基于Guava API实现异步通知和事件回调

本文节选自《设计模式就该这样学》

1　基于Java API实现通知机制

当小伙伴们在社区提问时，如果有设置指定用户回答，则对应的用户就会收到邮件通知，这就是观察者模式的一种应用场景。有些小伙伴可能会想到MQ、异步队列等，其实JDK本身就提供这样的API。我们用代码来还原这样一个应用场景，首先创建GPer类。

/**
 * JDK提供的一种观察者的实现方式，被观察者
 */
public class GPer extends Observable{
    private String name = "GPer生态圈";
    private static GPer gper = null;
    private GPer(){}

    public static GPer getInstance(){
        if(null == gper){
            gper = new GPer();
        }
        return gper;
    }
    public String getName() {
        return name;
    }
    public void publishQuestion(Question question){
        System.out.println(question.getUserName() + "在" + this.name + "上提交了一个问题。");
        setChanged();
        notifyObservers(question);
    }
}

然后创建问题Question类。

public class Question {
    private String userName;
    private String content;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }
}

接着创建老师Teacher类。

public class Teacher implements Observer {

    private String name;

    public Teacher(String name) {
        this.name = name;
    }

    public void update(Observable o, Object arg) {
        GPer gper = (GPer)o;
        Question question = (Question)arg;
        System.out.println("======================");
        System.out.println(name + "老师，你好！
" + 
"您收到了一个来自" + gper.getName() + "的提问，希望您解答。问题内容如下：
" +
                   question.getContent() + "
" + "提问者：" + question.getUserName());
    }
}

最后编写客户端测试代码。

    public static void main(String[] args) {
        GPer gper = GPer.getInstance();
        Teacher tom = new Teacher("Tom");
        Teacher jerry = new Teacher("Jerry");

        gper.addObserver(tom);
        gper.addObserver(jerry);

        //用户行为
        Question question = new Question();
        question.setUserName("张三");
        question.setContent("观察者模式适用于哪些场景？");

        gper.publishQuestion(question);
}

运行结果如下图所示。

2　基于Guava API轻松落地观察者模式

笔者向大家推荐一个实现观察者模式的非常好用的框架，API使用也非常简单，举个例子，首先引入Maven依赖包。

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>20.0</version>
</dependency>

然后创建侦听事件GuavaEvent。

/**
 * Created by Tom
 */
public class GuavaEvent {
    @Subscribe
    public void subscribe(String str){
        //业务逻辑
        System.out.println("执行subscribe方法,传入的参数是:" + str);
    }

}

最后编写客户端测试代码。

/**
 * Created by Tom
 */
public class GuavaEventTest {
    public static void main(String[] args) {
        EventBus eventbus = new EventBus();
        GuavaEvent guavaEvent = new GuavaEvent();
        eventbus.register(guavaEvent);
        eventbus.post("Tom");
    }

}

3　使用观察者模式设计鼠标事件响应API

再来设计一个业务场景，帮助小伙伴们更好地理解观察者模式。在JDK源码中，观察者模式的应用也非常多。例如java.awt.Event就是观察者模式的一种，只不过Java很少被用来写桌面程序。我们用代码来实现一下，以帮助小伙伴们更深刻地了解观察者模式的实现原理。首先，创建EventListener接口。

/**
 * 观察者抽象
 * Created by Tom.
 */
public interface EventListener {

}

创建Event类。

/**
 * 标准事件源格式的定义
 * Created by Tom.
 */
public class Event {
    //事件源，动作是由谁发出的
    private Object source;
    //事件触发，要通知谁（观察者）
    private EventListener target;
    //观察者的回应
    private Method callback;
    //事件的名称
    private String trigger;
    //事件的触发事件
    private long time;

    public Event(EventListener target, Method callback) {
        this.target = target;
        this.callback = callback;
    }

    public Object getSource() {
        return source;
    }

    public Event setSource(Object source) {
        this.source = source;
        return this;
    }

    public String getTrigger() {
        return trigger;
    }

    public Event setTrigger(String trigger) {
        this.trigger = trigger;
        return this;
    }

    public long getTime() {
        return time;
    }

    public Event setTime(long time) {
        this.time = time;
        return this;
    }

    public Method getCallback() {
        return callback;
    }

    public EventListener getTarget() {
        return target;
    }

    @Override
    public String toString() {
        return "Event{" +
                "source=" + source +
                ", target=" + target +
                ", callback=" + callback +
                ", trigger="" + trigger + """ +
                ", time=" + time +
                "}";
    }
}

