chrome源码之 ObserverList

ObserverList<ObserverType> is a container for a list of observers, it is an easy-to-use observer pattern for two related objects. 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.
If you want to deal with observer_list in multi-thread, just use ObserverListThreadSafe.

ObserverList

template <class ObserverType, bool check_empty = false>
class ObserverList : public ObserverListBase<ObserverType> {
	//...
};

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

Example

#include "base/observer_list.h"

class MyWidget;

class MyObserver 
{
public:
  MyObserver( MyWidget * widget );

  virtual void OnFoo(MyWidget* w) {
    //...
  }
  virtual void OnBar(MyWidget* w, int x, int y){
    //...
  }
};

class MyWidget {
public:
  void AddObserver(MyObserver* obs) {
    observer_list_.AddObserver(obs);
  }

  void RemoveObserver(MyObserver* obs) {
    observer_list_.RemoveObserver(obs);
  }

  void NotifyFoo() {
    FOR_EACH_OBSERVER(MyObserver, observer_list_, OnFoo(this));
  }

  void NotifyBar(int x, int y) {
    FOR_EACH_OBSERVER(MyObserver, observer_list_, OnBar(this, x, y));
  }

private:
  ObserverList<MyObserver> observer_list_;
};

// implement
MyObserver::MyObserver( MyWidget * widget ) 
{
  widget->AddObserver(this);
}

void Test()
{
  MyWidget widget;
  // two observers
  MyObserver observer1(&widget), observer2(&widget);

  widget.NotifyFoo();
  widget.NotifyBar(3,5);
}