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     Iterator(ObserverListBase<ObserverType>& list)
  83         : list_(list.AsWeakPtr()),
  84           index_(0),
  85           max_index_(list.type_ == NOTIFY_ALL ?
  86                      std::numeric_limits<size_t>::max() :
  87                      list.observers_.size()) {
  88       ++list_->notify_depth_;
  89     }
  90
  91     ~Iterator() {
  92       if (list_ && --list_->notify_depth_ == 0)
  93         list_->Compact();
  94     }
  95
  96     ObserverType* GetNext() {
  97       if (!list_)
  98         return NULL;
  99       ListType& observers = list_->observers_;
 100       // Advance if the current element is null
 101       size_t max_index = std::min(max_index_, observers.size());
 102       while (index_ < max_index && !observers[index_])
 103         ++index_;
 104       return index_ < max_index ? observers[index_++] : NULL;
 105     }
 106
 107    private:
 108     base::WeakPtr<ObserverListBase<ObserverType> > list_;
 109     size_t index_;
 110     size_t max_index_;
 111   };
 112
 113   ObserverListBase() : notify_depth_(0), type_(NOTIFY_ALL) {}
 114   explicit ObserverListBase(NotificationType type)
 115       : notify_depth_(0), type_(type) {}
 116
 117   // Add an observer to the list.  An observer should not be added to
 118   // the same list more than once.
 119   void AddObserver(ObserverType* obs) {
 120     if (std::find(observers_.begin(), observers_.end(), obs)
 121         != observers_.end()) {
 122       NOTREACHED() << "Observers can only be added once!";
 123       return;
 124     }
 125     observers_.push_back(obs);
 126   }
 127
 128   // Remove an observer from the list if it is in the list.
 129   void RemoveObserver(ObserverType* obs) {
 130     typename ListType::iterator it =
 131       std::find(observers_.begin(), observers_.end(), obs);
 132     if (it != observers_.end()) {
 133       if (notify_depth_) {
 134         *it = 0;
 135       } else {
 136         observers_.erase(it);
 137       }
 138     }
 139   }
 140
 141   bool HasObserver(ObserverType* observer) const {
 142     for (size_t i = 0; i < observers_.size(); ++i) {
 143       if (observers_[i] == observer)
 144         return true;
 145     }
 146     return false;
 147   }
 148
 149   void Clear() {
 150     if (notify_depth_) {
 151       for (typename ListType::iterator it = observers_.begin();
 152            it != observers_.end(); ++it) {
 153         *it = 0;
 154       }
 155     } else {
 156       observers_.clear();
 157     }
 158   }
 159
 160   size_t size() const { return observers_.size(); }
 161
 162  protected:
 163   void Compact() {
 164     observers_.erase(
 165         std::remove(observers_.begin(), observers_.end(),
 166                     static_cast<ObserverType*>(NULL)), observers_.end());
 167   }
 168
 169  private:
 170   friend class ObserverListThreadSafe<ObserverType>;
 171
 172   typedef std::vector<ObserverType*> ListType;
 173
 174   ListType observers_;
 175   int notify_depth_;
 176   NotificationType type_;
 177
 178   friend class ObserverListBase::Iterator;
 179
 180   DISALLOW_COPY_AND_ASSIGN(ObserverListBase);
 181 };
 182
 183 template <class ObserverType, bool check_empty = false>
 184 class ObserverList : public ObserverListBase<ObserverType> {
 185  public:
 186   typedef typename ObserverListBase<ObserverType>::NotificationType
 187       NotificationType;
 188
 189   ObserverList() {}
 190   explicit ObserverList(NotificationType type)
 191       : ObserverListBase<ObserverType>(type) {}
 192
 193   ~ObserverList() {
 194     // When check_empty is true, assert that the list is empty on destruction.
 195     if (check_empty) {
 196       ObserverListBase<ObserverType>::Compact();
 197       DCHECK_EQ(ObserverListBase<ObserverType>::size(), 0U);
 198     }
 199   }
 200
 201   bool might_have_observers() const {
 202     return ObserverListBase<ObserverType>::size() != 0;
 203   }
 204 };
 205
 206 #define FOR_EACH_OBSERVER(ObserverType, observer_list, func)         \
 207   do {                                                               \
 208     if ((observer_list).might_have_observers()) {                    \
 209       ObserverListBase<ObserverType>::Iterator it(observer_list);    \
 210       ObserverType* obs;                                             \
 211       while ((obs = it.GetNext()) != NULL)                           \
 212         obs->func;                                                   \
 213     }                                                                \
 214   } while (0)
 215
 216 #endif  // BASE_OBSERVER_LIST_H__