fix: messages list IndexOutOfBoundsException [WPB-5612]

[saleniuk]

WPB-5612 java.lang.IndexOutOfBoundsException:

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PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

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What's new in this PR?

Issues

We were getting IndexOutOfBoundsException when opening conversation screen.

Causes (Optional)

shouldHaveSmallBottomPadding didn't have any limit, only index > 0, but index could be higher than the items count and that's what was happening, because lazyPagingMessages.itemSnapshotList.items is a list of only presented data, excluding placeholders. It was working fine when the app was always fetching messages starting from the most recent one, but in case the user has multiple unread messages or opens the conversation from the message search, the app may fetch not the last page but can start from the middle and then indexes don't match.

For instance, if we have 100 total messages, page size is 20 and we have 30 unread messages, then the initially loaded batch should have first unread message, so it should contain for instance messages 20 to 39. ItemSnapshotList will have size of 100 (all available messages) and these fetched already elements will be at their proper indexes 20 to 39, all other elements are placeholders and they will be null, but ItemSnapshotList.items only contains these 20 non-null elements with indexes starting from 0 to 19, because it's just a sublist, and we can easily have IndexOutOfBounds when we want to get item with index 30 from that sublist, and items don't match - itemSnapshotList.items[10] is not the same item as itemSnapshotList[10] so the list could look wrong.

Solutions

Get elements using their indexes from the itemSnapshotList instead of itemSnapshotList.items, use derivedStateOf to reduce number of recompositions and remember with keys to update value of shouldHaveSmallBottomPadding or shouldShowHeader when a neighbouring element is no longer a placeholder.
The whole list was also recomposing when scrolling, so it could be reduced as well by creating dedicated states for isScrollInProgress and mostRecentMessage using derivedStateOf.

Testing

Test Coverage (Optional)

• I have added automated test to this contribution

How to Test

Have a lot of unread messages, like more than 50, and open conversation - it shouldn't crash and each message should look properly (without avatar, header and with small paddings in between if they are grouped messages from the same user).

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PR Post Submission Checklist for internal contributors (Optional)

• Wire's Github Workflow has automatically linked the PR to a JIRA issue

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PR Post Merge Checklist for internal contributors

• If any soft of configuration variable was introduced by this PR, it has been added to the relevant documents and the CI jobs have been updated.

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References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[github-actions]

Test Results

691 tests   691 ✔️  9m 37s ⏱️
  98 suites      0 💤
  98 files        0 ❌

Results for commit d3a5a91.

♻️ This comment has been updated with latest results.

[github-actions]

APKs built during tests are available here. Scroll down to Artifacts!

[AndroidBob]

Build 1952 succeeded.

The build produced the following APK's:

• com.wire.android-v4.5.0(8176789)(8176789)-staging-compat.apk

[yamilmedina]

Awesome test coverage ! 👏