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，它的所有依赖都会收到通知并自动更新。
示例—气象观测站的监测系统
已有一个WeatherData对象，负责追踪目前的天气状况（温度、湿度、气压）。建立一个应用， 有三种布告板，分别显示目前的状况、气象统计及简单的预报。当WeatherData对象获得最新的测量数据时，三种布告板必须实时更新。而且可以扩展，公布一组api，可让其他开发人员写出自己的气象布告板，并插入此应用。
有两种方式实现：自定义观察者和内置观察类
自定义观察者
- UML图表示
气象站三种布告板的设计-自定义观察者
- 代码演示
import java.util.ArrayList;
import java.util.List;
interface Subject{
void registerObserver(Observer o);
void removeObserver(Observer o);
void notifyObservers();
}
class WeatherData implements Subject{
private ArrayList observers;
private float temperature;
private float humidity;
private float pressure;
public WeatherData(){
observers = new ArrayList();
}
@Override
public void registerObserver(Observer o) {
observers.add(o);
}
@Override
public void removeObserver(Observer o) {
int i = observers.indexOf(o);
if (i >= 0) {
observers.remove(i);
}
}
@Override
public void notifyObservers() {
for (int i = 0; i < observers.size(); i++) {
Observer observer = (Observer)observers.get(i);
observer.update(temperature,humidity,pressure);
}
}
public void measurementsChanged(){
notifyObservers();
}
public void setMeasurements(float temperature, float humidity, float pressure){
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}
}
interface Observer{
void update(float temp, float humidity, float pressure);
}
interface DisplayElement{
void display();
}
class CurrentConditionsDisplay implements Observer,DisplayElement{
private float temperature;
private float humidity;
private Subject weatherData;
public CurrentConditionsDisplay(Subject weatherData){
this.weatherData = weatherData;
weatherData.registerObserver(this);
}
@Override
public void update(float temp, float humidity, float pressure) {
this.temperature = temp;
this.humidity = humidity;
display();
}
@Override
public void display() {
System.out.println("Current conditions: " + temperature
+ "F degrees and " + humidity + "% humidity" );
}
}
class StatisticsDisplay implements Observer, DisplayElement{
private List<Float> temperatureList;
private List<Float> humidityList;
private List<Float> pressureList;
private Subject weatherData;
public StatisticsDisplay(Subject weatherData){
this.weatherData = weatherData;
temperatureList = new ArrayList<Float>();
humidityList = new ArrayList<Float>();
pressureList = new ArrayList<Float>();
weatherData.registerObserver(this);
}
@Override
public void update(float temp, float humidity, float pressure) {
temperatureList.add(temp);
humidityList.add(humidity);
pressureList.add(pressure);
display();
}
@Override
public void display() {
System.out.println("Statistics Average Data: " + calAverage(temperatureList)
+ "F degrees and " + calAverage(humidityList) + "% humidity and average pressure is "
+ calAverage(pressureList));
}
private Float calAverage(List<Float> list){
Float sum = 0.0f;
for (Float item : list){
sum += item;
}
return sum / (float) list.size();
}
}
class ForecastDisplay implements Observer, DisplayElement{
private float temperature;
private float humidity;
private float pressure;
private Subject weatherData;
public ForecastDisplay(Subject weatherData){
this.weatherData = weatherData;
weatherData.registerObserver(this);
}
@Override
public void update(float temp, float humidity, float pressure) {
this.temperature = temp + (float) 1;
this.humidity = humidity - (float) 1;
this.pressure = pressure + 0.5f;
display();
}
@Override
public void display() {
System.out.println("Forecast conditions: " + temperature
+ "F degrees and " + humidity + "% humidity and pressure is "
+ pressure);
}
}
public class WeatherStation{
public static void main(String[] args) {
WeatherData weatherData = new WeatherData();
CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);
weatherData.setMeasurements(80,65,30.4f);
weatherData.setMeasurements(82,70,29.2f);
weatherData.setMeasurements(78,90,29.2f);
}
}
- 测试结果
Current conditions: 80.0F degrees and 65.0% humidity
Statistics Average Data: 80.0F degrees and 65.0% humidity and average pressure is 30.4
Forecast conditions: 81.0F degrees and 64.0% humidity and pressure is 30.9
Current conditions: 82.0F degrees and 70.0% humidity
Statistics Average Data: 81.0F degrees and 67.5% humidity and average pressure is 29.8
Forecast conditions: 83.0F degrees and 69.0% humidity and pressure is 29.7
Current conditions: 78.0F degrees and 90.0% humidity
Statistics Average Data: 80.0F degrees and 75.0% humidity and average pressure is 29.6
