Improve the warning given when using channel before it's defined

[tmeasday]

Issue: #1514 fixed an issue where a "fake" channel that threw a warning broke RN. This left us without a warning

What I did

Instead, let's throw directly and check for that in the RN initializer.

How to test

Check RN-vanilla, plus general testing (CI + CRA-ks).

Note

This is targeted to #1514, we should change the target when that is merged.

Sometimes it's possible things grab the channel and don't access it, which might be a problem.

Another approach would just be to log a warning rather than throwing.

[ndelangen]

LGTM, ready to merge @tmeasday

[tmeasday]

Merge to master @ndelangen ?

[ndelangen]

Ow I didn't notice this isn't actually based on master.

In that case, we first need to rebase on master, or merge into it's current base, and proceed from there.

This particular change look good to me, I haven't seen #1509-fix-react-native-example

[tmeasday]

That's been merged I think. Let me mess with the PR

[codecov-io]

Codecov Report

Merging #1515 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1515      +/-   ##
==========================================
- Coverage   20.84%   20.82%   -0.02%     
==========================================
  Files         251      251              
  Lines        5575     5579       +4     
  Branches      668      669       +1     
==========================================
  Hits         1162     1162              
- Misses       3900     3903       +3     
- Partials      513      514       +1

Impacted Files	Coverage Δ
lib/addons/src/index.js	0% <0%> (ø)	⬆️
app/react-native/src/preview/index.js	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update afeebe0...983dd74. Read the comment docs.

[usulpro]

This causes that my tests fail because I do:
https://github.com/storybooks/storybook/blob/6e90ede42e6376a4eab16838e632e020a9be6bb2/addons/info/src/index.js#L9

[tmeasday]

@usulpro -- why does it fail? In what context? Storyshots?

[usulpro]

@tmeasday it fails if I run Jest test which imports this file. Actually I already have fixed this just by removing this line inside the not covered code

[tmeasday]

A Jest test -- is it storyshots or something else?

If something else perhaps it needs to setup a fake channel like SS does?

[usulpro]

I faced it in #1501 (addon-info)
But the easiest way to reproduce it -- just insert const channel = addons.getChannel(); in *.test.js and run Jest.
Actually, I agree - fake channel it the best and right way to deal with it. In my case, I don't test part of code related to channel since it's a very small piece of code so I left it uncovered. Maybe I should write a test for that later.

[tmeasday]

Yeah i'm not sure. I couldn't figure out a better way to warn people about the addons.getChannel() = undefined problem (which otherwise leads to a vanilla "can't access X on undefined" issue), and how they might solve it

Fake/mock channel seems pretty reasonable. With jest you can do it with one line I reckon.

[ndelangen]

With 4.0 I will change channel to be a Promise