设计模式 - 观察者模式(Observer Pattern) 详细解释

观察者模式(Observer Pattern) 详细解释

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观察者模式(Observer Pattern): 定义了对象之间的一对多的依赖, 这样一来, 当一个对象改变状态时, 它的全部依赖者都会收到通知并自己主动更新.

用法:

1. 首先新建主题(subject)接口, 负责注冊(register)\删除(remove)\通知(notify)观察者; 观察者(observer)接口,
负责更新(update)数据;

主题(subject)接口: 注冊观察者(registerObserver), 删除观察者(removeObserver), 通知观察者(notifyObservers, 通知全部观察者);

观察者(observer)接口: 更新(update);

代码:

/**
* @time 2014年5月22日
*/
package observer;

/**
* @author C.L.Wang
*
*/
public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObervers();
}

/**
* @time 2014年5月22日
*/
package observer;

/**
* @author C.L.Wang
*
*/
public interface Observer {
public void update(float temp, float humidity, float pressure);
}

/**
* @time 2014年5月22日
*/
package observer;

/**
* @author C.L.Wang
*
*/
public interface DisplayElement {
public void display();
}

2. 实现主题(subject)接口, 通过列表(list)的形式注冊(register)和删除(remove)观察者,

通知(notify)观察者时, 循环调用全部已注冊的观察者的更新(update)动作;

通过接口函数(set), 封装通知(notify)动作, 传入參数并进行通知.

代码:

/**
* @time 2014年5月22日
*/
package observer;

import java.util.ArrayList;

/**
* @author C.L.Wang
*
*/
public class WeatherData implements Subject {

public ArrayList<Observer> observers;
private float temperature;
private float humidity; //湿度
private float pressure;

public WeatherData() {
observers = new ArrayList<Observer>();
}

/* (non-Javadoc)
* @see observer.Subject#registerObserver(observer.Observer)
*/
@Override
public void registerObserver(Observer o) {
// TODO Auto-generated method stub
observers.add(o);
}

/* (non-Javadoc)
* @see observer.Subject#removeObserver(observer.Observer)
*/
@Override
public void removeObserver(Observer o) {
// TODO Auto-generated method stub
int i = observers.indexOf(o);
if (i>=0) {
observers.remove(i);
}
}

/* (non-Javadoc)
* @see observer.Subject#notifyObervers()
*/
@Override
public void notifyObervers() {
// TODO Auto-generated method stub
for (int i=0; i<observers.size(); ++i) {
Observer observer = (Observer)observers.get(i);
observer.update(temperature, humidity, pressure);
}
}

public void measurementsChanged() {
notifyObervers();
}

public void setMeasurements(float temperature, float humidity, float pressure) {
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}

}

3. 实现观察者(observer)接口, 主要保存收到的数据, 并实现更新(update)动作, 即把数据保存在本地;

在构造函数中, 把自己注冊(register)入, 传入的主题(subject)參数, 使主题能够通知观察者.

代码:

/**
* @time 2014年5月22日
*/
package observer;

/**
* @author C.L.Wang
*
*/
public class CurrentConditionsDisplay implements Observer, DisplayElement {

private float temperature;
private float humidity;
private Subject weatherData;

public CurrentConditionsDisplay(Subject weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this);
}

/* (non-Javadoc)
* @see observer.DisplayElement#display()
*/
@Override
public void display() {
// TODO Auto-generated method stub
System.out.println("Current conditions: " + temperature +
"F degrees and " + humidity + "% humidity");
}

/* (non-Javadoc)
* @see observer.Observer#update(float, float, float)
*/
@Override
public void update(float temperature, float humidity, float pressure) {
// TODO Auto-generated method stub
this.temperature = temperature;
this.humidity = humidity;
display();
}

}

package observer;

import java.util.*;

public class ForecastDisplay implements Observer, DisplayElement {
private float currentPressure = 29.92f;
private float lastPressure;
private WeatherData weatherData;

public ForecastDisplay(WeatherData weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this);
}

public void update(float temp, float humidity, float pressure) {
lastPressure = currentPressure;
currentPressure = pressure;

display();
}

public void display() {
System.out.print("Forecast: ");
if (currentPressure > lastPressure) {
System.out.println("Improving weather on the way!");
} else if (currentPressure == lastPressure) {
System.out.println("More of the same");
} else if (currentPressure < lastPressure) {
System.out.println("Watch out for cooler, rainy weather");
}
}
}

package observer;

import java.util.*;

public class StatisticsDisplay implements Observer, DisplayElement {
private float maxTemp = 0.0f;
private float minTemp = 200;
private float tempSum= 0.0f;
private int numReadings;
private WeatherData weatherData;

public StatisticsDisplay(WeatherData weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this);
}

public void update(float temp, float humidity, float pressure) {
tempSum += temp;
numReadings++;

if (temp > maxTemp) {
maxTemp = temp;
}

if (temp < minTemp) {
minTemp = temp;
}

display();
}

public void display() {
System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
+ "/" + maxTemp + "/" + minTemp);
}
}

4. 測试, 创建不同的观察者(observer), 并把主题(subject)作为參数传入, 通知观察者.

代码:

/**
* @time 2014年5月22日
*/
package observer;

/**
* @author C.L.Wang
*
*/
public class WeatherStation {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
WeatherData weatherData = new WeatherData();

CurrentConditionsDisplay currentConditionsDisplay =
new CurrentConditionsDisplay(weatherData); //new的时候进行注冊

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);
}

}

5. 输出:

Current conditions: 80.0F degrees and 65.0% humidity
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Improving weather on the way!
Current conditions: 82.0F degrees and 70.0% humidity
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Watch out for cooler, rainy weather
Current conditions: 78.0F degrees and 90.0% humidity
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same

面向对象的原则:

为了设计对象和工作之间松散耦合的交互.