观察者（Observer）模式

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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:

二，自定义观察者模式代码实现

1，三个接口如下：

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：
1. 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）
2. 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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