java - RxJava Single.just() vs Single.fromCallable()？

Question

我想知道是否有人可以阐明这个问题，什么时候使用

Single.fromCallable( ()-> myObject )

代替

Single.just(myObject)

---

根据文档，Single.fromCallable():

 /**
 * Returns a {@link Single} that invokes passed function and emits its result for each new SingleObserver that subscribes.
 * <p>
 * Allows you to defer execution of passed function until SingleObserver subscribes to the {@link Single}.
 * It makes passed function "lazy".
 * Result of the function invocation will be emitted by the {@link Single}.
 * <dl>
 *   <dt><b>Scheduler:</b></dt>
 *   <dd>{@code fromCallable} does not operate by default on a particular {@link Scheduler}.</dd>
 * </dl>
 *
 * @param callable
 *         function which execution should be deferred, it will be invoked when SingleObserver will subscribe to the {@link Single}.
 * @param <T>
 *         the type of the item emitted by the {@link Single}.
 * @return a {@link Single} whose {@link SingleObserver}s' subscriptions trigger an invocation of the given function.
 */

和 Single.just() 的文档:

 /**
 * Returns a {@code Single} that emits a specified item.
 * <p>
 * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.just.png" alt="">
 * <p>
 * To convert any object into a {@code Single} that emits that object, pass that object into the
 * {@code just} method.
 * <dl>
 * <dt><b>Scheduler:</b></dt>
 * <dd>{@code just} does not operate by default on a particular {@link Scheduler}.</dd>
 * </dl>
 *
 * @param item
 *            the item to emit
 * @param <T>
 *            the type of that item
 * @return a {@code Single} that emits {@code item}
 * @see <a href="http://reactivex.io/documentation/operators/just.html">ReactiveX operators documentation: Just</a>
 */

Answer 1

通常，当您发射的东西不仅是一个对象，而且实际上是一些涉及繁重计算、I/O 或状态的方法调用的结果时，您会注意到不同之处。

Single.just(x) 立即在当前线程中评估 x，然后您将得到 x 的结果>，适用于所有订阅者。

Single.fromCallable(y) 在订阅时分别为每个订阅者调用 subscribeOn 调度器中的 y 可调用函数。

---

例如，如果您想将 I/O 操作卸载到后台线程，您可以使用

Single.fromCallable(() -> someIoOperation()).
    subscribeOn(Schedulers.io()).
    observeOn(AndroidSchedulers.mainThread()).
    subscribe(value -> updateUi(value), error -> handleError(error));

在这里使用 Single.just() 是行不通的，因为 someIoOperation() 将在当前线程上执行。