RxJava操作符——just

使用

如下代碼，Observable.just()方法創建一個Observable對象

調用subscribe()方法會發出一個包含“野猿新一”內容的事件

通過Action1.call()的回調我們將事件的內容打印出來

Observable.just("野猿新一")
        .subscribe(new Action1<String>() {
            @Override
            public void call(String s) {
                Log.d("Himmy", s);
            }
        });

源碼解析

RxJva現在已經更新到了3.0.0，整個項目的代碼量和複雜度都增加不少，爲了降低理解難度，本文解析的是RxJava1.0.0的源碼，其核心原理是一樣的

由如下源碼可知，just()方法中通過ScalarSynchronousObservable.create創建了一個Observable對象

public final static <T> Observable<T> just(final T value) {
    return ScalarSynchronousObservable.create(value);
}

 再看下ScalarSynchronousObservable這個類，其實是個Observable的子類，ScalarSynchronousObservable()方法內部其實是通過OnSubscribe來創建Observable

public final class ScalarSynchronousObservable<T> extends Observable<T> {

    public static final <T> ScalarSynchronousObservable<T> create(T t) {
        return new ScalarSynchronousObservable<T>(t);
    }

    private final T t;

    protected ScalarSynchronousObservable(final T t) {
        super(new OnSubscribe<T>() {

            @Override
            public void call(Subscriber<? super T> s) {
                /*
                 *  We don't check isUnsubscribed as it is a significant performance impact in the fast-path use cases.
                 *  See PerfBaseline tests and https://github.com/ReactiveX/RxJava/issues/1383 for more information.
                 *  The assumption here is that when asking for a single item we should emit it and not concern ourselves with 
                 *  being unsubscribed already. If the Subscriber unsubscribes at 0, they shouldn't have subscribed, or it will 
                 *  filter it out (such as take(0)). This prevents us from paying the price on every subscription. 
                 */
                s.onNext(t);
                s.onCompleted();
            }

        });
        this.t = t;
    }

    public T get() {
        return t;
    }

}

所以如下兩個方法創建的Observable結果是一樣的，只是just方法幫我們簡化了代碼

// 通過Observable.just創建
Observable observable1 = Observable.just("野猿新一");
// 通過Observable.create和OnSubscribe創建
Observable observable2 = Observable.create(new Observable.OnSubscribe<String>() {
    @Override
    public void call(Subscriber<? super String> subscriber) {
        subscriber.onNext("野猿新一");
        subscriber.onCompleted();
    }
});