Using OpenType font features in iOS - ios

Question

It seems that in iOS, OpenType font-features and their selectors are identified by numbers. For example we have these in Frameworks/CoreText/SFNTLayoutTypes.h:
enum {
kAllTypographicFeaturesType = 0,
kLigaturesType = 1,
...
kLanguageTagType = 39,
kCJKRomanSpacingType = 103,
kLastFeatureType = -1
}
/*
* Summary:
* Selectors for feature type kAllTypographicFeaturesType
*/
enum {
kAllTypeFeaturesOnSelector = 0,
kAllTypeFeaturesOffSelector = 1
};
...
We can list font-features using this:
extension UIFont {
typealias FontFeatureInfo = [String: Any]
func listFeatures() {
guard let fontFeatures = CTFontCopyFeatures(self) as? [FontFeatureInfo] else {
debugPrint("Could not copy font features.")
return
}
fontFeatures.forEach { fontFeatureInfo in
print(fontFeatureInfo)
}
}
}
and it prints something like this:
[
CTFeatureTypeName: All Typographic Features,
CTFeatureTypeIdentifier: 0,
CTFeatureTypeSelectors: <__NSArrayM 0x6000000e15c0>({
CTFeatureSelectorDefault = 1;
CTFeatureSelectorIdentifier = 0;
CTFeatureSelectorName = On;
})
]
[
CTFeatureTypeName: Ligatures,
CTFeatureTypeIdentifier: 1,
CTFeatureTypeSelectors: <__NSArrayM 0x6000000e1e90>({
CTFeatureSelectorIdentifier = 4;
CTFeatureSelectorName = "Rare Ligatures";
})
]
...
In web development we use these features by their string-based IDs, like ss01, dlig, salt, dnom and etc.
font-feature-settings: "ss01";
If we run code above for Inter typeface, there are many font-features that have CTFeatureOpenTypeTag: _opentype-tag_, but does not have any CTFeatureTypeIdentifier.
For example it shows:
[
CTFeatureOpenTypeTag: numr,
CTFeatureTypeSelectors: <__NSArrayM 0x6000000d8e40>({
CTFeatureOpenTypeValue = 0;
CTFeatureSelectorDefault = 1;
CTFeatureSelectorName = Off;
}, {
CTFeatureOpenTypeValue = 1;
CTFeatureSelectorName = On;
}),
CTFeatureTypeExclusive: 1,
CTFeatureTypeName: Numerators
]
[
CTFeatureOpenTypeTag: salt,
CTFeatureTypeName: Stylistic Alternates,
CTFeatureTypeSelectors: <__NSArrayM 0x6000000d9380>({
CTFeatureOpenTypeValue = 0;
CTFeatureSelectorDefault = 1;
CTFeatureSelectorName = Off;
}, {
CTFeatureOpenTypeValue = 1;
CTFeatureSelectorName = On;
}),
CTFeatureTypeExclusive: 1
]
The question is how can I use these features without numeric identifiers? How can I use combination of CTFeatureOpenTypeTag and CTFeatureOpenTypeValue to enable OpenType font-features?