我不会谈论 Java，但我在 C++ 中遇到过类似类型的问题。
这是我提出的解决方案的粗略草图。它是用 C++ 编写的，但基本思想应该很好地转化为 Java（或其他 OO 语言）：
- Observer 是一个带有键（枚举值，而不是字符串；这是一个速度折衷，因为键没有经过散列搜索，但这意味着没有简单的“字符串”名称，您必须提前定义它们）以便主题注册兴趣。因为是单例，所以一直存在。
- 每个主题都派生自一个公共基类。基类有一个抽象虚函数 Notify(...) 必须在派生类中实现，以及一个析构函数，当它被删除时，它会从 Observer 中删除（它总是可以到达）。
- 在观察者本身内部，如果在 Notify(...) 正在进行时调用 Detach(...)，则任何分离的 Subjects 最终都会出现在列表中。
- 当在 Observer 上调用 Notify(...) 时，它会创建主题列表的临时副本。当它迭代它时，它将它与最近分离的进行比较。如果目标不在其上，则在目标上调用 Notify(...)。否则，将被跳过。
- Observer 中的 Notify(...) 还跟踪处理级联调用的深度（A 通知 B、C、D，并且 D.Notify(...) 触发对 E 的 Notify(...) 调用， ETC。）
这是界面最终的样子：
/*
The Notifier is a singleton implementation of the Subject/Observer design
pattern. Any class/instance which wishes to participate as an observer
of an event can derive from the Notified base class and register itself
with the Notiifer for enumerated events.
Notifier derived classes MUST implement the notify function, which has
a prototype of:
void Notify(const NOTIFIED_EVENT_TYPE_T& event)
This is a data object passed from the Notifier class. The structure
passed has a void* in it. There is no illusion of type safety here
and it is the responsibility of the user to ensure it is cast properly.
In most cases, it will be "NULL".
Classes derived from Notified do not need to deregister (though it may
be a good idea to do so) as the base class destructor will attempt to
remove itself from the Notifier system automatically.
The event type is an enumeration and not a string as it is in many
"generic" notification systems. In practical use, this is for a closed
application where the messages will be known at compile time. This allows
us to increase the speed of the delivery by NOT having a
dictionary keyed lookup mechanism. Some loss of generality is implied
by this.
This class/system is NOT thread safe, but could be made so with some
mutex wrappers. It is safe to call Attach/Detach as a consequence
of calling Notify(...).
*/
class Notified;
class Notifier : public SingletonDynamic<Notifier>
{
public:
typedef enum
{
NE_MIN = 0,
NE_DEBUG_BUTTON_PRESSED = NE_MIN,
NE_DEBUG_LINE_DRAW_ADD_LINE_PIXELS,
NE_DEBUG_TOGGLE_VISIBILITY,
NE_DEBUG_MESSAGE,
NE_RESET_DRAW_CYCLE,
NE_VIEWPORT_CHANGED,
NE_MAX,
} NOTIFIED_EVENT_TYPE_T;
private:
typedef vector<NOTIFIED_EVENT_TYPE_T> NOTIFIED_EVENT_TYPE_VECTOR_T;
typedef map<Notified*,NOTIFIED_EVENT_TYPE_VECTOR_T> NOTIFIED_MAP_T;
typedef map<Notified*,NOTIFIED_EVENT_TYPE_VECTOR_T>::iterator NOTIFIED_MAP_ITER_T;
typedef vector<Notified*> NOTIFIED_VECTOR_T;
typedef vector<NOTIFIED_VECTOR_T> NOTIFIED_VECTOR_VECTOR_T;
NOTIFIED_MAP_T _notifiedMap;
NOTIFIED_VECTOR_VECTOR_T _notifiedVector;
NOTIFIED_MAP_ITER_T _mapIter;
// This vector keeps a temporary list of observers that have completely
// detached since the current "Notify(...)" operation began. This is
// to handle the problem where a Notified instance has called Detach(...)
// because of a Notify(...) call. The removed instance could be a dead
// pointer, so don't try to talk to it.
vector<Notified*> _detached;
int32 _notifyDepth;
void RemoveEvent(NOTIFIED_EVENT_TYPE_VECTOR_T& orgEventTypes, NOTIFIED_EVENT_TYPE_T eventType);
void RemoveNotified(NOTIFIED_VECTOR_T& orgNotified, Notified* observer);
public:
virtual void Reset();
virtual bool Init() { Reset(); return true; }
virtual void Shutdown() { Reset(); }
void Attach(Notified* observer, NOTIFIED_EVENT_TYPE_T eventType);
// Detach for a specific event
void Detach(Notified* observer, NOTIFIED_EVENT_TYPE_T eventType);
// Detach for ALL events
void Detach(Notified* observer);
/* The design of this interface is very specific. I could
* create a class to hold all the event data and then the
* method would just have take that object. But then I would
* have to search for every place in the code that created an
* object to be used and make sure it updated the passed in
* object when a member is added to it. This way, a break
* occurs at compile time that must be addressed.
*/
void Notify(NOTIFIED_EVENT_TYPE_T, const void* eventData = NULL);
/* Used for CPPUnit. Could create a Mock...maybe...but this seems
* like it will get the job done with minimal fuss. For now.
*/
// Return all events that this object is registered for.
vector<NOTIFIED_EVENT_TYPE_T> GetEvents(Notified* observer);
// Return all objects registered for this event.
vector<Notified*> GetNotified(NOTIFIED_EVENT_TYPE_T event);
};
/* This is the base class for anything that can receive notifications.
*/
class Notified
{
public:
virtual void Notify(Notifier::NOTIFIED_EVENT_TYPE_T eventType, const void* eventData) = 0;
virtual ~Notified();
};
typedef Notifier::NOTIFIED_EVENT_TYPE_T NOTIFIED_EVENT_TYPE_T;
注意：Notified 类有一个函数，Notify(...) 这里。因为 void* 不是类型安全的，所以我创建了其他版本，其中 notify 如下所示：
virtual void Notify(Notifier::NOTIFIED_EVENT_TYPE_T eventType, int value);
virtual void Notify(Notifier::NOTIFIED_EVENT_TYPE_T eventType, const string& str);
相应的 Notify(...) 方法已添加到通知程序本身。所有这些都使用一个函数来获取“目标列表”，然后在目标上调用适当的函数。这很好用，并且使接收器不必进行难看的演员表。
这似乎运作良好。该解决方案与源代码一起 发布在网络上。
