Stats Traffic: Optimize table view updates by integrating diffable data source

[staskus]

Description

Each time we update Stats data, we build a new tableViewModel() and reload all table view cells. It happens for each type of update we receive, which results in almost a dozen refreshes of table view cells happening in a short time. It can cause flickering of cell UI, or contribute to lag when scrolling.

Solution

Implemented native UITableViewDiffableDataSource for Traffic tab which would only update table view cells if data changes. In a gist, we only need to make cell models Hashable, and the system will take care of efficient updates:

1. Defined Snapshot and DataSource types which depend on StatsTrafficSection and StatsHashableImmuTableRow to integrate within existing ImmuTable
2. We have a different section for each PeriodType, so StatsTrafficSection only depends on it
3. We already define view models ImmuTableRow for cells, so StatsHashableImmuTableRow only depends on a Hashable version of ImmuTableRow
4. The diffable data source relies on both the identity and the equality of the items it manages. The identity is determined by the item's hash, and equality is determined by whether the item's content has changed. Row's identity depends on StatSection and on RowType. For example, periodAuthors section and StatsGhostTwoColumnImmutableRow row type.
5. Made all the rows we use within Traffic tab to conform to Hashable
6. Once it's done, we build Snapshot the same way we previously built ImmuTable view model. No changes in logic are necessary and we can follow a similar pattern
7. Made updates to SiteStatsPeriodTableViewController and ImmuTable to use a new DiffableDataSource and applySnapshot instead of calling tableView.reloadData

Now any cell is only reloaded when the data of this particular cell changes.

If these changes prove successful, I plan to apply the same changes to the Insights tab to avoid similar issues such as flickering their as well.

Video

Before

before.MP4

After (together with #22592)

after.MP4

Regression Notes

1. Potential unintended areas of impact

Days/Weeks/Months/Years tabs remain unaffected since they use SiteStatsPeriodTableViewControllerDeprecated.

The only unintended effect is implementing Hashable incorrectly. We want to make sure we identify each different period row uniquely but also detect when its content changes.

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

Manual testing

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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[dangermattic]

	1 Warning
⚠️	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

Generated by 🚫 Danger

[staskus]

Made the last little improvements, since I noticed a couple of issues after merging with the trunk:

1. Make sure AnyHashableImmuTableRow equality == is checked correctly, without involving hashValue. The working solution turned out to be wrapping ImmutableRow into AnyHashable and then comparing ==, without this type of erasure Swift doesn't allow to compare Hashable protocols for equality. To reiterate, all of this is required since hashValue is used to determine identity of a row while equality == is used to determine whether a row needs to be updated.
2. Wrote manual equality implementation for StatsTotalRowData. It took some time to realize that two objects that should be equal are not equal because UIImage object has changed although the image remained the same.

[guarani]

Thanks @staskus! The improvement in loading is noticeable! I couldn't spot any issues and don't have any feedback on the code.