HeadFirst设计模式读书笔记
观察者模式
一，简介
- 什么是观察者模式？
定义：观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，他的所有依赖者都会收到通知并自动更新。
理解：类似于报纸订阅，向某家报社（被观察者）订阅报纸，只要有新报纸出版，他就会把新报纸送到订阅者（观察者）手中。同时订阅者可以取消订阅这份报纸。 -
现在还是书中的例子：被观察者是气象数据WeatherData，观察者是气象显示板CurrentConditionDisplay，类图如下:
observer
二，自定义观察者模式代码实现
1，三个接口如下：
package interfaces;
public interface Subject {
public void registerObserver(Observer o);
}
public interface Observer {
public void update(float temp,float humidity,float pressure);
}
public interface DisplayElement {
public void display();
}
2，WeatherData的实现
package implementObjObserver;
import java.util.ArrayList;
import interfaces.Observer;
import interfaces.Subject;
public class WeatherData implements Subject{
private ArrayList observers;
private float temperature;
private float humidity;
private float pressure;
public WeatherData(){
observers = new ArrayList();
}
public void removeObserver(Observer o){
int i = observers.indexOf(o);
if(i>=0){
observers.remove(i);
}
}
public void notifyObservers(){
for(int i = 0; i < observers.size(); i++){
Observer observer =(Observer)observers.get(i);
observer.update(temperature, humidity, pressure);
}
}
@Override
public void registerObserver(Observer o) {
// TODO Auto-generated method stub
observers.add(o);
}
public void setMeasurements(float temperature,float humidity,float pressure){
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}
public void measurementsChanged(){
notifyObservers();
}
}
3，CurrentConditionDisplay代码如下
package implementObjObserver;
import interfaces.DisplayElement;
import interfaces.Observer;
import interfaces.Subject;
public class CurrentConditionsDisplay implements Observer,DisplayElement {
private float temperature;
private float humidity;
private Subject weatherData;
public CurrentConditionsDisplay(Subject weatherData) {
// TODO Auto-generated constructor stub
this.weatherData = weatherData;
weatherData.registerObserver(this);
}
@Override
public void update(float temp, float humidity, float pressure) {
// TODO Auto-generated method stub
this.temperature = temp;
this.humidity = humidity;
display();
}
@Override
public void display() {
// TODO Auto-generated method stub
System.out.println("Current conditions: " + temperature + "F degrees and "
+ humidity + "%humidity");
}
}
4，测试代码
package implementObjObserver;
public class WeatherStation {
public static void main(String[] args) {
WeatherData weatherData = new WeatherData();
CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
weatherData.setMeasurements(80, 12, 20.4f);
}
}
三，JDK中的观察者模式
1，简介
jdk中内置了观察者模式，但是Observable是一个类不是一个接口，那么久降低了其扩展性，因为一个类只能继承一个类。
2，需要注意的部分
- 增加了一个setChanged方法。
- 两种通知观察者的方式：
- notifyObservers();采用这种方式，观察者需要用拉的方式获取更新（在下面的代码中可以看到）
- notifyObservers(Object obj);这是一种推数据的方式，直接将一个object对象通知给每一个观察者。
四，使用JDK封装的观察者模式实现代码
1，不需要提供observer和observable接口，只需要提供一个每个显示板都需要实现display方法的接口
public interface DisplayElement {
public void display();
}
2，WeatherData需要继承jdk提供的Observable类，其中注册observer的list以及notifyObserver()具体的实现都已经被jdk内部实现好了。
package com.liu;
import java.util.Observable;
public class WeatherData extends Observable {
private float temperature;
private float humidity;
private float pressure;
public void messureChanged(){
setChanged();
notifyObservers();
}
public void setMessurements(float tem, float humi,float pre){
this.temperature = tem;
this.humidity = humi;
this.pressure = pre;
messureChanged();
}
//因为采用的是notifyObservers()方法，所以采用的是拉的方式更新数据，所以需要提供get方法。
public float getTemperature(){
return temperature;
}
public float getHumidity(){
return humidity;
}
public float getPressure(){
return pressure;
}
}
3，观察者实现jdk提供的Observer接口，那么会使用内部封装好的update()方法，所以需要注意类型强转。
package com.liu;
import java.util.Observable;
import java.util.Observer;
public class CurrentConditionsDisplay implements Observer, DisplayElement {
private Observable observable;
private float temperature;
private float humidity;
public CurrentConditionsDisplay(Observable observable) {
// TODO Auto-generated constructor stub
this.observable = observable;
observable.addObserver(this);
}
@Override
public void display() {
// TODO Auto-generated method stub
System.out.println("Current conditions: " + temperature + "F degrees and "
+ humidity + "%humidity");
}
@Override
public void update(Observable o, Object arg) {
// TODO Auto-generated method stub
if(o instanceof WeatherData){
WeatherData weatherData = (WeatherData)o;
this.temperature = weatherData.getTemperature();
this.humidity = weatherData.getHumidity();
display();
}
}
}
4，代码结果测试
public class WeatherStation {
public static void main(String[] args) {
WeatherData weatherData = new WeatherData();
CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
weatherData.setMessurements(12, 12, 12);
}
}
五，JDK中提到的还有一个notifyObserver(Object obj)的方法可以通知观察者
1，创建一个Datas类
package com.liu;
public class Datas {
private float temperature;
private float humidity;
private float pressure;
public Datas() {
// TODO Auto-generated constructor stub
}
public float getTemperature() {
return temperature;
}
public void setTemperature(float temperature) {
this.temperature = temperature;
}
public float getHumidity() {
return humidity;
}
public void setHumidity(float humidity) {
this.humidity = humidity;
}
public float getPressure() {
return pressure;
}
public void setPressure(float pressure) {
this.pressure = pressure;
}
}
2，被观察者需要改为：
import java.util.Observable;
public class WeatherData extends Observable {
private float temperature;
private float humidity;
private float pressure;
private Datas data = new Datas();
public void messureChanged(){
setChanged();
notifyObservers(data);
}
public void setMessurements(float tem, float humi,float pre){
data.setTemperature(tem);
data.setHumidity(humi);
data.setPressure(pre);
messureChanged();
}
}
3，观察者：
package com.liu;
import java.util.Observable;
import java.util.Observer;
public class CurrentConditionsDisplay implements Observer, DisplayElement {
private Observable observable;
private float temperature;
private float humidity;
public CurrentConditionsDisplay(Observable observable) {
// TODO Auto-generated constructor stub
this.observable = observable;
observable.addObserver(this);
}
@Override
public void display() {
// TODO Auto-generated method stub
System.out.println("Current conditions: " + temperature + "F degrees and "
+ humidity + "%humidity");
}
@Override
public void update(Observable o, Object obj) {
// TODO Auto-generated method stub
if(o instanceof WeatherData){
WeatherData weatherData = (WeatherData)o;
Datas data = (Datas)obj;
this.temperature = data.getTemperature();
this.humidity = data.getHumidity();
display();
}
}
}
4，测试代码不变
public class WeatherStation {
public static void main(String[] args) {
WeatherData weatherData = new WeatherData();
CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
weatherData.setMessurements(14, 12, 12);
}
}
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