How to check if text contains only numbers?

I've tried many cases, but none work for me. I tried:

if resultTitles[indexPath.row].rangeOfCharacterFromSet(badCharacters) == nil {
    let badCharacters = NSCharacterSet.decimalDigitCharacterSet().invertedSet
    print("Index: \(indexPath.row)")
}

also tried to

if ((resultTitles[0].toInt()) != nil) {
    print("ERROR")
}

So, how can I check that my text contains only numbers?

I find this solution, in Swift 3, to be cleaner

import Foundation

CharacterSet.decimalDigits.isSuperset(of: CharacterSet(charactersIn: yourString))

You just need to check whether the Set of the characters of your String is subset of the Set containing the characters from 0 to 9.

extension String {
    var isNumeric: Bool {
        guard self.characters.count > 0 else { return false }
        let nums: Set<Character> = ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
        return Set(self.characters).isSubset(of: nums)
    }
}

"".isNumeric // false
"No numbers here".isNumeric // false
"123".isNumeric // true
"Hello world 123".isNumeric // false

Expanding on the fantastic work from Luca above, here is the answer given written by way of Swift 5.

extension String {
    var isNumeric: Bool {
        guard self.count > 0 else { return false }
        let nums: Set<Character> = ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
        return Set(self).isSubset(of: nums)
    }
}

We have to check whether every character of the string is a digit or not and string must not be empty.

extension String {
   var isNumeric: Bool {
     return !(self.isEmpty) && self.allSatisfy { $0.isNumber }
   }
}

allSatisfy(_ predicate: (Character) throws -> Bool) rethrows -> Bool is a method which returns a Boolean value indicating whether every element of a sequence satisfies a given predicate.

For reference: https://developer.apple.com/documentation/swift/array/2994715-allsatisfy

Example: