观察者模式

一 定义

观察者模式也叫做发布订阅模式，是一种使用率非常高的设计模式。

定义：定义对象间一种一对多的依赖关系，使得每当一个对象改变状态，则所有依赖于它的对象都会得到通知并被自动更新。

二 模式结构

角色介绍：

• Subject:抽象主题，也就是被观察者的角色，抽象主题角色把所有观察者对象的引用保存在一个集合里，每个主题都可以有任意数量的观察者，抽象主题提供一个接口，可以增加和删除观察者对象。
• ConcreteSubject：具体主题，该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发生改变时，给所有注册过得观察者发送通知，具体主题角色又被叫做具体观察者角色。
• Observer：抽象观察者，该角色是观察者的抽象类，它定义了一个更新接口，使得在得到主题更改通知时更新自己。
• ConcreteObserver:具体观察者，该角色实现抽象观察者角色所定义的更新接口，以便在主题的状态发生变化时更新自身的状态。

三 实例

观察者模式的通用代码如下：

• 抽象观察者Observer接口，主要完成在收到通知后更新自己。

public interface Observer {

    // 更新方法
     void update();
}

• 具体的观察者ConcreteObserver，实现抽象观察者中定义的接口，在收到通知后更新自己。

public class ConcreteObserver implements Observer {
    @Override
    public void update() {
        System.out.println("接收到更新通知，更新！");
    }
}

• 被观察者角色Subject，主要是定义了一个被观察者数组，实现注册，移除观察者，通知观察者更新等。

public abstract class Subject {

    // 定义一个观察者数组
    private List<Observer> mObservers=new ArrayList<>();

    /**
     * 注册观察者对象
     * @param o
     */
    public void addObserver(Observer o){
        this.mObservers.add(o);
    }


    /**
     * 移除观察者对象
     * @param o
     */
    public void deleteObserver(Observer o){
        this.mObservers.remove(o);
    }


    /**
     * 通知所有观察者更新
     */
    public void notifyObserver(){
        for (Observer observer:mObservers){
            observer.update();
        }
     }
}

• 具体的被观察者ConcreteObserver，实现自己的业务逻辑。

public class ConcreteSubject extends Subject{

    /**
     * 具体的业务逻辑
     */
    public void doSomething(){
        super.notifyObserver();
    }
}

• 测试代码

// 创建一个被观察者
        ConcreteSubject subject=new ConcreteSubject();
        // 定义一个观察者
        Observer observer=new ConcreteObserver();
        // 注册观察者
        subject.addObserver(observer);
        // 开始观察
        subject.doSomething();

具体结果就不贴了，注册观察者后，被观察者会发送通知，通知到观察者，观察者收到通知后，更新自己。

下面，我们以订阅开发者论坛的技术文章为例，Android 开发者网站是google专门向android开发者提供技术支持的专业社区，开发者可以通过邮件订阅的方式订阅上面的文章。在这里，开发者相当于观察者，Android 开发者网站相当于被观察者。

实际上，针对观察者模式，Java向我们提供了观察者和被观察对象，在Java的实用工具类库java.util包下，我们可以直接使用，下面可以看下源码。

• 观察者Observer接口，主要定义了一个可用于数据更新的接口。

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);
}

• 被观察者Observable，可以看到，Observable源码也是实现了addObserver，deleteObserver，notifyObservers等方法。

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
             */
            // Android-changed: Call out to hasChanged() to figure out if something changes.
            // Upstream code avoids calling the nonfinal hasChanged() from the synchronized block,
            // but that would break compatibility for apps that override that method.
            // if (!changed)
            if (!hasChanged())
                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();
    }
}

以下是具体的代码：

• 观察者角色，观察者实现了Observer接口，只要实现一个update()方法就能接受到更新。

public class Developer implements Observer {

    private String name;

    public Developer(String name){
        this.name=name;
    }

    @Override
    public void update(Observable o, Object arg) {
        System.out.println("你好，"+name+",Android开发网站更新啦，内容："+arg);
    }
}

• 被观察者角色，实现Observable，在这里可以实现自己的业务逻辑，本实例实现了一个发布新技术的方法。

public class DevelopTribune extends Observable {

    // 自己的业务逻辑
    public void postNewPublish(String content){
        // 标识状态发生改变
        setChanged();
        // 通知所有观察者
        notifyObservers(content);
    }
}

• 测试代码

// 被观察者
       DevelopTribune developTribune=new DevelopTribune();
       // 观察者
       Developer developer1=new Developer("Lucy");
       Developer developer2=new Developer("James");
       Developer developer3=new Developer("Jackson");
       // 注册观察者
       developTribune.addObserver(developer1);
       developTribune.addObserver(developer2);
       developTribune.addObserver(developer3);

       // 发布更新
       developTribune.postNewPublish("新技术来袭！");

• 运行结果

  
  

四 优缺点

优点：

• 观察者和被观察者之间是抽象耦合，应对业务变化，非常容易扩展。
• 增强系统灵活性，可扩展性。
  缺点：
  在运用观察者模式需要考虑效率问题，即存在一个被观察者和多个观察者的时候，开发和调试会比较复杂，如果一个观察者卡壳，会影响整体的执行效率。

五 使用场景

• 关联行为场景，关联行为要是可拆分的，而不是组合关系。
• 事件多级触发场景
• 跨系统信息交换场景