Remove UIAccessibilityTraitKeyboardKey to fix touch typing

[vashworth]

UIAccessibilityTraitKeyboardKey was added in #4575 so that TextInputSemanticsObject supported VoiceOver gestures for text editing features, such as pinch to select text, and up/down fling to move cursor. After experimenting with it, I found that those features were still available even after removing UIAccessibilityTraitKeyboardKey.

Fixes flutter/flutter#94465.

In Touch Typing Mode:

touch_typing.mov

In Standard Typing Mode:

standard_typing.mov

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[vashworth]

The original PR was made in 2018, so it could be that this changed in a newer version of iOS or something about how we do text input has changed? It's unclear to me. I only have an iOS 17 device to test with. @chunhtai Do you think it's worth wrapping in an @available to limit to iOS 17 since that's the only one I was able to test?

[chunhtai]

Can you talk a bit more on why removing the trait fixes the issue? What is the observed behavior before and after?

[vashworth]

Can you talk a bit more on why removing the trait fixes the issue? What is the observed behavior before and after?

Yes, the issue is that when touch typing is on, accessibilityActivate is triggered after only one tap. Whereas with standard typing, it's triggered after double tapping.

engine/shell/platform/darwin/ios/framework/Source/accessibility_text_entry.mm

Lines 262 to 267 in 33c828f

	- (BOOL)accessibilityActivate {
	if (![self isAccessibilityBridgeAlive]) {
	return false;
	}
	return [[self textInputSurrogate] accessibilityActivate];
	}

This is happening due to Flutter adding the UIAccessibilityTraitKeyboardKey accessibility trait to TextInputSemanticsObject.

engine/shell/platform/darwin/ios/framework/Source/accessibility_text_entry.mm

Lines 307 to 312 in 33c828f

	// Adding UIAccessibilityTraitKeyboardKey to the trait list so that iOS treats it like
	// a keyboard entry control, thus adding support for text editing features, such as
	// pinch to select text, and up/down fling to move cursor.
	UIAccessibilityTraits results = [super accessibilityTraits] |
	[self textInputSurrogate].accessibilityTraits |
	UIAccessibilityTraitKeyboardKey;

According to Apple's documentation UIAccessibilityTraitKeyboardKey tells iOS that the "accessibility element behaves like a keyboard key". And if we look at the different typing modes, Touch typing reacts on a single tap on a keyboard key whereas Standard typing reacts on a double tap on a keyboard key:

Standard typing: Select a key by swiping left or right on the keyboard, then double-tap to enter the character.
Touch typing: Touch a key on the keyboard to select it, then lift your finger to enter the character.

So by treating TextInputSemanticsObject as a keyboard key, it reacts to a single tap when touch typing is enabled.

By removing UIAccessibilityTraitKeyboardKey, TextInputSemanticsObject is no longer treated as a keyboard key and therefore does not react on single touch when Touch typing is enabled. You can observe this by being in Touch typing mode, closing the keyboard, and then clicking on the text field. It will not open the keyboard on a single tap, which is the correct behavior. It will open on double tap. The videos I included in the description also show that the pinch to select text and up/down fling to move cursor functionality still works.

[chunhtai]

Thanks for explanation, yes this makes sense. According to the doc, that trait should probably not be on the Textfield semantics object.

[chunhtai]

LGTM