观察者模式(Observer)

让你的对象知悉现况

使用自定义的Subject(主题)与Observer(观察者模式)

设计原则

找出程序中会变化的方面,然后将其和固定不变的部分分离

在观察者模式中,会改变的事主题的状态,以及观察者的数目和类型.用这个模式,你可以改变依赖于主题状态的对象,却不改变主题.这就叫提前规划

针对接口编程,不针对实现编程

主题与观察者都使用接口:观察者利用主题的接口向主题注册,二主题利用观察者接口通知观察者.这样可以让两者之前运作正常,同时具有松耦合的优点

多用组合,少用继承

观察者模式利用"组合"将许多观察者组合进主题中.对象之前的这种关系不是通过继承产生的,而是在运行时利用组合的方式而产生的.

自己实现观察者模式

我们先定义主题接口

/**
 * 主题接口
 */publicinterfaceSubject {
publicvoidregisterObserver(Observer o);
publicvoidremoveObserver(Observer o);
publicvoidnotifyObservers();

}

定义观察者接口

/**
 * 观察者接口
 */publicinterfaceObserver {
publicvoidupdate(float temp,float humidity,float pressure);
}

显示元素接口

/**
 * 显示元素接口
 */publicinterfaceDisplayElement {
publicvoiddisplay();
}

编写公告板实现,实现了观察者接口与显示元素接口

/**
 * 公告板实现
 */publicclassCurrentConditionDisplayimplementsObserver,DisplayElement {
privatefloat temperature;
privatefloat humidity;
private Subject weatherData;

/**
     * 构造器需要weatherData对象(也就是主题)作为注册之用
     * @param weatherData:天气对象
     */publicCurrentConditionDisplay(Subject weatherData){
this.weatherData=weatherData;
        weatherData.registerObserver(this);
    }

/**
     * display()方法就只是把最近的问的和湿度显示出来
     */@Override
publicvoiddisplay() {
        System.out.println("Current conditions:"+temperature+"F degree and "+humidity+"% humidity");
    }

/**
     * 当update被调用时,我们把温度和湿度保存起来,然后调用display
     * @param temp
     * @param humidity
     * @param pressure
     */@Override
publicvoidupdate(float temp,float humidity,float pressure) {
this.temperature=temp;
this.humidity=humidity;
            display();
    }
}

天气数据实现主题接口

import java.util.ArrayList;

/**
 * 天气数据类实现了Subject(主题)接口
 */publicclassWeatherDataimplementsSubject {
private ArrayList<Observer> observers;
privatefloat temperature;
privatefloat humidity;
privatefloat pressure;

publicWeatherData(){
        observers=new ArrayList<>();
    }

    @Override
publicvoidregisterObserver(Observer o) {
        observers.add(o);
    }

    @Override
publicvoidremoveObserver(Observer o) {
int i=observers.indexOf(o);
if(i>=0){
            observers.remove(i);
        }
    }

    @Override
publicvoidnotifyObservers() {
for (Observer observer : observers) {
            observer.update(temperature, humidity, pressure);
        }
    }

/**
     * 此方法会在气象值变化时被调用
     */publicvoidmeasurementsChanged(){
        notifyObservers();
    }

publicvoidsetMeasurements(float temperature,float humidity,float pressure){
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
        measurementsChanged();
    }
}

来个测试吧

publicclass WeatherStation {
publicstaticvoidmain(String[] args){
        WeatherData weatherData=new WeatherData();

        CurrentConditionDisplay currentConditionDisplay=new CurrentConditionDisplay(weatherData);
        weatherData.setMeasurements(80,65,30.4f);
    }
}

#输出
Current conditions:80.0F degree and 65.0% humidity

使用java自带的Observer

定义显示元素接口

package Observable;

publicinterfaceDisplayElement {
publicvoiddisplay();
}

实现观察者接口和显示元素接口

package Observable;

import java.util.Observable;
import java.util.Observer;

/**
 * 天气状况布告板
 * Created by jimersylee on
 */publicclassCurrentConditionDisplayimplementsObserver,DisplayElement {
    Observable observable;
privatefloat temperature;
privatefloat humidity;

publicCurrentConditionDisplay(Observable observable){
this.observable=observable;
        observable.addObserver(this);
    }

publicvoidupdate(Observable obs,Object arg){
if(obsinstanceof WeatherData){
            WeatherData weatherData=(WeatherData)obs;
this.temperature=weatherData.getTemperature();
this.humidity=weatherData.getHumidity();
            display();
        }
    }

publicvoiddisplay(){
        System.out.println("Current conditions:"+temperature+"F degrees and "+humidity+"% humidity");
    }

}

实现观察者抽象类

package Observable;

import java.util.Observable;

/**
 * 使用java.util内置的观察者模式实现
 * Created by jimersylee
 */publicclassWeatherDataextendsObservable {
privatefloat temperature;
privatefloat humidity;
privatefloat pressure;

publicWeatherData(){

    }

publicvoidmeasurementsChanged(){
        setChanged();
        notifyObservers();
    }

publicfloatgetTemperature(){
return temperature;
    }

publicfloatgetHumidity(){
return humidity;
    }

publicfloatgetPressure(){
return pressure;
    }

publicvoidsetMeasurements(float temperature,float humidity,float pressure){
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
        measurementsChanged();
    }

}

写个测试吧

package Observable;

publicclass WeatherStation {
publicstaticvoidmain(String[] args){
        WeatherData weatherData=new WeatherData();

        CurrentConditionDisplay currentConditionDisplay=new CurrentConditionDisplay(weatherData);

        weatherData.setMeasurements(80,30,33.2f);
    }
}

#输出
Current conditions:80.0F degrees and 30.0% humidity

项目地址

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