RxJava Single.just（）与Single.fromCallable（）吗？
问题内容：
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.
*/
以及以下文件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>
*/
问题答案：
在您提到的用例中，实际上没有重大区别。
现在想象我们需要通过函数调用动态创建对象吗？
fun getTimeObject() {
val timeInMillis = System.currentTimeMillis()
return TimeObject(timeInMillis)
}
然后，Single.just(getTimeObject())结果具有新订户时Single将发出相同的结果Long。
但是，使用时Single.fromcallable(()-> getTimeObject())，结果Single将具有一个不同的Long值，表示当它有一个新的订户时，以毫秒为单位的当前时间。
这是因为fromCallable每次有新订阅者 Lazily时都要 执行lambda 。
