Added border curve style prop ("Squircle" effect - iOS only)

[eric-edouard]

Summary

NOTE: PR is based on #32017 which went stale for quite a long time but can now safely be closed

Since iOS 13+, it is possible to change the corner curve property on iOS in order to smoothen border radius and make it more "rounded" (also called "squircle")
Here's an article explaining in details what it is.
This property is also built in figma, but currently there is no way to implement this directly with react-native despite it being available natively on iOS.

Many open source react-native libraries were created in order to simulate this behaviour:
react-native-super-ellipse-mask
react-native-squircle-view
react-native-figma-squircle

But they rely on creating an SVG shape with the smoothed corners and masking the view behind. This makes it not very performant (flickering on mounting was a common side-effect)

This PR aims at implementing the property natively.

PR for the docs update: facebook/react-native-website#2785

Changelog

[iOS] [Added] - Added borderCurve style prop for smooth border radius (squircle effect)

Test Plan

We used the RNTester app and added an example with cornerCurve set to 'continuous' (only on iOS).

As the difference is quite subtle, we also made some more tests to better illustrate the difference (these are not in the RN-tester app):

We overlapped two views with position: absolute, the one in the background has a red background and has cornerRadius set to false, and the one in the foreground is set to true. We can clearly see where the borders differs on the corners.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	7,821,839	+1,558
android	hermes	armeabi-v7a	7,213,962	+630
android	hermes	x86	8,133,894	+1,135
android	hermes	x86_64	8,111,836	+963
android	jsc	arm64-v8a	9,700,247	+1,631
android	jsc	armeabi-v7a	8,454,450	+692
android	jsc	x86	9,650,465	+1,203
android	jsc	x86_64	10,248,242	+1,022

Base commit: f3db6cc
Branch: main

[janicduplessis]

Nice! PR looks good to me.

Sorry this is taking forever to merge

cc @cortinico

[janicduplessis]

@eric-edouard Did you test if this work with the Fabric renderer? I think RN tester uses it by default now.

Can you also rebase, I think CI was broken recently and was fixed here 94341f0.

[janicduplessis]

I think similar logic could be used here https://github.com/facebook/react-native/blob/main/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm#L582.

[eric-edouard]

@janicduplessis you're correct, this doesn't work with Fabric yet. Will take a look into it! Thanks

[eric-edouard]

@janicduplessis I've adapted the logic to have it work with Fabric, works well on my end with the same testing strategy as in the PR description.
I mostly duplicated the logic found in both BorderStyle (It being a string prop and an enum in native) and BorderRadius (as it affects corners)
it did seem like a lot to add in many places for such a small feature 😄 but couldn't see how to make it simpler.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: f3db6cc
Branch: main

[eric-edouard]

@janicduplessis quick bump 🙂

[cipolleschi]

Cold you rebase this? I can see test_windows fail, but they should not be affected by your PR, and the CI is green on main. After the rebase, I'll import it!

[eric-edouard]

@cipolleschi rebased + tested again on RNTester app after merge, all good

[eric-edouard]

@cipolleschi quick bump!

[facebook-github-bot]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[cipolleschi]

Sorry if this took a while to be imported. Thanks again for the contribution.
Can I ask if you can rebase it and fix the small comment I left?

Besides these, it looks very good.

[eric-edouard]

@cipolleschi No worries! Fixed the new line and aligned with main. Let me know if there's anything else

[facebook-github-bot]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @eric-edouard in 8993ffc.

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