An NSNumberFormatter for iOS that usefully implements attributedStringForObjectValue…

AttributedNumberFormatter.swift

import Foundation

/// A number formatter that usefully implements `attributedString(for:withDefaultAttributes:)`, which iOS's NumberFormatter does not.
final class AttributedNumberFormatter: NumberFormatter {
    override func attributedString(for obj: Any, withDefaultAttributes defaultAttributes: [NSAttributedString.Key : Any]? = nil) -> NSAttributedString? {

        guard
            let number = obj as? NSNumber,
            let plain = string(from: number)
            else { return nil }

        let activeAttributes: [String: Any]?
        switch (number, number.compare(0)) {
        case (NSDecimalNumber.notANumber, _):
            activeAttributes = textAttributesForNotANumber
        case (kCFNumberPositiveInfinity, _):
            activeAttributes = textAttributesForPositiveInfinity
        case (kCFNumberNegativeInfinity, _):
            activeAttributes = textAttributesForNegativeInfinity
        case (_, .orderedSame):
            activeAttributes = textAttributesForZero
        case (_, .orderedAscending):
            activeAttributes = textAttributesForNegativeValues
        case (_, .orderedDescending):
            activeAttributes = textAttributesForPositiveValues
        }

        // Seems to be how the Mac implementation works.
        var attributes = defaultAttributes ?? [:]
        for (k, v) in activeAttributes ?? [:] {
            attributes[NSAttributedString.Key(k)] = v
        }

        return NSAttributedString(string: plain, attributes: attributes)
    }
}