DateTimePicker: Replace react-dates and moment with useLilius and date-fns

[noisysocks]

What?

Follows https://github.com/WordPress/gutenberg/pull/41385.

Updates DatePicker to use useLilius instead of react-dates to render its calendar and removes moment as a dependency from @wordpress/components in favour of date-fns.

https://github.com/its-danny/use-lilius https://github.com/date-fns/date-fns

Why?

Problems with react-dates

• It contains lots of different components, most of which we do not use. We've had problems with tree shaking in the past.
• It uses moment which is quite large and no longer actively developed.
• It requires hacks.
  • We have to use useEffect to set an aria-label on the day button.
  • We have to do weird stuff to keep focus within the calendar when navigating between months.
  • We have to force re-render by remounting the component using key when the date is changed. (This makes the whole thing feel really slow.)
• Limited support for using @wordpress/components components. Many of the UI elements can't be swapped out with @wordpress/components components. This results in a less consistent UI and more CSS shipped to the browser.

Considering react-calendar

I looked at using react-calendar in https://github.com/WordPress/gutenberg/pull/41385. This is a smaller alternative to react-dates that uses date-fns.

Pros:

• Much fewer bytes.
• No hacks required.
• Uses date-fns, allowing us to remove moment as a dependency.

Cons:

• Limited support for using @wordpress/components components. Many of the UI elements can't be swapped out with @wordpress/components components. This results in a less consistent UI and more CSS shipped to the browser.

Considering useLilius

I then looked at useLilius per a suggestion by @stokesman. This is a "headless" library that uses date-fns. It offers a single hook that can be used to display a calendar and not any actual UI components.

Pros:

• Barely any bytes.
• No hacks required.
• Uses date-fns, allowing us to remove moment as a dependency.
• Headless, so we can use @wordpress/components components and Emotion for all of the UI.

Cons:

• Doesn't implement keyboard navigation using the arrow keys.

Considering Temporal

The Temporal API is now stage 3, so I considered a completely bespoke implementation using Temporal.

Pros:

• Much better data model. Temporal.PlainDate is perfectly suited for implementing a calendar as it does not contain time or timezone information, both of which are not relevant in the context of choosing a date from a calendar.
• Future proof. Temporal will assumedly be a native browser API in due course.

Cons:

• Slightly more upfront work.
• There's literally a big yellow banner at the top of the proposal saying don't use Temporal in production.
• Requires a heavy polyfill. No browser currently implements Temporal. What's more, Temporal is much more difficult to tree shake than date-fns because all of the functions are implemented as methods on e.g. Temporal.PlainDate.prototype. This means the polyfill we ship is quite large.

Conclusion: useLilius + date-fns

I settled on using useLilius and rolling our own implementation of keyboard navigation. This approach offers the most upfront benefit and is relatively straightforward to implement.

This approach doesn't preclude a later switch to Temporal. When the Temporal proposal is more developed and implemented by at least one widely used browser (e.g. Chrome), it will be less likely that a user needs to download a heavy polyfill. We should then be able to easily implement our own version of useLilius that uses Temporal.PlainDate. If you look at the source of useLilius you'll see that it's quite small.

How?

• @wordpress/components: Remove moment and react-dates dependencies. Add date-fns and use-lilius.
• DateTimePicker: Use VStack so that we can remove all CSS.
• TimePicker: Swap moment functions for date-fns functions.
• DatePicker
  • Use useLilius instead of react-dates.
  • Use @wordpress/components and Emotion instead of CSS.
  • Use dateI18n so that dates are correctly translated.
  • Use wp.date.getSettings() so that dates appear in the selected format.
  • Add custom implementation of keyboard navigation.

Testing Instructions

1. Create or edit a post.
2. Open the Publish popover in the document settings sidebar.
3. Have at it. Things should work mostly the same as they did before. Be sure to test keyboard navigation and using screen reader.

Screenshots or screencast

Before:

https://user-images.githubusercontent.com/612155/182987676-dbd31296-eff6-47ad-9d9e-799e137b2b68.mp4

After:

https://user-images.githubusercontent.com/612155/182987811-486a1e81-824f-4581-b8f6-f832b8f82c12.mp4

I don't know if it comes across on video but it feels much snappier to use because we're no longer remounting the component 🙂

[github-actions[bot]]

Size Change: -38.7 kB (-3%)

Total Size: 1.24 MB

Filename	Size	Change
build/components/index.min.js	198 kB	-33.7 kB (-15%)	👏
build/components/style-rtl.css	11.6 kB	-2.48 kB (-18%)	👏
build/components/style.css	11.6 kB	-2.47 kB (-18%)	👏
build/compose/index.min.js	12 kB	-1 B (0%)
build/date/index.min.js	32 kB	+5 B (0%)

ℹ️ View Unchanged

Filename	Size
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_{compressed-size-action}

[ramonjd]

I've been testing this using keyboard and mouse interactions in LTR and RTL langs and it's working great!

• date selector from calendar and input fields works as expected
• custom start of the week setting appears as expected
• publish date updates with my changes

🎉

[noisysocks]

Thanks so much @ciampo! I've opened https://github.com/WordPress/gutenberg/pull/43495 to address your notes.

• The component's chosen timezone seem to be tightly coupled with the WordPress editor's settings — we should work on a way to decouple this, and maybe have the desired timezone offset as a prop?

DatePicker doesn't actually use the WordPress editor timezone because we are explicitly passing a timezone each time we call dateI18n.

We are relying on the date format that you get from wp.date.__experimentalGetSettings().formats.date, though. I'll have a play with some options here—maybe injecting formatI18n as a prop would be a good idea.

• It would be great if the @wordpress/componentspackage didn't import any date library at all (i.e.date-fns), and instead used @wordpress/date` as its date utility library

I wrote my thoughts on this in https://github.com/WordPress/gutenberg/pull/43005#discussion_r952040389. In short I think both packages are complimentary.

• Of course, we should look into migrating @wordpress/date away from moment too, but that's a whole different story

Yes!