Q:

Is an Emoji (or Unicode Character) available on a given device?

A:

Maybe. ← Compare against this character!

This article goes into details on how to compare the emoji or Unicode rendition against a known, invalid image.

This approach is very similar to the algorithm I implemented back in 1993, which was the object of a patent application.

It is also the response proposed to Stack Overflow question #41318999, «Is there a way to know if an Emoji is supported in iOS?»

•  SO-41318999

•  SO-9352753

•  SO-31363211

Method: Compare character images

1. Convert the character into a Portable Network Graphic image, lossless
2. Extract the png data
3. Compare the data against a known invalid image
4. If there is a match, then the character is invalid

This approach leverages on the fact that the OS uses the same default character for unknown unicode character.

Step 1: character to png

func png(ofSize fontSize: CGFloat) -> Data? {
    let attributes = [NSAttributedStringKey.font:
                      UIFont.systemFont(ofSize: fontSize)]
    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
}

Step 2: Compare against a known invalid character

func unicodeAvailable() -> Bool {
    if let refUnicodePng = Character.refUnicodePng,
        let myPng = self.png(ofSize: Character.refUnicodeSize) {
        return refUnicodePng != myPng
    }
    return false
}

► Find the complete project on GitHub (Swift + Xcode) and run your own Unit Tests to experiment.