HeadFirst設計模式讀書筆記
觀察者模式
一线椰，簡介
- 什么是觀察者模式胞谈？
定義：觀察者模式定義了對象之間的一對多依賴，這樣一來憨愉，當一個對象改變狀態(tài)時烦绳，他的所有依賴者都會收到通知并自動更新。
理解：類似于報紙訂閱配紫，向某家報社（被觀察者）訂閱報紙径密，只要有新報紙出版，他就會把新報紙送到訂閱者（觀察者）手中躺孝。同時訂閱者可以取消訂閱這份報紙享扔。 -
現(xiàn)在還是書中的例子：被觀察者是氣象數(shù)據WeatherData，觀察者是氣象顯示板CurrentConditionDisplay括细，類圖如下:
observer
二伪很，自定義觀察者模式代碼實現(xiàn)
1，三個接口如下：
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奋单，WeatherData的實現(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锉试，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);
}
}
三览濒，JDK中的觀察者模式
1呆盖，簡介
jdk中內置了觀察者模式，但是Observable是一個類不是一個接口贷笛，那么久降低了其擴展性应又，因為一個類只能繼承一個類。
2乏苦，需要注意的部分
- 增加了一個setChanged方法株扛。
- 兩種通知觀察者的方式：
- notifyObservers();采用這種方式尤筐，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）
- notifyObservers(Object obj);這是一種推數(shù)據的方式，直接將一個object對象通知給每一個觀察者洞就。
四盆繁，使用JDK封裝的觀察者模式實現(xiàn)代碼
1，不需要提供observer和observable接口旬蟋，只需要提供一個每個顯示板都需要實現(xiàn)display方法的接口
public interface DisplayElement {
public void display();
}
2油昂，WeatherData需要繼承jdk提供的Observable類，其中注冊observer的list以及notifyObserver()具體的實現(xiàn)都已經被jdk內部實現(xiàn)好了倾贰。
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();
}
//因為采用的是notifyObservers()方法冕碟，所以采用的是拉的方式更新數(shù)據，所以需要提供get方法匆浙。
public float getTemperature(){
return temperature;
}
public float getHumidity(){
return humidity;
}
public float getPressure(){
return pressure;
}
}
3安寺，觀察者實現(xiàn)jdk提供的Observer接口，那么會使用內部封裝好的update()方法吞彤，所以需要注意類型強轉我衬。
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);
}
}
五饰恕，JDK中提到的還有一個notifyObserver(Object obj)的方法可以通知觀察者
1，創(chuàng)建一個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井仰，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埋嵌，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);
}
}