No localized low-zoom labels when system language is Simplified Chinese

[1ec5]

When the system language is set to Simplified Chinese, labels of countries, states and provinces, and oceans and seas disappear instead of being localized into Simplified Chinese. This is because +[MGLVectorTileSource preferredMapboxStreetsLanguageForPreferences:] assumes all Streets source layers offer the same languages. In fact, this documentation says (in a rather confusing way) that name_zh-Hans is unavailable for the country_label, state_label, and marine_label layers.

The language fallback logic being added for #11867 could address this issue, but a more targeted fix would involve special-casing these three source layers.

/ref #11867
/cc @friedbunny @nickidlugash @langsmith

[lilykaiser]

@lloydsheng do you have time to investigate this? cc @chriswu42

[lloydsheng]

@lilykaiser Happy to help investigate this issue. cc @suntony

[lloydsheng]

@1ec5 I checked the tile data, These layers did have no name_zh-Hans field.

country_label	marine_label	state_label

My initial thought was that can we add the name_zh-Hans field to those layers from the data side? If can't, I have 2 options to fix it from SDK side.

1. Check the layer when setting layer's text, if it is one of country_label, state_label,marine_label, then check the localizedText expression, fallback to name_zh if it is name_zh-Han.

2. As we have the expression now, we could add a logic with the expression to check all the name field, fallback to name_zh if it is name_zh-Han and the value of it is empty.

@julianrex @1ec5 Glad to hear your thougts about this.

[1ec5]

My initial thought was that can we add the name_zh-Hans field to those layers from the data side?

Unfortunately, GL doesn’t currently support adding fields to vector tiles at runtime. (This is a concept sometimes called “data join”.) Better to alias one field with another in the expressions instead.

Check the layer when setting layer's text, if it is one of country_label, state_label,marine_label, then check the localizedText expression, fallback to name_zh if it is name_zh-Han.

The navigation SDK has similar logic that determines the locale to pass into -[NSExpression mgl_expressionLocalizedIntoLocale:] based on the layer’s source layer identifier.

The special case would have to live in -[MGLStyle localizeLabelsIntoLocale:], because -[NSExpression mgl_expressionLocalizedIntoLocale:] doesn’t know the current source name’s locale (and giving it that context would make little sense and would break backwards compatibility). If we put the special case in -[MGLStyle localizeLabelsIntoLocale:], then the caller of -[NSExpression mgl_expressionLocalizedIntoLocale:] is responsible for making sure the locale is appropriate for the current layer.

As we have the expression now, we could add a logic with the expression to check all the name field, fallback to name_zh if it is name_zh-Han and the value of it is empty.

The fallback could be implemented by wrapping key path expressions in mgl_coalesce() functions. This is the more robust approach. We need to take this approach anyways for #11867, since all the name_ fields in Streets source v8 are optional. The tricky part is that localizing the style a second time would needlessly complicate the expression. Localizing an already localized expression with the same locale could turn:

NSExpression(format: "mgl_coalesce({name_zh-Hans, name_zh})")

into:

NSExpression(format: """
    mgl_coalesce({
        mgl_coalesce({name_zh-Hans, name_zh}),
        mgl_coalesce({name_zh-Hans, name_zh})
    })
    """)

With a different locale the second time, it would become:

NSExpression(format: """
    mgl_coalesce({
        name_en,
        name_en
    })
    """)

If we could positively identify a coalescing expression as the result of an existing localization operation, then we might be able to replace the expression instead of more deeply nesting it, somewhere around here:

mapbox-gl-native/platform/darwin/src/NSExpression+MGLAdditions.mm

Lines 1519 to 1530 in 192c611

	NSArray *arguments = self.arguments;
	NSArray *localizedArguments = MGLLocalizedCollection(arguments, locale);
	if (localizedArguments != arguments) {
	return [NSExpression expressionForFunction:localizedOperand
	selectorName:self.function
	arguments:localizedArguments];
	}
	if (localizedOperand != operand) {
	return [NSExpression expressionForFunction:localizedOperand
	selectorName:self.function
	arguments:self.arguments];
	}

[lloydsheng]

@1ec5 Thanks for your feedback 👍. I've pushed some changes to fix this issue #12387. I think it also made the SDK compatible with name fields of Mapbox Streets v8.

/cc @chriswu42 @suntony

[1ec5]

If we could positively identify a coalescing expression as the result of an existing localization operation, then we might be able to replace the expression instead of more deeply nesting it

We could append a dummy key path expression to the end of the coalescing array, something like com.mapbox.expressions.localized. Right around here, before localizing the arguments, we could check whether the dummy key path appears in the coalescing expression:

mapbox-gl-native/platform/darwin/src/NSExpression+MGLAdditions.mm

Line 1414 in cb714d5

NSArray *localizedArguments = MGLLocalizedCollection(arguments, locale);

If it does, then we can replace the expression with a new localized one instead of relocalizing the existing expression (which would cause it to become more deeply nested):

NSExpression *localizedCoalescingExpressionMarker = [NSExpression expressionForKeyPath:@"com.mapbox.expressions.localized"];

if ([self.function isEqualToString:@"mgl_coalesce:"]) {
    // Check if the coalescing expression was the result of previously localizing an expression.
    NSArray *coalescedExpressions = (NSArray *)self.arguments.firstObject;
    if ([coalescedExpressions isKindOfClass:[NSArray array]] &&
        [coalescedExpressions.lastObject isEqual:localizedCoalescingExpressionMarker]) {
        // If so, replace the expression instead of relocalizing it.
        return [[NSExpression expressionForKeyPath:@"name"]
                mgl_expressionLocalizedIntoLocale:locale];
    }
}

NSArray *localizedArguments = MGLLocalizedCollection(arguments, locale);
// …

[1ec5]

Fixed in #12387.