Observer源码
Observer是一个interface，只有一个update方法，两个参数，参数1位被观察者，参数2为被观察者额外传递的参数。
/**
* A class can implement the <code>Observer</code> interface when it
* wants to be informed of changes in observable objects.
*
* @author Chris Warth
* @see java.util.Observable
* @since JDK1.0
*/
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);
}
Observable 源码
Observable是一个普通Java类，维护两个成员变量：
- 状态改变标记位：changed；
- observers维护所有观察者 内部操作observers的代码都采用了同步机制，不会有线程安全问题
public class Observable {
private boolean changed = false;
private final ArrayList<Observer> observers;
/** Construct an Observable with zero Observers. */
public Observable() {
observers = new ArrayList<>();
}
/**
* 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 (!observers.contains(o)) {
observers.add(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) {
observers.remove(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.
*/
Observer[] arrLocal;
synchronized (this) {
/* We don't want the Observer doing callbacks into
* arbitrary Observables while holding its own Monitor.
* The code where we extract each Observable from
* the ArrayList 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 (!hasChanged())
return;
arrLocal = observers.toArray(new Observer[observers.size()]);
clearChanged();
}
for (int i = arrLocal.length-1; i>=0; i--)
arrLocal[i].update(this, arg);
}
/**
* Clears the observer list so that this object no longer has any observers.
*/
public synchronized void deleteObservers() {
observers.clear();
}
/**
* 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 observers.size();
}
}
使用实例
NumObserable是一个被观察者，当它的成员变量data的数值发生变化时，会通知所有的观察者。 NumObserable.java
package com.zj.observer;
import java.util.Observable;
public class NumObservable extends Observable {
private int data = 0;
public int getData() {
return data;
}
public void setData(int i) {
data = i;
setChanged();
notifyObservers();
}
}
NumObserver是观察者。当它的被观察者(NumObserable)执行了notifyObservers()后，它会执行uodate()方法。 NumObserver.java
package com.zj.observer;
import java.util.Observable;
import java.util.Observer;
public class NumObserver implements Observer{
public void update(Observable o, Object arg) {
NumObservable myObserable=(NumObservable) o;
System.out.println("Data has changed to " +myObserable.getData());
}
}
测试类SingleTest，在这里将观察者加入到被观察者的观察列表中。
package com.zj.observer;
public class SingleTest {
public static void main(String[] args) {
NumObservable number = new NumObservable();
number.addObserver(new NumObserver());
number.setData(1);
number.setData(2);
number.setData(3);
}
结果：
Data has changed to 1
Data has changed to 2
Data has changed to 3