android监听机制，应该是一种观察者模式。
摘抄网上教程，观察者模式的结构如下：
其中涉及的角色有：
● 抽象主题(Subject)角色：抽象主题角色把所有对观察者对象的引用保存在一个聚集（比如ArrayList对象）里，每个主题都可以有任何数量的观察者。抽象主题提供一个接口，可以增加和删除观察者对象，抽象主题角色又叫做抽象被观察者(Observable)角色。
● 具体主题(ConcreteSubject)角色：将有关状态存入具体观察者对象；在具体主题的内部状态改变时，给所有登记过的观察者发出通知。具体主题角色又叫做具体被观察者(Concrete Observable)角色。
● 抽象观察者(Observer)角色：为所有的具体观察者定义一个接口，在得到主题的通知时更新自己，这个接口叫做更新接口。
● 具体观察者(ConcreteObserver)角色：存储与主题的状态自恰的状态。具体观察者角色实现抽象观察者角色所要求的更新接口，以便使本身的状态与主题的状态 像协调。如果需要，具体观察者角色可以保持一个指向具体主题对象的引用。
android4.4 InCallUI中，用一个类CallList来维护当前所有的Call信息，当某个call状态发生改变之后，要通知到UI进行界面更新；这个类似于一个被观察者：
public void addListener(Listener listener) {
Preconditions.checkNotNull(listener);
mListeners.add(listener);
}
public void removeListener(Listener listener) {
Preconditions.checkNotNull(listener);
mListeners.remove(listener);
}
private void notifyListenersOfChange() {
for (Listener listener : mListeners) {
listener.onCallListChange(this);
}
}
可以添加一些列观察者，并负责notify到他们。
在被观察者类中定义了一些接口。
/**
* Listener interface for any class that wants to be notified of changes
* to the call list.
*/
public interface Listener {
/**
* Called when a new incoming call comes in.
* This is the only method that gets called for incoming calls. Listeners
* that want to perform an action on incoming call should respond in this method
* because {@link #onCallListChange} does not automatically get called for
* incoming calls.
*/
public void onIncomingCall(Call call);
/**
* Called anytime there are changes to the call list. The change can be switching call
* states, updating information, etc. This method will NOT be called for new incoming
* calls and for calls that switch to disconnected state. Listeners must add actions
* to those method implementations if they want to deal with those actions.
*/
public void onCallListChange(CallList callList);
/**
* Called when a call switches to the disconnected state. This is the only method
* that will get called upon disconnection.
*/
public void onDisconnect(Call call);
}
观察者要实现这些接口，以便得到通知后，要做相应的处理工作。
class InCallPresenter implements CallList.Listener {
/**
* Called when a call becomes disconnected. Called everytime an existing call
* changes from being connected (incoming/outgoing/active) to disconnected.
*/
@Override
public void onDisconnect(Call call) {
hideDialpadForDisconnect();
maybeShowErrorDialogOnDisconnect(call);
// We need to do the run the same code as onCallListChange.
onCallListChange(CallList.getInstance());
if (isActivityStarted()) {
mInCallActivity.dismissKeyguard(false);
}
}
}