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

Question

我想知道是否有人可以阐明这个问题，何时使用

Single.fromCallable( ()-> myObject )

代替

Single.just(myObject)

从文档中，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.

 */

和文档Single.just()：

 /**

 * 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

在您提到的用例中，实际上没有重大区别。现在想象一下我们需要通过函数调用动态创建对象吗？fun getTimeObject() {&nbsp; &nbsp; val timeInMillis = System.currentTimeMillis()&nbsp; &nbsp; return TimeObject(timeInMillis)}然后，当它有一个新订阅者时，Single.just(getTimeObject())结果Single将发出相同的信息Long。然而，当它有一个新订阅者时Single.fromcallable(()-> getTimeObject())，结果Single将发出一个不同的Long指示以毫秒为单位的当前时间。那是因为fromCallable每当它有一个新订阅者Lazily 时，它就会执行它的 lambda 。

Answer 2

当你有一个类似的函数时，你应该使用fromCallable()MyObject myFunction() {&nbsp; &nbsp; // some login here&nbsp; &nbsp; return new MyObject();}然后你可以像这样从这个函数创建Single：Single.fromCallable(() -> myFunction());Single.just(myObject)只是在没有任何逻辑的情况下发出您的对象。因此，当您想要发出特定项目时，无需使用fromCallable()。