[iOS] Add fallbacks for name fields

[lloydsheng]

Aiming to fix this issue #12164 and compatible with name fields of Mapbox Streets v8.

[1ec5]

Thanks, this is a much more elegant approach to implementing v8 support than what I had been working on for #11867. (#11867 covers some additional things like VoiceOver that are unrelated to localization.)

In addition to the feedback below, we should figure out the edge case mentioned in #12164 (comment), that the same expression may be localized more than once. When localizing a function expression, perhaps we check whether the function is mgl_coalesce: and the argument array consists entirely of name_ key path expressions; if so, then we can replace the coalescing function instead of localizing each argument as we normally do.

[1ec5]

As this documentation states, name_en can be null in Streets source v8, in which case we need to fall back to name.

[1ec5]

Should we also have name_zh-Hant attempt to fall back to name_zh-Hans and vice versa, for Streets source v8?

[1ec5]

Apparently the same mapbox.mapbox-streets-v7 tileset on api.mapbox.cn uses name_zh-CN instead of name_zh-Hans: mapbox/mapbox-navigation-ios#1554. Could we address that here by opportunistically falling back to name_zh-CN if name_zh-Hans is nil?

/cc @m-stephen

[1ec5]

Fixed an issue causing country and ocean labels to disappear after calling -[MGLStyle localizeLabelsIntoLocale:] when the system language is set to Simplified Chinese.

[1ec5]

This can be written more succinctly as:

[NSExpression expressionWithFormat:@"mgl_coalesce({%K, name_zh, name})", localizedKeyPath]

[1ec5]

Of these four key paths, only name_zh-CN really needs to be escaped using %K, due to the hyphen. But this is better because it preserves the correct fallback order in the code.

A more generic approach would be to build up an array of key path expressions, then create a coalescing expression in only one place. Something like this:

NSArray *keyPaths;
if (…) {
    keyPaths = @[localizedKeyPath, @"name_zh-CN", @"name_zh", @"name"];
} else if (…) {
    …
}

NSMutableArray *keyPathExpressions = [NSMutableArray array];
for (NSString *keyPath in keyPaths) {
    [keyPathExpressions addObject:[NSExpression expressionForKeyPath:keyPath]];
}

return [NSExpression expressionWithFormat:@"mgl_coalesce(%@)", keyPathExpressions];

This might make it easier to address the edge case in #12387 (review) with the solution in #12164 (comment):

[keyPathExpressions addObject:
 [NSExpression expressionForKeyPath:@"com.mapbox.expressions.localized"]];

Given the complexity of handling the relocalization case, we can address it in a subsequent PR if you think it’s out of scope for this PR.

[lloydsheng]

@1ec5 Good suggestion, but I think it might happen in another PR.

[1ec5]

Surround -[MGLStyle localizeLabelsIntoLocale:] in backticks so that jazzy will automatically link it to the method’s documentation.

[1ec5]

Note that label localization is currently disabled for Streets source v8 because this code only recognizes v6 and v7:

mapbox-gl-native/platform/darwin/src/MGLVectorTileSource.mm

Line 144 in cb714d5

return [identifiers containsObject:@"mapbox.mapbox-streets-v7"] || [identifiers containsObject:@"mapbox.mapbox-streets-v6"];

But this fallback is good to have in preparation for #11867 anyhow.