feat(ngTransclude & $compile): fix treatment of unfilled optional slots

[petebacondarwin]

Closes #13426

[petebacondarwin]

@PascalPrecht - hey look at this!

[0x-r4bbit]

@petebacondarwin awesome! Does this also keep the original contents of the DOM where we transclude into?

E.g.

<span ng-transclude="fooSlot">
This is default DOM and going to be replaced if `fooSlot` is defined, if not this stays here
</span>

[petebacondarwin]

Yep! That is the second commit in the PR.

[0x-r4bbit]

Argh. I just've just read the commit messages instead of reading the code haha. Awesome! Thanks for the work!

[petebacondarwin]

See https://ci.angularjs.org/job/angular.js-pete/989/artifact/build/docs/api/ng/directive/ngTransclude#multi-slot-transclusion
for a running example

[Narretz]

how about just else?

[petebacondarwin]

We have to differentiate between null and undefined. I will add a comment to clarify.

[Narretz]

Okay, I just noticed there's no difference in test behavior.

[Narretz]

feat($compile): expose transclusion $$slotson the$transclude`

Isn't the commit message a bit contradictory? It's called a feature but $$slots indicates a private API.

And a question, is it possible to keep the content with the default behavior (no slots, transclude is true or content)? Since people have requested something similar for the default ngTransclude: #11839
I think we could extend the 'content' value to make it optional by adding ? and applying the same behavior.

[petebacondarwin]

We could make $$slots -> $slots.

[petebacondarwin]

Regarding default content for the default transclusion, perhaps we could special case "" in the transclude map:

transclude: {
  'required': 'requiredSlot',
  'optional': '?optionalSlot',
  '': '?' // this would make the default transclusion optional too...
}

and if you wanted optional default transclusion only then you would put:

transclude: { '': '?' }

[Narretz]

Then we should also document $slots, I guess.

For default transchlusion. Okay, I forgot you can have default transclusion and slots. The proposed API for that looks a bit silly, but it's probably okay. But we can also decide on that after 1.5

[petebacondarwin]

Hmm it is a bit awkward to make the default transclusion optional. I think we should punt that feature to post-1.5.0

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[Narretz]

Looks like the last commit caused some doc e2e tests to fail. :o Other than that, feel free to merge. We can tweak it after the fact.