVER_LIST_H_
   7
   8 #include <algorithm>
   9 #include <limits>
  10 #include <vector>
  11
  12 #include "base/basictypes.h"
  13 #include "base/logging.h"
  14 #include "base/memory/weak_ptr.h"
  15
  16 ///////////////////////////////////////////////////////////////////////////////
  17 //
  18 // OVERVIEW:
  19 //
  20 //   A container for a list of observers.  Unlike a normal STL vector or list,
  21 //   this container can be modified during iteration without invalidating the
  22 //   iterator.  So, it safely handles the case of an observer removing itself
  23 //   or other observers from the list while observers are being notified.
  24 //
  25 // TYPICAL USAGE:
  26 //
  27 //   class MyWidget {
  28 //    public:
  29 //     ...
  30 //
  31 //     class Observer {
  32 //      public:
  33 //       virtual void OnFoo(MyWidget* w) = 0;
  34 //       virtual void OnBar(MyWidget* w, int x, int y) = 0;
  35 //     };
  36 //
  37 //     void AddObserver(Observer* obs) {
  38 //       observer_list_.AddObserver(obs);
  39 //     }
  40 //
  41 //     void RemoveObserver(Observer* obs) {
  42 //       observer_list_.RemoveObserver(obs);
  43 //     }
  44 //
  45 //     void NotifyFoo() {
  46 //       FOR_EACH_OBSERVER(Observer, observer_list_, OnFoo(this));
  47 //     }
  48 //
  49 //     void NotifyBar(int x, int y) {
  50 //       FOR_EACH_OBSERVER(Observer, observer_list_, OnBar(this, x, y));
  51 //     }
  52 //
  53 //    private:
  54 //     ObserverList<Observer> observer_list_;
  55 //   };
  56 //
  57 //
  58 ///////////////////////////////////////////////////////////////////////////////
  59
  60 template <typename ObserverType>
  61 class ObserverListThreadSafe;
  62
  63 template <class ObserverType>
  64 class ObserverListBase
  65     : public base::SupportsWeakPtr<ObserverListBase<ObserverType> > {
  66  public:
  67   // Enumeration of which observers are notified.
  68   enum NotificationType {
  69     // Specifies that any observers added during notification are notified.
  70     // This is the default type if non type is provided to the constructor.
  71     NOTIFY_ALL,
  72
  73     // Specifies that observers added while sending out notification are not
  74     // notified.
  75     NOTIFY_EXISTING_ONLY
  76   };
  77
  78   // An iterator class that can be used to access the list of observers.  See
  79   // also the FOR_EACH_OBSERVER macro defined below.
  80   class Iterator {
  81    public:
  82     explicit Iterator(ObserverListBase<ObserverType>* list);
  83     ~Iterator();
  84     ObserverType* GetNext();
  85
  86    private:
  87     base::WeakPtr<ObserverListBase<ObserverType> > list_;
  88     size_t index_;
  89     size_t max_index_;
  90   };
  91
  92   ObserverListBase() : notify_depth_(0), type_(NOTIFY_ALL) {}
  93   explicit ObserverListBase(NotificationType type)
  94       : notify_depth_(0), type_(type) {}
  95
  96   // Add an observer to the list.  An observer should not be added to
  97   // the same list more than once.
  98   void AddObserver(ObserverType* obs);
  99
 100   // Remove an observer from the list if it is in the list.
 101   void RemoveObserver(ObserverType* obs);
 102
 103   // Determine whether a particular observer is in the list.
