Detect when a unicode character cannot be displayed correctly

Question

Some unicode characters cannot be displayed on iOS but are displayed correctly on macOS. Similarly, some unicode characters that iOS can display cannot be displayed on watch

Answer 1

Compare against the known, undefined character U+1FFF:

/// - Parameter font: a UIFont
/// - Returns: true if glyph exists
func glyphAvailable(forFont font:UIFont) -> Bool {
    if let refUnicodePng = Character("\u{1fff}").png(forFont: font),
        let myPng = self.png(forFont: font) {
        return refUnicodePng != myPng
    }
    return false
}

using a png bitmap:

/// - Parameter font: a UIFont
/// - Returns: an optional png representation
func png(forFont font: UIFont) -> Data? {
    let attributes = [NSAttributedStringKey.font: font]
    let charStr = "\(self)" as NSString
    let size = charStr.size(withAttributes: attributes)

    UIGraphicsBeginImageContext(size)
    charStr.draw(at: CGPoint(x: 0,y :0), withAttributes: attributes)

    var png:Data? = nil
    if let charImage = UIGraphicsGetImageFromCurrentImageContext() {
        png = UIImagePNGRepresentation(charImage)
    }

    UIGraphicsEndImageContext()
    return png
}

Answered here.