Forecast conditions: 79.0F degrees and 89.0% humidity and pressure is 29.7
内置观察类
- UML图表示
气象站三种布告板的设计-内置观察类
- 代码演示
import java.util.ArrayList;
import java.util.List;
import java.util.Observable;
import java.util.Observer;
class WeatherData2 extends Observable{
private float temperature;
private float humidity;
private float pressure;
public void measurementsChanged(){
setChanged();
notifyObservers();
}
public void setMeasurements(float temperature, float humidity, float pressure){
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}
float getTemperature() {
return temperature;
}
float getHumidity() {
return humidity;
}
float getPressure(){
return pressure;
}
}
class CurrentConditionsDisplay2 implements Observer,DisplayElement{
private float temperature;
private float humidity;
private Observable observable;
public CurrentConditionsDisplay2(Observable observable){
this.observable = observable;
observable.addObserver(this);
}
@Override
public void display() {
System.out.println("Current conditions: " + temperature
+ "F degrees and " + humidity + "% humidity" );
}
@Override
public void update(Observable o, Object arg) {
if (o instanceof WeatherData2){
WeatherData2 weatherData2 = (WeatherData2) o;
this.temperature = weatherData2.getTemperature();
this.humidity = weatherData2.getHumidity();
display();
}
}
}
class StatisticsDisplay2 implements Observer, DisplayElement{
private Observable observable;
private List<Float> temperatureList;
private List<Float> humidityList;
private List<Float> pressureList;
public StatisticsDisplay2(Observable observable){
this.observable = observable;
temperatureList = new ArrayList<Float>();
humidityList = new ArrayList<Float>();
pressureList = new ArrayList<Float>();
observable.addObserver(this);
}
@Override
public void display() {
System.out.println("Statistics Average Data: " + calAverage(temperatureList)
+ "F degrees and " + calAverage(humidityList) + "% humidity and average pressure is "
+ calAverage(pressureList));
}
private Float calAverage(List<Float> list){
Float sum = 0.0f;
for (Float item : list){
sum += item;
}
return sum / (float) list.size();
}
@Override
public void update(Observable o, Object arg) {
if (o instanceof WeatherData2){
WeatherData2 weatherData2 = (WeatherData2) o;
temperatureList.add(weatherData2.getTemperature());
humidityList.add(weatherData2.getHumidity());
pressureList.add(weatherData2.getPressure());
display();
}
}
}
class ForecastDisplay2 implements Observer, DisplayElement{
private float temperature;
private float humidity;
private float pressure;
private Observable observable;
public ForecastDisplay2(Observable observable){
this.observable = observable;
observable.addObserver(this);
}
@Override
public void display() {
System.out.println("Forecast conditions: " + temperature
+ "F degrees and " + humidity + "% humidity and pressure is "
+ pressure);
}
@Override
public void update(Observable o, Object arg) {
if (o instanceof WeatherData2){
WeatherData2 weatherData2 = (WeatherData2) o;
this.temperature = weatherData2.getTemperature() + (float) 1;
this.humidity = weatherData2.getHumidity() - (float) 1;
this.pressure = weatherData2.getPressure() + 0.5f;
display();
}
}
}
public class WeatherStation2 {
public static void main(String[] args) {
WeatherData2 weatherData2 = new WeatherData2();
CurrentConditionsDisplay2 currentConditionsDisplay = new CurrentConditionsDisplay2(weatherData2);
StatisticsDisplay2 statisticsDisplay = new StatisticsDisplay2(weatherData2);
ForecastDisplay2 forecastDisplay = new ForecastDisplay2(weatherData2);
weatherData2.setMeasurements(80,65,30.4f);
weatherData2.setMeasurements(82,70,29.2f);
weatherData2.setMeasurements(78,90,29.2f);
}
}
- 测试结果
Forecast conditions: 81.0F degrees and 64.0% humidity and pressure is 30.9
Statistics Average Data: 80.0F degrees and 65.0% humidity and average pressure is 30.4
Current conditions: 80.0F degrees and 65.0% humidity
Forecast conditions: 83.0F degrees and 69.0% humidity and pressure is 29.7
Statistics Average Data: 81.0F degrees and 67.5% humidity and average pressure is 29.8
Current conditions: 82.0F degrees and 70.0% humidity
Forecast conditions: 79.0F degrees and 89.0% humidity and pressure is 29.7
Statistics Average Data: 80.0F degrees and 75.0% humidity and average pressure is 29.6
Current conditions: 78.0F degrees and 90.0% humidity
结论
两种方式都可以实现观察者模式，各有特色。两种模式都能实现“推”和“拉”的方式。
- 自定义观察者：可以实现用接口开发，而不是实现开发，实现方式比较自由，但需要自定义的代码多。
- 内置观察类：不能实现接口开发，必须用继承的方式开发，但不需要自己实现注册、取消等代码。
