base/observer_list.h

// Copyright (c) 2011 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_

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

#include "base/basictypes.h"
#include "base/logging.h"
#include "base/memory/weak_ptr.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:
//     base::ObserverList<Observer> observer_list_;
//   };
//
//
///////////////////////////////////////////////////////////////////////////////

namespace base {

template <typename ObserverType>
class ObserverListThreadSafe;

template <class ObserverType>
class ObserverListBase
    : public SupportsWeakPtr<ObserverListBase<ObserverType>> {
 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:
    explicit Iterator(ObserverListBase<ObserverType>* list);
    ~Iterator();
    ObserverType* GetNext();

   private:
    WeakPtr<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.  An observer should not be added to
  // the same list more than once.
  void AddObserver(ObserverType* obs);

  // Remove an observer from the list if it is in the list.
  void RemoveObserver(ObserverType* obs);

  // Determine whether a particular observer is in the list.
  bool HasObserver(const ObserverType* observer) const;

  void Clear();

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

  void Compact();

 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>
ObserverListBase<ObserverType>::Iterator::Iterator(
    ObserverListBase<ObserverType>* list)
    : list_(list->AsWeakPtr()),
      index_(0),
      max_index_(list->type_ == NOTIFY_ALL ? std::numeric_limits<size_t>::max()
                                           : list->observers_.size()) {
  ++list_->notify_depth_;
}

template <class ObserverType>
ObserverListBase<ObserverType>::Iterator::~Iterator() {
  if (list_.get() && --list_->notify_depth_ == 0)
    list_->Compact();
}

template <class ObserverType>
ObserverType* ObserverListBase<ObserverType>::Iterator::GetNext() {
  if (!list_.get())
    return nullptr;
  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_++] : nullptr;
}

template <class ObserverType>
void ObserverListBase<ObserverType>::AddObserver(ObserverType* obs) {
  DCHECK(obs);
  if (std::find(observers_.begin(), observers_.end(), obs)
      != observers_.end()) {
    NOTREACHED() << "Observers can only be added once!";
    return;
  }
  observers_.push_back(obs);
}

template <class ObserverType>
void ObserverListBase<ObserverType>::RemoveObserver(ObserverType* obs) {
  DCHECK(obs);
  typename ListType::iterator it =
    std::find(observers_.begin(), observers_.end(), obs);
  if (it != observers_.end()) {
    if (notify_depth_) {
      *it = nullptr;
    } else {
      observers_.erase(it);
    }
  }
}

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

template <class ObserverType>
void ObserverListBase<ObserverType>::Clear() {
  if (notify_depth_) {
    for (typename ListType::iterator it = observers_.begin();
      it != observers_.end(); ++it) {
      *it = nullptr;
    }
  } else {
    observers_.clear();
  }
}

template <class ObserverType>
void ObserverListBase<ObserverType>::Compact() {
  observers_.erase(
      std::remove(observers_.begin(), observers_.end(), nullptr),
      observers_.end());
}

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);
    }
  }

  bool might_have_observers() const {
    return ObserverListBase<ObserverType>::size() != 0;
  }
};

#define FOR_EACH_OBSERVER(ObserverType, observer_list, func)             \
  do {                                                                   \
    if ((observer_list).might_have_observers()) {                        \
      base::ObserverListBase<ObserverType>::Iterator it_inside_observer_macro( \
          &observer_list);                                               \
      ObserverType* obs;                                                 \
      while ((obs = it_inside_observer_macro.GetNext()) != nullptr)      \
        obs->func;                                                       \
    }                                                                    \
  } while (0)

}  // namespace base

#endif  // BASE_OBSERVER_LIST_H_