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 watchOS. This is due to different built-in fonts installed on these platforms.

When a character cannot be displayed it appears as a ? inside a box, like so:

I've also seen some characters display as an alien instead (not sure why the difference):

Is there a way to know when a specific unicode character will not be displayed properly given a string of the unicode character such as "ᄥ"?

I am in need of a solution that works for both iOS and watchOS.

Answer 1

You can use CTFontGetGlyphsForCharacters() to determine if a font has a glyph for a particular code point (note that supplementary characters need to be checked as surrogate pairs):

CTFontRef font = CTFontCreateWithName(CFSTR("Helvetica"), 12, NULL);
const UniChar code_point[] = { 0xD83C, 0xDCA1 };  // U+1F0A1
CGGlyph glyph[] = { 0, 0 };
bool has_glyph = CTFontGetGlyphsForCharacters(font, code_point, glyph, 2);

Or, in Swift:

let font = CTFontCreateWithName("Helvetica", 12, nil)
var code_point: [UniChar] = [0xD83C, 0xDCA1]
var glyphs: [CGGlyph] = [0, 0]
let has_glyph = CTFontGetGlyphsForCharacters(font, &code_point, &glyph, 2)

If you want to check the complete set of fallback fonts that the system will try to load a glyph from, you will need to check all of the fonts returned by CTFontCopyDefaultCascadeListForLanguages(). Check the answer to this question for information on how the fallback font list is created.