（设计模式）观察者

Posted 2021-04-25 多元思维的开发者

观察者模式：

观察者模式也叫发布与订阅模式，主要有两种对象：

1、主题（被观察对象）；2、观察者。观察者可以订阅任意主题；一旦主题更新变动，就会通知已经订阅的所有观察者。

例子：

在jdk的util中，定义了一种比较灵活的发布与订阅的接口。这两个其实就只有参考的意义，因为大多数都需要根据具体的场景重新设计被观察者和观察者的接口。

因为一些实际的被观察者对象需要具有特定的属性和方法，但是在观察者的update接口中，参数的类型是Observable，那么就代表需要进行一个类型的转换才行。

1、观察者接口：

2、可观察类：

相应属性：

// 更新标志，用于push和观察者来的判断private boolean changed = false;
// 观察者集合private Vector<Observer> obs;
/** Construct an Observable with zero Observers. */
public Observable() { obs = new Vector<>();}

观察者注册与移除方法：

/** * 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(); // 是否已经注册过 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);}

通知观察者的方法入口：

/** * 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 */  // 只有changed为true才可以通知所有观察者 if (!changed) return; arrLocal = obs.toArray();  // 清除更新标志，设置为false clearChanged(); }  // 从最后一个观察者开始通知（具体的变动主题，更新的内容） for (int i = arrLocal.length-1; i>=0; i--) ((Observer)arrLocal[i]).update(this, arg);}

两个基本但挺有用的方法，setChanged和hasChange方法，用于对被观察者（主题）的状态进行管理。

/** * 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() { // 在这里可以周期性存储一定量的的变更内容，然后设置change并通知所有观察者。 changed = true; // 按道理，这里应该有一个notifyObservee方法的调用}
/** * 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() { /** * 返回主题的内容是否更新了 * 可以pull，观察者拉结合使用。然后可观察者提供返回更新数据的接口 */ return changed;}

设计分析

1、jdk的观察者模式还是设计的不错，但是只能作为模板参考。因为通用性不高，很多时候主题都是具备很多属性，并且update的方法需要重新定义入参类型才能满足具体的业务需求。

2、观察者模式完全可以提供push和pull两种方式；拉方式，那么被观察者需要提供能够知道内容更新和获取更新数据的方法。

模式分析：

1、将主题（被观察者）和观察者之间松耦合，主题的内容怎么来、经过怎样的处理都不影响观察者的观察行为。只要主题的状态发生变化，就会通知所有的观察者主题发生变化。

2、除了被动通知观察者，观察者还可以主动拉取主题最新状态的信息，也就是推拉模式。