Java 源码解读 - 观察者模式

JDK 封装好的原生的观察者模式，使用起来很简单，只需要

1. 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
2. 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