設(shè)計(jì)模式系列—觀察者（Observer）模式

HeadFirst設(shè)計(jì)模式讀書筆記

觀察者模式

一，簡介

  1. 什么是觀察者模式？
    定義：觀察者模式定義了對象之間的一對多依賴，這樣一來，當(dāng)一個(gè)對象改變狀態(tài)時(shí)，他的所有依賴者都會(huì)收到通知并自動(dòng)更新。
    理解：類似于報(bào)紙訂閱，向某家報(bào)社（被觀察者）訂閱報(bào)紙，只要有新報(bào)紙出版，他就會(huì)把新報(bào)紙送到訂閱者（觀察者）手中。同時(shí)訂閱者可以取消訂閱這份報(bào)紙。

  2. 現(xiàn)在還是書中的例子：被觀察者是氣象數(shù)據(jù)WeatherData，觀察者是氣象顯示板CurrentConditionDisplay，類圖如下:


    observer

二，自定義觀察者模式代碼實(shí)現(xiàn)

1，三個(gè)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的實(shí)現(xiàn)

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代碼如下

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，測試代碼

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中的觀察者模式

1，簡介

jdk中內(nèi)置了觀察者模式，但是Observable是一個(gè)類不是一個(gè)接口，那么久降低了其擴(kuò)展性，因?yàn)橐粋€(gè)類只能繼承一個(gè)類。

2，需要注意的部分

  • 增加了一個(gè)setChanged方法。
  • 兩種通知觀察者的方式：
  1. notifyObservers();采用這種方式，觀察者需要用拉的方式獲取更新（在下面的代碼中可以看到）
  2. notifyObservers(Object obj);這是一種推數(shù)據(jù)的方式，直接將一個(gè)object對象通知給每一個(gè)觀察者。

四，使用JDK封裝的觀察者模式實(shí)現(xiàn)代碼

1，不需要提供observer和observable接口，只需要提供一個(gè)每個(gè)顯示板都需要實(shí)現(xiàn)display方法的接口

public interface DisplayElement {
public void display();
}

2，WeatherData需要繼承jdk提供的Observable類，其中注冊observer的list以及notifyObserver()具體的實(shí)現(xiàn)都已經(jīng)被jdk內(nèi)部實(shí)現(xiàn)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?yàn)椴捎玫氖莕otifyObservers()方法，所以采用的是拉的方式更新數(shù)據(jù)，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，觀察者實(shí)現(xiàn)jdk提供的Observer接口，那么會(huì)使用內(nèi)部封裝好的update()方法，所以需要注意類型強(qiáng)轉(zhuǎn)。

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，代碼結(jié)果測試

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中提到的還有一個(gè)notifyObserver(Object obj)的方法可以通知觀察者

1，創(chuàng)建一個(gè)Datas類

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，被觀察者需要改為：

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，觀察者：

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，測試代碼不變

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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