1.什么是观察者模式?
有时被称作发布/订阅模式，观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使它们能够自动更新自己。
上面的解释太书面化了，举个生活中例子，小区中有100户业主向杂志社订阅了一本月刊杂志，每月的10号会发送给业主。这当中100户就是观察者，杂志社就是被观察者，订阅杂志的行为就是观察者模式中的订阅，每月10号就是观察者模式中的发布。==观察者模式的核心概念是一对多==。
2.观察者模式解决的问题
将一个系统分割成一个一些类相互协作的类有一个不好的副作用，那就是需要维护相关对象间的一致性。我们不希望为了维持一致性而使各类紧密耦合，这样会给维护、扩展和重用都带来不便。观察者就是解决这类的耦合关系的
3.观察者模式中的各个角色
Observer(抽象接口):观察者抽象接口，一般定义了观察者的抽象行为
Observerable(抽象类):被观察者抽象类，含有一个存储观察者对象的集合
ConcreteObserver(具体的观察者):实现Observer接口
ConcreteObserverable(具体的被观察者):继承Observerable抽象类
4.观察者模式的代码实现
观察者模式的代码一般有2种，可以完全由自己代码实现逻辑，也可以借助java api来实现。
自己实现代码如下：
Observer
public interface Observer<T> {
void update(T t);
}
Observerable
public abstract class Observerable {
//用来存储观察者的集合
private List<Observer> obervers = new ArrayList<>();
/**
* 订阅事件
* @param observer
*/
public void subscribe(Observer observer){
obervers.add(observer);
}
/**
* 取消订阅
* @param observer
*/
public void unSubscribe(Observer observer){
obervers.remove(observer);
}
/**
* 通知广播
* @param obj
*/
public void notification(Object obj){
for (Observer o : obervers) {
o.update(obj);
}
}
}
ConcreteObserver
public class ReaderOberver implements Observer<String> {
private String name;
public ReaderOberver(String name){
this.name = name;
}
@Override
public void update(String s) {
System.out.println(name+" get message is "+s);
}
}
ConcreteObserverable
public class CompanyObserverable extends Observerable {
public void sendMessage(String message){
System.out.println("company send message : " +message);
notification(message);
}
}
执行测试代码
public class Main {
public static void main(String[] args) {
//1.创建一个被观察者对象
CompanyObserverable observerable = new CompanyObserverable();
//2.创建两个观察者对象
ReaderOberver readA = new ReaderOberver("Read A");
ReaderOberver readB = new ReaderOberver("Read B");
//3.把这两个观察者对象注册到被观察者对象的集合中，俗称订阅
observerable.subscribe(readA);
observerable.subscribe(readB);
//4.被观察者触发广播
observerable.sendMessage(" The Message ");
}
}
执行结果
company send message : The Message
Read A get message is The Message
Read B get message is The Message
总结：上面的例子很好的实现了观察者模式，重点就是一对多，但是这种写法是线程不安全的。所以介绍下java api的写法，它解决了线程安全的问题。
用java api实现观察者模式主要使用以下两个类
import java.util.Observable;
import java.util.Observer;
先看使用
ConcreteObserver
public class ReaderObserver implements Observer {
private String name;
public ReaderObserver(String name){
this.name = name;
}
@Override
public void update(Observable o, Object arg) {
System.out.println(name +" get message is "+ arg.toString()+" from "+((CompanyObserverable)o).getName());
}
}
ConcreteObserverable
public class CompanyObserverable extends Observable {
private String name;
public CompanyObserverable(String name){
this.name = name;
}
public String getName() {
return name;
}
public void sendMessage(String message){
System.out.println(name+" send message :"+message);
setChanged();
notifyObservers(message);
}
}
执行测试代码
public class Main {
public static void main(String[] args){
//1.创建一个被观察者对象
CompanyObserverable observerable = new CompanyObserverable("company");
ReaderObserver observerA = new ReaderObserver("observerA");
ReaderObserver observerB = new ReaderObserver("observerB");
//2.创建两个观察者对象
observerable.addObserver(observerA);
observerable.addObserver(observerB);
//4.被观察者触发广播
observerable.sendMessage(" The message ");
}
}
执行结果
company send message : The message
observerB get message is The message from company
observerA get message is The message from company
两种写法的结果是一样的，那么我们去看看系统的Observer和Observerable的源码，看看他是怎么工作的，为什么能解决线程安全的问题，代码不多，我就直接贴出来了
Observable.java
public class Observable {
private boolean changed = false;
private Vector<Observer> obs;
/** Construct an Observable with zero Observers. */
public Observable() {
obs = new Vector<>();
}
/**
* Adds an observer to the set of observers for this object, provided
* that it is not the same as some observer already in the set.