创建EventContext类。

/**
 * 被观察者的抽象
 * Created by Tom.
 */
public abstract class EventContext {
    protected Map<String,Event> events = new HashMap<String,Event>();

    public void addListener(String eventType, EventListener target, Method callback){
        events.put(eventType,new Event(target,callback));
    }

    public void addListener(String eventType, EventListener target){
        try {
            this.addListener(eventType, target, 
target.getClass().getMethod("on"+toUpperFirstCase(eventType), Event.class));
        }catch (NoSuchMethodException e){
            return;
        }
    }

    private String toUpperFirstCase(String eventType) {
        char [] chars = eventType.toCharArray();
        chars[0] -= 32;
        return String.valueOf(chars);
    }

    private void trigger(Event event){
        event.setSource(this);
        event.setTime(System.currentTimeMillis());

        try {
            if (event.getCallback() != null) {
                //用反射调用回调函数
                event.getCallback().invoke(event.getTarget(), event);
            }
        }catch (Exception e){
            e.printStackTrace();
        }
    }

    protected void trigger(String trigger){
        if(!this.events.containsKey(trigger)){return;}
        trigger(this.events.get(trigger).setTrigger(trigger));
    }
}

然后创建MouseEventType接口。

/**
 * Created by Tom.
 */
public interface MouseEventType {
    //单击
    String ON_CLICK = "click";

    //双击
    String ON_DOUBLE_CLICK = "doubleClick";

    //弹起
    String ON_UP = "up";

    //按下
    String ON_DOWN = "down";

    //移动
    String ON_MOVE = "move";

    //滚动
    String ON_WHEEL = "wheel";

    //悬停
    String ON_OVER = "over";

    //失去焦点
    String ON_BLUR = "blur";

    //获得焦点
    String ON_FOCUS = "focus";
}

创建Mouse类。

/**
 * 具体的被观察者
 * Created by Tom.
 */
public class Mouse extends EventContext {

    public void click(){
        System.out.println("调用单击方法");
        this.trigger(MouseEventType.ON_CLICK);
    }

    public void doubleClick(){
        System.out.println("调用双击方法");
        this.trigger(MouseEventType.ON_DOUBLE_CLICK);
    }

    public void up(){
        System.out.println("调用弹起方法");
        this.trigger(MouseEventType.ON_UP);
    }

    public void down(){
        System.out.println("调用按下方法");
        this.trigger(MouseEventType.ON_DOWN);
    }

    public void move(){
        System.out.println("调用移动方法");
        this.trigger(MouseEventType.ON_MOVE);
    }

    public void wheel(){
        System.out.println("调用滚动方法");
        this.trigger(MouseEventType.ON_WHEEL);
    }

    public void over(){
        System.out.println("调用悬停方法");
        this.trigger(MouseEventType.ON_OVER);
    }

    public void blur(){
        System.out.println("调用获得焦点方法");
        this.trigger(MouseEventType.ON_BLUR);
    }

    public void focus(){
        System.out.println("调用失去焦点方法");
        this.trigger(MouseEventType.ON_FOCUS);
    }
}

创建回调方法MouseEventLisenter类。

/**
 * 观察者
 * Created by Tom.
 */
public class MouseEventListener implements EventListener {


    public void onClick(Event e){
        System.out.println("===========触发鼠标单击事件==========" + "
" + e);
    }

    public void onDoubleClick(Event e){
        System.out.println("===========触发鼠标双击事件==========" + "
" + e);
    }

    public void onUp(Event e){
        System.out.println("===========触发鼠标弹起事件==========" + "
" + e);
    }

    public void onDown(Event e){
        System.out.println("===========触发鼠标按下事件==========" + "
" + e);
    }

    public void onMove(Event e){
        System.out.println("===========触发鼠标移动事件==========" + "
" + e);
    }

    public void onWheel(Event e){
        System.out.println("===========触发鼠标滚动事件==========" + "
" + e);
    }

    public void onOver(Event e){
        System.out.println("===========触发鼠标悬停事件==========" + "
" + e);
    }

    public void onBlur(Event e){
        System.out.println("===========触发鼠标失去焦点事件==========" + "
" + e);
    }

    public void onFocus(Event e){
        System.out.println("===========触发鼠标获得焦点事件==========" + "
" + e);
    }

}

最后编写客户端测试代码。

    public static void main(String[] args) {
        EventListener listener = new MouseEventListener();

        Mouse mouse = new Mouse();
        mouse.addListener(MouseEventType.ON_CLICK,listener);
        mouse.addListener(MouseEventType.ON_MOVE,listener);

        mouse.click();
        mouse.move();
    }
		

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