Java设计模式-监听器模式

监听器模式有三个要素——事件源、事件对象、监听器。

事件源:顾名思义,事件发生的源头,比如点击的按钮,属于被监听的对象;
事件对象:这个经常和事件源混淆,它经常被用来包装事件源,切记,它毕竟是个事件,比如点击事件,和事件源的区别自己感受,木有栗子;
监听器:这个是监听器模式的核心,定义事件发生后的动作,通常事件对象作为监听器中定义的函数入参。

下面举个简单的栗子:
故事背景是,小明是个不讲卫生的孩子,他妈妈很担心他的健康,规定必须饭前洗手。
定义一个熊孩子。熊孩子就是被监听的对象,是事件源,一切事件都是事件源发出,这似乎是句废话。

public class Child {
    private String name;
    private RemindListener remindListener;
 
    public Child(String name){
        this.name = name;
    }
    public void eat() {
        if(null!=remindListener){
            remindListener.remind(new RemindWashingHandsEvent(this));
        }
        System.out.println("Child eat...");
    }
 
    public void addListener(RemindListener listener){
        remindListener = listener;
    }
}

接下来是看看事件对象,事件对象正如上面所述,包装了事件源。我们在这里定义一个饭前洗手事件。

public class RemindWashingHandsEvent {
    private Child child;
 
    public RemindWashingHandsEvent(Child child){
        this.child = child;
    }
}

事件对象定义了事件的属性、状态。
紧接着是定义事件发生后,监听器的动作,在这里是提醒洗手。

public class RemindListener {
    public void remind(RemindWashingHandsEvent remindWashingHandsEvent){
        System.out.println("listen to mom, washing hands before eating...");
    }
}

注意,监听器主要封装了动作,仅此而已。
以上代码,只是为了说明监听器模式原理,代码通俗,不太优雅。
下面继承或实现java标准库,又随手写了一对代码,夜深了,有时间再解释。

public class Kid{
    private String name;
    private List liteners;
 
    public Kid(String name) {
        this.name = name;
        this.liteners = Lists.newArrayList();
    }
 
    public void eat(){
 
        for(Listener listener:liteners){
            if(listener instanceof WashingHandsListener){
                WashingHandsListener washingHandsListener = (WashingHandsListener) listener;
                washingHandsListener.fireAfterEventInvoked(new WashingHandsEvent(this,"洗手"));
            }
        }
        System.out.println("吃饭...");
    }
 
    public void addListener(Listener listener){
        liteners.add(listener);
    }
     
}
public class Event extends EventObject {
    /**
     * Constructs a prototypical Event.
     *
     * @param source The object on which the Event initially occurred.
     * @throws IllegalArgumentException if source is null.
     */
    public Event(Object source) {
        super(source);
    }
}
public class WashingHandsEvent extends Event{
    private String eventName;
    /**
     * Constructs a prototypical Event.
     *
     * @param source The object on which the Event initially occurred.
     * @throws IllegalArgumentException if source is null.
     */
    public WashingHandsEvent(Object source,String eventName) {
        super(source);
        this.eventName = eventName;
    }
 
    public String getEventName() {
        return eventName;
    }
 
    public void setEventName(String eventName) {
        this.eventName = eventName;
    }
}
public interface Listener extends java.util.EventListener{
    public void fireAfterEventInvoked(Event event);
}
public class WashingHandsListener implements Listener{
    @Override
    public void fireAfterEventInvoked(Event event) {
        WashingHandsEvent washingHandsEvent = (WashingHandsEvent) event;
        System.out.println("饭前准备"+ washingHandsEvent.getEventName());
    }
}
public class Test {
    public static void main(String[] args) {
        Kid xiaoming = new Kid("xiaoming");
        xiaoming.addListener(new WashingHandsListener());
        xiaoming.eat();
    }
}

输出结果:

image

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