Implement Uni.onItem().ifNotNull()

[cescoffier]

This group checks if the item is not null. If the item is null default results are provided (null, empty Multi...)

This PR also contains various cosmetic fixes.

Related to #109

[kenfinnigan]

I must be not grokking as I don't get how this would pass.

uni has null and therefore will return yolo! on subscription.

So I expected that r would return YOLO! and not yolo!.

[cescoffier]

The uni produces null, so ifNotNull().apply(String::toUpperCase) is not called, and null is propagated downstream. As a consequence .onItem().ifNull().continueWith("yolo!") is called, yolo! is emitted.

Does that make sense?

[Ladicek]

But if you switch the 2 lines, it will become YOLO!, right?

(Because it's a pipeline, even though it really doesn't look like one.)

[cescoffier]

Exactly.

[kenfinnigan]

Ah, so are you saying onItem().ifNull() in the uni is never called because the previous line already consumed the onItem()?

If so, how would that line ever get executed?

[cescoffier]

Each line is a stage, so there is an item emitted on every line.

Uni.createFrom().item(() -> null); // emits `null`
  .onItem().ifNotNull().apply(String::toUpperCase) // skipped because the item is null, so emits (another) `null`
  .onItem().ifNull().continueWith("yolo!") // called because the received item is `null`, and emits "yolol!"

[kenfinnigan]

Maybe my confusion is that the code is actually duplicated? In that the same onItem() calls are added to the same uni which means it's a single chain of 4 possible outcomes, as opposed to 2 chains with 2 possible outcomes each.

Am I reading that right now?

[cescoffier]

So, yes the code is duplicated, as the first one (does nothing - no one subscribes) is just there for the snippet inclusion in the doc.

The second one is actually checking that it does the right thing.

[kenfinnigan]

If I'm understanding, you could remove lines 33 and 34 and just do uni.await() right, as the onItem() entries are still in the Uni?

[cescoffier]

No, every line returns a new Uni. So if line 27 we affect a new variable, and reuse it, then yes.