设计模式-行为型模式-观察者模式

观察者模式

观察者模式有时也被称为发布/订阅模式，该模式定义了一种一对多的依赖关系，可以让多个观察者对象同时监听某一个主题对象，当主题对象的状态发生变化时，通知所有观察者对象，使它们能够自动更新自己。例如某个Text文本框的text属性绑定到X数据源上，当X数据源发生更改时自动同步更新Text的值。被观察者要维护一个观察者的List，逐个通知可能存在性能的问题（比如List很长，或者update的逻辑耗时等）。

//观察者接口
public interface IWatcher {
    public void update(String str);
}

//被观察者接口
public interface IWatched {
    public void addWatcher(IWatcher watcher);
    public void removeWatcher(IWatcher watcher);
    public void notifyWatchers(String str);
}

//具体的观察者
public class ConcreteWatcher implements IWatcher {

    @Override
    public void update(String str) {
        System.out.println(str);
    }
}

//具体的被观察者
public class ConcreteWatched implements IWatched {
    private List<IWatcher> watchers = new ArrayList<>();

    @Override
    public void addWatcher(IWatcher watcher) {
        watchers.add(watcher);
    }

    @Override
    public void removeWatcher(IWatcher watcher) {
        watchers.remove(watcher);
    }

    @Override
    public void notifyWatchers(String str) {
        
　　　　// 通知观察者
　　　　 for (IWatcher iWatcher : watchers) {
            iWatcher.update(str);
        }
    }
}

package ObserverPattern;

public class Main {
    public static void executeObserverPattern() {
       System.out.println("Observer Pattern:");
       IWatched watched = new ConcreteWatched();

       IWatcher watcher1 = new ConcreteWatcher();
       IWatcher watcher2 = new ConcreteWatcher();
       ConcreteWatcher watcher3 = new ConcreteWatcher();

       watched.addWatcher(watcher1);
       watched.addWatcher(watcher2);
       watched.addWatcher(watcher3);

       watched.notifyWatchers("Hello World!!!");
    }
}