Add findChildIndexCallback examples

[gildaswise]

The documentation for using findChildIndexCallback recommends using indexOf, but that causes this line to throw in debug mode, and when using SliverList, it breaks the render.

This PR changes the usage to check if the index is not negative before using it, and changes to return null instead if the child wasn't able to be found.

There's the related issue #107123, but this doesn't actually fix it.

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This PR has been updated to add the snippets that were used in the findChildIndexCallback comment as examples with proper tests, as well as updating the comment to reference the new examples.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
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[Piinks]

Hi @gildaswise thanks for contributing! It looks like we should fix the issue with indexOf returning invalid indices, rather than changing our samples to reflect a workaround. What do you think?

[gildaswise]

Hey Kate, thanks for reviewing!

It looks like we should fix the issue with indexOf returning invalid indices

I didn't understand if there's something to be fixed. The findChildIndexCallback expects a nullable index as a return, and unless we change the processing to ignore negative indexes, I believe the correct usage should still be checking for the return of indexOf before returning in the callback, since it should support not finding the index.

[gildaswise]

I just noticed that the findChildIndexCallback says this:

This callback should take an input [Key], and it should return the index of the child element with that associated key, or null if not found.

So indeed the recommended callback in the documentation was wrong, it's not supposed to return -1 if not found, but null as I changed.

[Piinks]

Ah thank you for explaining. I see what you mean.

I think we should likely change these snippets to be dartpad examples. That way we can write a test for each to ensure this works as expected and does not regress in the future. Those examples live in examples/api, and are embedded using {@tool dartpad} instead of {@tool snippet}.

[gildaswise]

I think we should likely change these snippets to be dartpad examples.

I can try to do this in this PR then, will look into it once I have time this week. Thank you!

[goderbauer]

(triage): @gildaswise Do you still have plans to continue working on this PR in the near future?

[gildaswise]

@goderbauer Sorry about the delay, been a rough few weeks for me. I'll try to add the dartpad examples this week for sure, and if I can't, I'll ping you here

[Piinks]

Thanks so much for contributing this! Converting these to dartpads make them even more helpful as interactive examples, plus the tests you added ensure they aren't accidentally broken in the future. :)

[gildaswise]

Thanks so much for contributing this! Converting these to dartpads make them even more helpful as interactive examples, plus the tests you added ensure they aren't accidentally broken in the future. :)

Thank you for the review, Kate! I can't merge it myself but the CI passed now, please let me know if I need to do something else.

[Piinks]

Just waiting on a second review for this to land, I will reach out again to see if we can get one. :)

[cbracken]

nit: add a period at the end of the comment and remove the blank line so it's snug against main.

Style guide ref: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#use-correct-grammar

[cbracken]

Nit: Add a period at the end of the comment.

Style guide ref: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#use-correct-grammar