Make section diffingKey parameter required

[ronaldsmartin]

Changes in this pull request

Issue fixed: n/a

As discussed with @benasher44 and @noahsark769, this updates the initializers for TableSectionViewModel and CollectionSectionViewModel so that the diffingKey argument is required.

This prevents accidental misuse of the automatic diffing API, which is possible if you rely on the current default parameter value. 😅

No new behavior here, just a breaking API change. Tests are updated accordingly (no new ones).

Note on the parameter positioning

Previously diffingKey was the last parameter in each initializer. I chose to make it the first in the argument list:

• The Swift API design guidelines tell us:

Prefer to locate parameters with defaults toward the end of the parameter list

• If we keep diffingKey close to the bottom as the last required parameter, it divides the required argument for the collection/table view models from the optional arguments for header and footer like so 🤮

TableSectionViewModel(
    cellViewModels: ...,
    diffingKey: nil,
    headerViewModel: ...,
    footerViewModel: ...
)

I like it at the beginning better because it kind of looks like a section name. Open to discussion if there are any objections, though!

Checklist

• All tests pass. Demo project builds and runs.
• I added tests, an experiment, or detailed why my change isn't tested.
• I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
• I have reviewed the contributing guide

[jessesquires]

👍