Reader: Unsubscribe from Post object changes when toolbar disappears to avoid crashes

[staskus]

Fixes #22116

ReaderDetailToolbar can remain active in one tab while changes to the Post object can happen in another tab. To avoid issues, unsubscribe to Post changes when the view disappears.

I could not reproduce the crash but based on the logs the crash would happen during logout when accessing likesCount in ReaderDetailToolbar. It looks like Post observer is triggered during the logout process and the app crashes when we try to access no longer valid Post object. It happens because we remove Post observer in ReaderDetailToolbar.deinit() which isn't called when we switch from Reader tab to Me tab.

Solution

Connect Post observing to ReaderDetailViewController life cycle.

The other option I considered is moving all the observation logic to ReaderDetailViewController and then call specific methods of ReaderDetailToolbar to update the view. Do you think that would be preferable?

To test:

Open Reader Details, and then switch tabs, for example:

1. Open Reader
2. Search for a site
3. Select site
4. Select post
5. Switch to another tab
6. Confirm unsubscribePostChanges is called
7. Come back to Reader tab
8. Confirm subscribePostChanges is called

Regression Notes

1. Potential unintended areas of impact

When Reader Detail view disappears, we stop observing likes and comments. If that wasn't intentional, then it could be an indented area of impact.

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist:

• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

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📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

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[dvdchr]

I haven't been able to reproduce the crash so far, but I've tested the changes from this PR and didn't notice any issues with the expected flow. LGTM!

Thanks for tackling this, @staskus ! 🙇🏼‍♂️