观察者模式

观察者模式的定义

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，则所有依赖它的对象都会得到通知并自动更新。不好理解？打个比方，
比如你订了报纸，每天更新了就会通知你。报亭和用户们就是一对多的关系。

观察者模式实现

观察者模式主要由观察者和被观察者组成
Observer 观察者，接口
Observable 被观察者，类
当被观察者更新时，会通知观察者。
观察者模式主要实现是接口回调，我们来简单编写个观察者模式的Demo

观察者
public class Coder implements Observer {
    public String name;
    public Coder(String name) {
        this.name=name;
    }


    @Override
    public void update(Observable o, Object arg) {
        System.out.println("HI,"+name+",更新了"+arg);
    }
}

被观察者
public class AndroidDev extends Observable {
    public void postNewPublication(String content) {
        //设置标志位
        setChanged();
        //通知Observer
        notifyObservers(content);
    }
}

public class Test {
    public static void main(String [] args){
        System.out.println("123");
        AndroidDev dev=new AndroidDev();
        Coder coder1=new Coder("1");
        Coder coder12=new Coder("2");
        Coder coder13=new Coder("3");

        dev.addObserver(coder1);
        dev.addObserver(coder12);
        dev.addObserver(coder13);

        dev.postNewPublication("更新了Android8.0");
    }
}

下面是运行结果

123
HI,3,更新了更新了Android8.0
HI,2,更新了更新了Android8.0
HI,1,更新了更新了Android8.0

程序员们订阅了Android开发者，当Android版本有更新的时候会通知所有订阅了的程序员。我们来看下Observer 和Observable 的源码。

源码分析

Observer

public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an Observable object's
     * notifyObservers method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the notifyObservers
     *                 method.
     */
    void update(Observable o, Object arg);
}

是不是很简单，就一个update方法。在看下Observable 类

public class Observable {
    private boolean changed = false;
    private Vector obs;

    /** Construct an Observable with zero Observers. */

    public Observable() {
        obs = new Vector<>();
    }


    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 null 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
     * hasChanged method, then notify all of its observers
     * and then call the clearChanged method to
     * indicate that this object has no longer changed.
     * 

     * Each observer has its update method called with two
     * arguments: this observable object and null. In other
     * words, this method is equivalent to:
     * 

     * notifyObservers(null)
     *
     * @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
     * hasChanged method, then notify all of its observers
     * and then call the clearChanged method to indicate
     * that this object has no longer changed.
     * 

     * Each observer has its update method called with two
     * arguments: this observable object and the arg 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 Observable object as having been changed; the
     * hasChanged method will now return true.
     */
    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 hasChanged method will now return false.
     * This method is called automatically by the
     * notifyObservers 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  true if and only if the setChanged
     *          method has been called more recently than the
     *          clearChanged method on this object;
     *          false otherwise.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#setChanged()
     */
    public synchronized boolean hasChanged() {
        return changed;
    }

    /**
     * Returns the number of observers of this Observable object.
     *
     * @return  the number of observers of this object.
     */
    public synchronized int countObservers() {
        return obs.size();
    }

setChanged()：设置更新的标志位
addObserver（）：添加Observer对象
notifyObservers(Object arg) ：遍历集合中的Observier，并通知更新。

这样看起来是不是容易多了，先写到这。。。