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！\n" + "您收到了一个来自" + gper.getName() + "的提问，希望您解答。问题内容如下：\n" + question.getContent() + "\n" + "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==========" + "\n" + e); } public void onDoubleClick(Event e){ System.out.println("===========触发鼠标双击事件==========" + "\n" + e); } public void onUp(Event e){ System.out.println("===========触发鼠标弹起事件==========" + "\n" + e); } public void onDown(Event e){ System.out.println("===========触发鼠标按下事件==========" + "\n" + e); } public void onMove(Event e){ System.out.println("===========触发鼠标移动事件==========" + "\n" + e); } public void onWheel(Event e){ System.out.println("===========触发鼠标滚动事件==========" + "\n" + e); } public void onOver(Event e){ System.out.println("===========触发鼠标悬停事件==========" + "\n" + e); } public void onBlur(Event e){ System.out.println("===========触发鼠标失去焦点事件==========" + "\n" + e); } public void onFocus(Event e){ System.out.println("===========触发鼠标获得焦点事件==========" + "\n" + 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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