最簡單的觀察者模式

//被觀察者
public class Observable {
    public interface Listener {
        void call();
    }

    Observable.Listener mListener = null;

    public static Observable create(Listener listener) {
        mListener = listener;
    }

    public void someMethod() {
        mListener.call();
    }
}

//創建被觀察者對象, 傳入Observable.Listener對象
Observable observable = Observable.create(new Observable.Listener() {

    @override
    void call() {
      //do something
   }

抽象出來一個觀察者接口(Subscriber), 在調用被觀察者的方法時被作為參數回調

public class Observable {

    public interface onSubscribe {
        void call(Subscriber subscriber);
    }

    Observable.onSubscribe mOnSubscribe = null;

    public static Observable create(onSubscribe pOnSubscribe) {
        mOnSubscribe = pOnSubscribe;
    }

    public void subscribe(Subscriber subscribe) {
        mOnSubscribe.call(subscribe);  //把subscribe作為call方法的參數回調
    }
}

//觀察者
public interface Subscriber {
    void onNext(String string);
}

Observable observable = Observable.create(new Observable.onSubscribe() {

    @override
    void call(Subscriber subscriber) {
        //do something
        subscriber.next("1");
        subscriber.next("2");
        subscriber.next("3");
    }
});

這時如果新建一個Subscriber對象, 并將它傳參到observable對象的subscribe方法中:

Subscriber subscriber = new Subscriber() {
    @override
    void onNext(String string) {
      // 打印string
   }
};
observable.subscribe(subscriber);

//subscriber對象打印出
"1"
"2"
"3"

將代碼連貫起來

Observable.create(new Observable.onSubscribe() {
    @override
    void call(Subscriber subscriber) {
        //do something
        subscriber.next("1");
        subscriber.next("2");
        subscriber.next("3");
    }
}).subscribe(new Subscriber() {
    @override
    void onNext(String string) {
      // 打印string
   }
});

RxJava提供了一些Observable的創建方法, 不需要實現Observable.onSubscribe接口

Observable.just(1, 2, 3, 4) 
.subscribe(new Action1<Integer>() {
   @Override public void call(Integer number) {
       Log.d(tag, "number:" + number);
   }
 });

其中, 在RxJava中Subscriber是Observer的子類