 104   bool HasObserver(const ObserverType* observer) const;
 105
 106   void Clear();
 107
 108  protected:
 109   size_t size() const { return observers_.size(); }
 110
 111   void Compact();
 112
 113  private:
 114   friend class ObserverListThreadSafe<ObserverType>;
 115
 116   typedef std::vector<ObserverType*> ListType;
 117
 118   ListType observers_;
 119   int notify_depth_;
 120   NotificationType type_;
 121
 122   friend class ObserverListBase::Iterator;
 123
 124   DISALLOW_COPY_AND_ASSIGN(ObserverListBase);
 125 };
 126
 127 template <class ObserverType>
 128 ObserverListBase<ObserverType>::Iterator::Iterator(
 129     ObserverListBase<ObserverType>* list)
 130     : list_(list->AsWeakPtr()),
 131       index_(0),
 132       max_index_(list->type_ == NOTIFY_ALL ? std::numeric_limits<size_t>::max()
 133                                            : list->observers_.size()) {
 134   ++list_->notify_depth_;
 135 }
 136
 137 template <class ObserverType>
 138 ObserverListBase<ObserverType>::Iterator::~Iterator() {
 139   if (list_.get() && --list_->notify_depth_ == 0)
 140     list_->Compact();
 141 }
 142
 143 template <class ObserverType>
 144 ObserverType* ObserverListBase<ObserverType>::Iterator::GetNext() {
 145   if (!list_.get())
 146     return nullptr;
 147   ListType& observers = list_->observers_;
 148   // Advance if the current element is null
 149   size_t max_index = std::min(max_index_, observers.size());
 150   while (index_ < max_index && !observers[index_])
 151     ++index_;
 152   return index_ < max_index ? observers[index_++] : nullptr;
 153 }
 154
 155 template <class ObserverType>
 156 void ObserverListBase<ObserverType>::AddObserver(ObserverType* obs) {
 157   DCHECK(obs);
 158   if (std::find(observers_.begin(), observers_.end(), obs)
 159       != observers_.end()) {
 160     NOTREACHED() << "Observers can only be added once!";
 161     return;
 162   }
 163   observers_.push_back(obs);
 164 }
 165
 166 template <class ObserverType>
 167 void ObserverListBase<ObserverType>::RemoveObserver(ObserverType* obs) {
 168   DCHECK(obs);
 169   typename ListType::iterator it =
 170     std::find(observers_.begin(), observers_.end(), obs);
 171   if (it != observers_.end()) {
 172     if (notify_depth_) {
 173       *it = nullptr;
 174     } else {
 175       observers_.erase(it);
 176     }
 177   }
 178 }
 179
 180 template <class ObserverType>
 181 bool ObserverListBase<ObserverType>::HasObserver(
 182     const ObserverType* observer) const {
 183   for (size_t i = 0; i < observers_.size(); ++i) {
 184     if (observers_[i] == observer)
 185       return true;
 186   }
 187   return false;
 188 }
 189
 190 template <class ObserverType>
 191 void ObserverListBase<ObserverType>::Clear() {
 192   if (notify_depth_) {
 193     for (typename ListType::iterator it = observers_.begin();
 194       it != observers_.end(); ++it) {
 195       *it = nullptr;
 196     }
 197   } else {
 198     observers_.clear();
 199   }
 200 }
 201
 202 template <class ObserverType>
 203 void ObserverListBase<ObserverType>::Compact() {
 204   observers_.erase(
 205       std::remove(observers_.begin(), observers_.end(), nullptr),
 206       observers_.end());
 207 }
 208
 209 template <class ObserverType, bool check_empty = false>
 210 class ObserverList : public ObserverListBase<ObserverType> {
 211  public:
 212   typedef typename ObserverListBase<ObserverType>::NotificationType
 213       NotificationType;
 214
 215   ObserverList() {}
 216   explicit ObserverList(NotificationType type)
 217       : ObserverListBase<ObserverType>(type) {}
 218
 219   ~ObserverList() {
 220     // When check_empty is true, assert that the list is empty on destruction.
 221     if (check_empty) {
 222       ObserverListBase<ObserverType>::Compact();
 223       DCHECK_EQ(ObserverListBase<ObserverType>::size(), 0U);
 224     }
 225   }
 226
 227   bool might_have_observers() const {
 228     return ObserverListBase<ObserverType>::size() != 0;
 229   }
 230 };
 231
 232 #define FOR_EACH_OBSERVER(ObserverType, observer_list, func)             \
 233   do {                                                                   \
 234     if ((observer_list).might_have_observers()) {                        \
 235       ObserverListBase<ObserverType>::Iterator it_inside_observer_macro( \
 236           &observer_list);                                               \
 237       ObserverType* obs;                                                 \
 238       while ((obs = it_inside_observer_macro.GetNext()) != nullptr)         \
 239         obs->func;                                                       \
 240     }                                                                    \
 241   } while (0)
 242
 243 #endif  // BASE_OBSERVER_LIST_H_