String in Swift does not support UTF16 surrogate in default.

From Swift Official Guide

A Unicode scalar is any Unicode code point in the range U+0000 to U+D7FF inclusive or U+E000 to U+10FFFF inclusive. Unicode scalars do not include the Unicode surrogate pair code points, which are the code points in the range U+D800 to U+DFFF inclusive.

Therefore, I wrote a support class for it. :)

SwiftSurrogate on Github

Now you can decode Swift String from UTF16 surrogate pair like this.

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var emoji1 = SwiftySurrogate.decodeFromSurrogatePair(surrogatePair: "D83D:DCC9")
var emoji2 = SwiftySurrogate.decodeFromSurrogatePair(high: 0xD83C, low: 0xDF80)