Add support for Increase Contrast on iOS

[miquelbeltran]

Link to the design document: https://docs.google.com/document/d/1kePVlqWvJu5Ph0RL6wgg67F3SATmsJ8QY5N0S1MvaGg/edit?usp=sharing

Description

Read the "Increase Contrast" setting from iOS accessibility settings and pass it together with the rest of accessibility flags.

According to the Apple documentation, the method UIAccessibilityDarkerSystemColorsEnabled() exposes the value of the "Increase Contrast" setting in the Accessibility settings.

https://developer.apple.com/documentation/uikit/1615087-uiaccessibilitydarkersystemcolor

From UIInterface.h:

/* The value of the "high contrast" Accessibility setting is available via `UIAccessibilityDarkerSystemColorsEnabled()`,
 * and is also expressed as the UIAccessibilityContrast trait.
 */
typedef NS_ENUM(NSInteger, UIAccessibilityContrast) {
    UIAccessibilityContrastUnspecified = -1,
    UIAccessibilityContrastNormal,
    UIAccessibilityContrastHigh,
} API_AVAILABLE(ios(13.0), tvos(13.0)) API_UNAVAILABLE(watchos);

The good thing about UIAccessibilityDarkerSystemColorsEnabled is that it is available from iOS 8.0, and it is not exclusive of iOS 13 like the UIAccessibilityContrast trait.

The value is passed as a flag together with the rest of accessibility
settings.

Later on, the flag is exposed in the window.accessibilitySettings and can be read from the MediaQuery class in Flutter (a PR on the Flutter project will be done as well)

Related Issues

Reference ticket: flutter/flutter#48418

Blocks PR on flutter/flutter: flutter/flutter#48486

Tests

I couldn't find any equivalent test for any of the other platform specific accessibility flags.

I tested this locally with an iOS simulator.

[miquelbeltran]

After reading #15370 I guess this is also considered a breaking change and needs to go through the process described here: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes

Probably means you will close this PR.

Nevertheless it was fun to try to fix it!

[Piinks]

Hi @miquelbeltran! Welcome 🎉 and thanks for the contribution!
While this may be a breaking change, that does not necessarily mean it will be closed. I encourage you to read the breaking change policy you mentions, and consider if you would like to continue. There is a path to landing your change, just with some extra steps in this special case. :)

[miquelbeltran]

Thanks @Piinks! I created the design document, added a link here and posted it on the #hackers-devrel channel, let's hope it all goes through :)

I will resolve the conflicts, since another accessibility flag has been reverted, to keep the change clean.

[miquelbeltran]

The failing "analyze" is because a change also has to be made on the Flutter Framework repo.

Analyzing 3 directories...                                      

  error • Missing concrete implementation of 'getter AccessibilityFeatures.highContrast' • packages/flutter_test/test/window_test.dart:252:7 • non_abstract_class_inherits_abstract_member

Which requires to add the highContrast flag in the FakeAccessibilityFeatures test class.

[Piinks]

I've left a note on the framework-side PR about splitting it into 2 smaller PRs. The first should unblock this one, and then you can land the other after this. I've also shared the design doc around to see if it can get more feedback.
If you haven't already, feel free to check out https://github.com/flutter/flutter/wiki/Chat and join in on the conversation! :)

[Piinks]

Thanks @Piinks! I created the design document, added a link here and posted it on the #hackers-devrel channel, let's hope it all goes through :)

Oh! You are on the server. Sorry! I must have missed it. 😝

[LongCatIsLooong]

Thank you for the pull request @miquelbeltran! Looks like https://github.com/flutter/engine/pull/10791/files is a different attempt at this andUIAccessibilityDarkerSystemColorsEnabled seems to be doing the same thing as UIAccessibilityContrast. Is there any official documentation that mentions the correspondence between the two?

[miquelbeltran]

Hi! It is explained in the PR description.

This is from the iOS UIKit Framework source code (UIInterface.h)

The value of the "high contrast" Accessibility setting is available via UIAccessibilityDarkerSystemColorsEnabled(),
and is also expressed as the UIAccessibilityContrast trait.

The UiAccessibilityContrast is iOS 13 only while UIAccessibilityDarkerSystemColorsEnabled exists from iOS 8.

There's more info on the description of this pr.

[miquelbeltran]

I've updated the design document with a clarification on why using UIAccessibilityDarkerSystemColorsEnabled instead of UIAccessibilityContrast as well.

[Piinks]

This is waiting on flutter/flutter#48486

[LongCatIsLooong]

@miquelbeltran thanks for the explanation (and adding it to the doc)!
Is it possible to get different results from these two ways? I.e., UITraitCollection can be overridden by the parent view controller and since UIAccessibilityDarkerSystemColorsEnabled() reads from system settings it may not be aware of the override?

[miquelbeltran]

@LongCatIsLooong you're welcome!

Is it possible to get different results from these two ways?

I think yes, as you say you can override the UITraitCollection. Seems that the UIAccessibilityContrast is the only one accessibility setting present in there.

I personally think that is UIAccessibilityDarkerSystemColorsEnabled the recommended way to go. Although maybe a mixed solution is to use UIAccessibilityDarkerSystemColorsEnabled on iOS 8 to 12, and use the UIAccessibilityContrast trait in iOS 13.

What do you think?

[LongCatIsLooong]

I'm leaning towards handling it differently in iOS 13. I believe some work was already done in #10791 for iOS 13 high contrast, but accessibilityFeatures might be a more suitable channel to use (instead of settings).

[LongCatIsLooong]

@miquelbeltran according to the wiki https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes, the design doc for the breaking change needs to be reviewed by dev rel folks in chat. Do you mind posting the design doc there?

[miquelbeltran]

I did it a while ago

[LongCatIsLooong]

Hi @miquelbeltran sorry for the delay! It looks like the changes are no longer considered breaking. Could you update flutter/flutter#48486?

[miquelbeltran]

Thanks @LongCatIsLooong I've updated now the other PR with the requested changes. Let me know if something else is missing.

[LongCatIsLooong]

Could you fix https://github.com/flutter/engine/pull/15343/checks?check_run_id=385448037 by following the steps in the error message? Not sure what's with the web engine build failure, seems unrelated. Maybe updating the branch will help?

[miquelbeltran]

Thanks, I didn't see the formatting issue, fixed that and also rebased to current master.