base/observer_list.h

// Copyright (c) 2010 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.

#ifndef BASE_OBSERVER_LIST_H__
#define BASE_OBSERVER_LIST_H__
#pragma once

#include <algorithm>
#include <limits>
#include <vector>

#include "base/basictypes.h"
#include "base/logging.h"

///////////////////////////////////////////////////////////////////////////////
//
// OVERVIEW:
//
//   A container for a list of observers.  Unlike a normal STL vector or list,
//   this container can be modified during iteration without invalidating the
//   iterator.  So, it safely handles the case of an observer removing itself
//   or other observers from the list while observers are being notified.
//
// TYPICAL USAGE:
//
//   class MyWidget {
//    public:
//     ...
//
//     class Observer {
//      public:
//       virtual void OnFoo(MyWidget* w) = 0;
//       virtual void OnBar(MyWidget* w, int x, int y) = 0;
//     };
//
//     void AddObserver(Observer* obs) {
//       observer_list_.AddObserver(obs);
//     }
//
//     void RemoveObserver(Observer* obs) {
//       observer_list_.RemoveObserver(obs);
//     }
//
//     void NotifyFoo() {
//       FOR_EACH_OBSERVER(Observer, observer_list_, OnFoo(this));
//     }
//
//     void NotifyBar(int x, int y) {
//       FOR_EACH_OBSERVER(Observer, observer_list_, OnBar(this, x, y));
//     }
//
//    private:
//     ObserverList<Observer> observer_list_;
//   };
//
//
///////////////////////////////////////////////////////////////////////////////

template <typename ObserverType>
class ObserverListThreadSafe;

template <class ObserverType>
class ObserverListBase {
 public:
  // Enumeration of which observers are notified.
  enum NotificationType {
    // Specifies that any observers added during notification are notified.
    // This is the default type if non type is provided to the constructor.
    NOTIFY_ALL,

    // Specifies that observers added while sending out notification are not
    // notified.
    NOTIFY_EXISTING_ONLY
  };

  // An iterator class that can be used to access the list of observers.  See
  // also the FOR_EACH_OBSERVER macro defined below.
  class Iterator {
   public:
    Iterator(ObserverListBase<ObserverType>& list)
        : list_(list),
          index_(0),
          max_index_(list.type_ == NOTIFY_ALL ?
                     std::numeric_limits<size_t>::max() :
                     list.observers_.size()) {
      ++list_.notify_depth_;
    }

    ~Iterator() {
      if (--list_.notify_depth_ == 0)
        list_.Compact();
    }

    ObserverType* GetNext() {
      ListType& observers = list_.observers_;
      // Advance if the current element is null
      size_t max_index = std::min(max_index_, observers.size());
      while (index_ < max_index && !observers[index_])
        ++index_;
      return index_ < max_index ? observers[index_++] : NULL;
    }

   private:
    ObserverListBase<ObserverType>& list_;
    size_t index_;
    size_t max_index_;
  };

  ObserverListBase() : notify_depth_(0), type_(NOTIFY_ALL) {}
  explicit ObserverListBase(NotificationType type)
      : notify_depth_(0), type_(type) {}

  // Add an observer to the list.
  void AddObserver(ObserverType* obs) {
    DCHECK(find(observers_.begin(), observers_.end(), obs) == observers_.end())
        << "Observers can only be added once!";
    observers_.push_back(obs);
  }

  // Remove an observer from the list.
  void RemoveObserver(ObserverType* obs) {
    typename ListType::iterator it =
      std::find(observers_.begin(), observers_.end(), obs);
    if (it != observers_.end()) {
      if (notify_depth_) {
        *it = 0;
      } else {
        observers_.erase(it);
      }
    }
  }

  bool HasObserver(ObserverType* observer) const {
    for (size_t i = 0; i < observers_.size(); ++i) {
      if (observers_[i] == observer)
        return true;
    }
    return false;
  }

  void Clear() {
    if (notify_depth_) {
      for (typename ListType::iterator it = observers_.begin();
           it != observers_.end(); ++it) {
        *it = 0;
      }
    } else {
      observers_.clear();
    }
  }

  size_t size() const { return observers_.size(); }

 protected:
  void Compact() {
    typename ListType::iterator it = observers_.begin();
    while (it != observers_.end()) {
      if (*it) {
        ++it;
      } else {
        it = observers_.erase(it);
      }
    }
  }

 private:
  friend class ObserverListThreadSafe<ObserverType>;

  typedef std::vector<ObserverType*> ListType;

  ListType observers_;
  int notify_depth_;
  NotificationType type_;

  friend class ObserverListBase::Iterator;

  DISALLOW_COPY_AND_ASSIGN(ObserverListBase);
};

template <class ObserverType, bool check_empty = false>
class ObserverList : public ObserverListBase<ObserverType> {
 public:
  typedef typename ObserverListBase<ObserverType>::NotificationType
      NotificationType;

  ObserverList() {}
  explicit ObserverList(NotificationType type)
      : ObserverListBase<ObserverType>(type) {}

  ~ObserverList() {
    // When check_empty is true, assert that the list is empty on destruction.
    if (check_empty) {
      ObserverListBase<ObserverType>::Compact();
      DCHECK_EQ(ObserverListBase<ObserverType>::size(), 0U);
    }
  }
};

#define FOR_EACH_OBSERVER(ObserverType, observer_list, func)  \
  do {                                                        \
    ObserverListBase<ObserverType>::Iterator it(observer_list);   \
    ObserverType* obs;                                        \
    while ((obs = it.GetNext()) != NULL)                      \
      obs->func;                                              \
  } while (0)

#endif  // BASE_OBSERVER_LIST_H__