import Foundation

@propertyWrapper
struct Trimmed {

    var wrappedValue: String {
        didSet {
            wrappedValue = wrappedValue.trimmingCharacters(in: charactersToTrim)
        }
    }

    var charactersToTrim: CharacterSet = .whitespacesAndNewlines
}

struct Tester {
    @Trimmed var trimmedString: String = ""
}

var tester = Tester()
tester.trimmedString = "     ab c    "
print(tester.trimmedString) // prints "ab c"