public interface Subscription {
    /**
     * Stops the receipt of notifications on the {@link Subscriber} that was registered when this Subscription
     * was received.
     * <p>
     * This allows unregistering an {@link Subscriber} before it has finished receiving all events (i.e. before
     * onCompleted is called).
     */
    void unsubscribe();

    /**
     * Indicates whether this {@code Subscription} is currently unsubscribed.
     *
     * @return {@code true} if this {@code Subscription} is currently unsubscribed, {@code false} otherwise
     */
    boolean isUnsubscribed();
}

Observable.subscribe()方法可以返回一个Subscription的对象,即我们每次订阅都会返回.

• 只需要调用unsubscribe就可以取消订阅

• subscription.isUnsubscribed() 用于判断是否取消订阅

  protected void unsubscribe() {
      if (subscription != null && !subscription.isUnsubscribed()) {
          subscription.unsubscribe();
      }
  }