Basic Caesar Cipher in Swift 3.1
Using the unicodeScalars view of the Swift String is a good start,
but the code can be simplified:


There is no need to create an array first, you can call map
directly on the unicodeScalars collection.
The conversion to Character is not needed, the array of shifted
unicode scalars can be converted directly back to a string
(attribution to this answer on Stack Overflow).


Also the variable name unicodePoints is misleading, because that
is an array of Character in your code.

Your method then becomes

func encrypt(message: String, shift: Int) -&gt; String {
    let unicodeScalars = message.unicodeScalars.map { UnicodeScalar(Int($0.value) + shift)! }
    return String(String.UnicodeScalarView(unicodeScalars))
}


However, this is probably not yet what you want. Example:

let message = "Attack at dawn"
print(encrypt(message: message, shift: 2))    // Cvvcem"cv"fcyp
print(encrypt(message: message, shift: 10))   // K~~kmu*k~*nkx


All characters are shifted by the given amount so that a space
becomes a quotation mark, and some letters are transformed to non-letters. Also

print(encrypt(message: message, shift: -100))


crashes at runtime, because the unicode scalar value is shifted to
an invalid value.

The traditional Caesar Cipher transforms only letters in the range
A...Z, and rotates them, so that shifting "Z" by 2 becomes "B".
Lowercase letters can be transformed to uppercase, but other characters
are usually ignored (either removed or left unchanged).

Here is a possible implementation with an inner function
shiftLetter() which transforms a single letter.
This inner function is used as argument in

msg.unicodeScalars.map(shiftLetter)


to transform the entire String:

func encrypt(message: String, shift: Int) -&gt; String {

    func shiftLetter(ucs: UnicodeScalar) -&gt; UnicodeScalar {
        let firstLetter = Int(UnicodeScalar("A").value)
        let lastLetter = Int(UnicodeScalar("Z").value)
        let letterCount = lastLetter - firstLetter + 1

        let value = Int(ucs.value)
        switch value {
        case firstLetter...lastLetter:
            // Offset relative to first letter:
            var offset = value - firstLetter
            // Apply shift amount (can be positive or negative):
            offset += shift
            // Transform back to the range firstLetter...lastLetter:
            offset = (offset % letterCount + letterCount) % letterCount
            // Return corresponding character:
            return UnicodeScalar(firstLetter + offset)!
        default:
            // Not in the range A...Z, leave unchanged:
            return ucs
        }
    }

    let msg = message.uppercased()
    return String(String.UnicodeScalarView(msg.unicodeScalars.map(shiftLetter)))
}


Examples:

let message = "Attack at dawn"
print(encrypt(message: message, shift: 2))    // CVVCEM CV FCYP
print(encrypt(message: message, shift: 10))   // KDDKMU KD NKGX
print(encrypt(message: message, shift: -100)) // EXXEGO EX HEAR