How do i convert HexString To ByteArray in Swift 3

I'm was trying to convert hexString to Array of Bytes([UInt8]) I searched everywhere but couldn't find a solution. Below is my swift 2 code

func stringToBytes(_ string: String) -> [UInt8]? {
    let chars = Array(string)
    let length = chars.count
    if length & 1 != 0 {
        return nil
    }
    var bytes = [UInt8]()
    bytes.reserveCapacity(length/2)
    for var i = 0; i < length; i += 2 {
        if let a = find(hexChars, chars[i]),
            let b = find(hexChars, chars[i+1]) {
            bytes.append(UInt8(a << 4) + UInt8(b))
        } else {
            return nil
        }
    }
    return bytes
} 

Example Hex

Hex : "7661706f72"

expectedOutput : "vapor"

This code can generate the same output as your swift 2 code.

func stringToBytes(_ string: String) -> [UInt8]? {
    // let length = string.characters.count
    // Updating / replacing previous (now commented) line for Swift 5 (2023-12)
    // No longer need reference characters.
    let length = string.count
    if length & 1 != 0 {
        return nil
    }
    var bytes = [UInt8]()
    bytes.reserveCapacity(length/2)
    var index = string.startIndex
    for _ in 0..<length/2 {
        let nextIndex = string.index(index, offsetBy: 2)
        if let b = UInt8(string[index..<nextIndex], radix: 16) {
            bytes.append(b)
        } else {
            return nil
        }
        index = nextIndex
    }
    return bytes
}

let bytes = stringToBytes("7661706f72")
print(String(bytes: bytes!, encoding: .utf8)) //->Optional("vapor")