Android 学习记录 - 观察者模式

概述JDK封装好的原生的观察者模式，使用起来很简单，只需要被观察者extendsObservable，使用addObserver(Observerobserver)添加观察者，使用setChange()+notifityObservers(Objectdata)通知更新观察者implementsObserver，实现update(Observableobservable,Objectda

JDK 封装好的原生的观察者模式，使用起来很简单，只需要

被观察者 extends Observable，使用 addobserver(Observer observer) 添加观察者，使用 setChange() + notifityObservers(Object data) 通知更新观察者 implements Observer，实现 update(Observable observable, Object data) 方法即可

源码也很简单，我们通过源码了解其机制：

观察者：

package java.util;/** * {@code Observer} is the interface to be implemented by objects that * receive notification of updates on an {@code Observable} object. * * @see Observable */public interface Observer {    /**     * This method is called if the specifIEd {@code Observable} object's     * {@code notifyObservers} method is called (because the {@code Observable}     * object has been updated.     *     * @param observable     *            the {@link Observable} object.     * @param data     *            the data passed to {@link Observable#notifyObservers(Object)}.     */    voID update(Observable observable, Object data);}

是一个接口，有一个方法接收更新，更新的参数是产生更新的被观察者 Observable 和更新发出的信息 Object

被观察者：

package java.util;/** * Observable is used to notify a group of Observer objects when a change * occurs. On creation, the set of observers is empty. After a change occurred, * the application can call the {@link #notifyObservers()} method. This will * cause the invocation of the {@code update()} method of all registered * Observers. The order of invocation is not specifIEd. This implementation will * call the Observers in the order they registered. Subclasses are completely * free in what order they call the update methods. * * @see Observer */

上面意思是被观察者用于在变化发生时通知一组观察者对象。
在创建被观察者时，是没有观察者的，需要通过注册（addobserver(Observer observer)）。
通过 notifyObservers() 调用所有注册（获得了引用）的观察者的 update() 方法。
调用没有特定次序，会按照观察者的注册顺序调用。子类可以重写 notifyObservers() 按自定义次序更新。

public class Observable {    List<Observer> observers = new ArrayList<Observer>();    boolean changed = false;    /**     * Constructs a new {@code Observable} object.     */    public Observable() {    }    /**     * Adds the specifIEd observer to the List of observers. If it is already     * registered, it is not added a second time.     *     * @param observer     *            the Observer to add.     */    public voID addobserver(Observer observer) {        if (observer == null) {            throw new NullPointerException("observer == null");        }        synchronized (this) {            if (!observers.contains(observer))                observers.add(observer);        }    }

observers：声明 Observer 类型的 List 用于存放注册的 Observer
changed：
无参构造函数用以生成被观察者
addobserver()：添加观察者，判断如果为空，抛出空对象异常；使用 synchronized(this) {} 上锁，使用同步方式添加，能保证不会二次添加；使用 contains() 方法判断对象是否已经注册过

    /**     * Clears the changed flag for this {@code Observable}. After calling     * {@code clearChanged()}, {@code hasChanged()} will return {@code false}.     */    protected voID clearChanged() {        changed = false;    }    /**     * Returns the number of observers registered to this {@code Observable}.     *     * @return the number of observers.     */    public int countObservers() {        return observers.size();    }    /**     * Removes the specifIEd observer from the List of observers. Passing null     * won't do anything.     *     * @param observer     *            the observer to remove.     */    public synchronized voID deleteObserver(Observer observer) {        observers.remove(observer);    }    /**     * Removes all observers from the List of observers.     */    public synchronized voID deleteObservers() {        observers.clear();    }

clearChanged()：设更新标志为 false，清除更新
countObservers()：返回 List 的长度
deleteObserver()：移除 List 中指定的观察者
deleteObservers()：清空 List 中所有观察者

    /**     * Returns the changed flag for this {@code Observable}.     *     * @return {@code true} when the changed flag for this {@code Observable} is     *         set, {@code false} otherwise.     */    public boolean hasChanged() {        return changed;    }    /**     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}     * method for every observer in the List of observers using null as the     * argument. Afterwards, calls {@code clearChanged()}.     * <p>     * Equivalent to calling {@code notifyObservers(null)}.     */    public voID notifyObservers() {        notifyObservers(null);    }    /**     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}     * method for every Observer in the List of observers using the specifIEd     * argument. Afterwards calls {@code clearChanged()}.     *     * @param data     *            the argument passed to {@code update()}.     */    @SuppressWarnings("unchecked")    public voID notifyObservers(Object data) {        int size = 0;        Observer[] arrays = null;        synchronized (this) {            if (hasChanged()) {                clearChanged();                size = observers.size();                arrays = new Observer[size];                observers.toArray(arrays);            }        }        if (arrays != null) {            for (Observer observer : arrays) {                observer.update(this, data);            }        }    }    /**     * Sets the changed flag for this {@code Observable}. After calling     * {@code setChanged()}, {@code hasChanged()} will return {@code true}.     */    protected voID setChanged() {        changed = true;    }}

hasChanged()：返回更新标志，用以保证更新时通知
notifyObservers()：调用 notifyObservers(null) 方法，没有数据传输
notifyObservers(Oject data)：当 hasChanged() 为 true 时，进入通知逻辑：一、使用 clearChanged() 清除这次更新；二、将 List 转化为 array 数组，这是为了使用 for(Oject object: arrays) 对注册的观察者进行遍历，效率比较高效；三、使用 update() 传递 Oject 或空值进行数据传输
setChanged()：通知时要使用 setChanged() 使更新标志生效

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