* The order in which notifications will be delivered to multiple
* observers is not specified. See the class comment.
*
* @param o an observer to be added.
* @throws NullPointerException if the parameter o is null.
*/
public synchronized void addObserver(Observer o) {
if (o == null)
throw new NullPointerException();
if (!obs.contains(o)) {
obs.addElement(o);
}
}
/**
* Deletes an observer from the set of observers of this object.
* Passing <CODE>null</CODE> to this method will have no effect.
* @param o the observer to be deleted.
*/
public synchronized void deleteObserver(Observer o) {
obs.removeElement(o);
}
/**
* If this object has changed, as indicated by the
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to
* indicate that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and <code>null</code>. In other
* words, this method is equivalent to:
* <blockquote><tt>
* notifyObservers(null)</tt></blockquote>
*
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers() {
notifyObservers(null);
}
/**
* If this object has changed, as indicated by the
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to indicate
* that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and the <code>arg</code> argument.
*
* @param arg any object.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers(Object arg) {
/*
* a temporary array buffer, used as a snapshot of the state of
* current Observers.
*/
Object[] arrLocal;
synchronized (this) {
/* We don't want the Observer doing callbacks into
* arbitrary code while holding its own Monitor.
* The code where we extract each Observable from
* the Vector and store the state of the Observer
* needs synchronization, but notifying observers
* does not (should not). The worst result of any
* potential race-condition here is that:
* 1) a newly-added Observer will miss a
* notification in progress
* 2) a recently unregistered Observer will be
* wrongly notified when it doesn't care
*/
if (!changed)
return;
arrLocal = obs.toArray();
clearChanged();
}
for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}
/**
* Clears the observer list so that this object no longer has any observers.
*/
public synchronized void deleteObservers() {
obs.removeAllElements();
}
/**
* Marks this <tt>Observable</tt> object as having been changed; the
* <tt>hasChanged</tt> method will now return <tt>true</tt>.
*/
protected synchronized void setChanged() {
changed = true;
}
/**
* Indicates that this object has no longer changed, or that it has
* already notified all of its observers of its most recent change,
* so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
* This method is called automatically by the
* <code>notifyObservers</code> methods.
*
* @see java.util.Observable#notifyObservers()
* @see java.util.Observable#notifyObservers(java.lang.Object)
*/
protected synchronized void clearChanged() {
changed = false;
}
/**
* Tests if this object has changed.
*
* @return <code>true</code> if and only if the <code>setChanged</code>
* method has been called more recently than the
* <code>clearChanged</code> method on this object;
* <code>false</code> otherwise.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#setChanged()
*/
public synchronized boolean hasChanged() {
return changed;
}
/**
* Returns the number of observers of this <tt>Observable</tt> object.
*
* @return the number of observers of this object.
*/
public synchronized int countObservers() {
return obs.size();
}
}
Observer.java
public interface Observer {
/**
* This method is called whenever the observed object is changed. An
* application calls an <tt>Observable</tt> object's
* <code>notifyObservers</code> method to have all the object's
* observers notified of the change.
*
* @param o the observable object.
* @param arg an argument passed to the <code>notifyObservers</code>
* method.
*/
void update(Observable o, Object arg);
}
Observer这个借口非常简单，跟我们自己定义基本没差。而Observable这个类明显比我们定义要复杂一点，我们看重点
先看成员变量
private boolean changed = false;
private Vector<Observer> obs;
changed属性用来标记观察者集合中的书籍有没有改变，所以我们在发布前调用了setChanged()，否则不能正常发布。
obs属性用来存储观察者对象集合，这里用到了Vector集合对象类型，看到这里应该明白为什么系统提供的方案是线程安全的。
注：用CopyOnWriteArrayList这个集合框架类可以替代Vector会更好。
5.观察者模式在android中运用
1.观察者模式之回调模式
在android中回调模式无处不在，不管是系统api代码还是我们自己的业务代码，几乎天天都在写回调。其实回调模式就是一种简单的观察者模式，区别是回调模式是1对1的，观察者模式是1对n的。举一个android中最常见的例子，Button和OnClickListener之间的关系，Button就是被观察者，OnClickListener就是观察者的接口定义，OnClickListener的实现就是真正的观察者，只不过Button并没有存储OnClickListener实现的集合，只有存储着一个实例而已。
2.listview中的notifyDataChanged方法
notifyDataChanged这方法对于做android的人来说，熟悉了不能再熟悉了，但似乎从来没有归根探究。其实他内部就是用到了观察者模式用于当数据改变中通知view更新操作，由于源码比较复杂，这里就不做展开了。
