UTF16View

You can access a string’s view of UTF-16 code units by using its utf16 property. A string’s UTF-16 view encodes the string’s Unicode scalar values as 16-bit integers.

let flowers = "Flowers 💐"
for v in flowers.utf16 {
    print(v)
}
// 70
// 108
// 111
// 119
// 101
// 114
// 115
// 32
// 55357
// 56464

Unicode scalar values that make up a string’s contents can be up to 21 bits long. The longer scalar values may need two UInt16 values for storage. Those “pairs” of code units are called surrogate pairs.

let flowermoji = "💐"
for v in flowermoji.unicodeScalars {
    print(v, v.value)
}
// 💐 128144

for v in flowermoji.utf16 {
    print(v)
}
// 55357
// 56464

To convert a String.UTF16View instance back into a string, use the String type’s init(_:) initializer.

let favemoji = "My favorite emoji is 🎉"
if let i = favemoji.utf16.firstIndex(where: { $0 >= 128 }) {
    let asciiPrefix = String(favemoji.utf16[..<i])!
    print(asciiPrefix)
}
// Prints "My favorite emoji is "

UTF16View Elements Match NSString Characters

The UTF-16 code units of a string’s utf16 view match the elements accessed through indexed NSString APIs.

print(flowers.utf16.count)
// Prints "10"

let nsflowers = flowers as NSString
print(nsflowers.length)
// Prints "10"

Unlike NSString, however, String.UTF16View does not use integer indices. If you need to access a specific position in a UTF-16 view, use Swift’s index manipulation methods. The following example accesses the fourth code unit in both the flowers and nsflowers strings:

print(nsflowers.character(at: 3))
// Prints "119"

let i = flowers.utf16.index(flowers.utf16.startIndex, offsetBy: 3)
print(flowers.utf16[i])
// Prints "119"

Although the Swift overlay updates many Objective-C methods to return native Swift indices and index ranges, some still return instances of NSRange. To convert an NSRange instance to a range of String.Index, use the Range(_:in:) initializer, which takes an NSRange and a string as arguments.

let snowy = "❄️ Let it snow! ☃️"
let nsrange = NSRange(location: 3, length: 12)
if let range = Range(nsrange, in: snowy) {
    print(snowy[range])
}
// Prints "Let it snow!"