Swift4 StringのアクセスをsubscriptでIntによって可能にした

StringExtensions.swift

extension String {

    /// String[index]
    ///
    /// - Parameter i: Index of String
    public subscript(i: Int) -> String {
        let index = self.index(startIndex, offsetBy: i)
        return String(self[index])
    }

    /// String[startIndex..<endIndex]
    ///
    /// - Parameter range: CountableRange<Int> (ex. 0..<5)
    public subscript(range: CountableRange<Int>) -> String {
        let start = index(startIndex, offsetBy: range.lowerBound)
        let end = index(startIndex, offsetBy: range.upperBound)
        return String(self[start..<end])
    }

    /// String[startIndex...endIndex]
    ///
    /// - Parameter range: CountableClosedRange<Int> (ex. 0...5)
    public subscript(range: CountableClosedRange<Int>) -> String {
        let start = index(startIndex, offsetBy: range.lowerBound)
        let end = index(startIndex, offsetBy: range.upperBound)
        return String(self[start...end])
    }

    /// String[startIndex...]
    ///
    /// - Parameter range: CountablePartialRangeFrom<Int> (ex. 0...)
    public subscript(range: CountablePartialRangeFrom<Int>) -> String {
        let start = index(startIndex, offsetBy: range.lowerBound)
        return String(self[start...])
    }

    /// String[...endIndex]
    ///
    /// - Parameter range: PartialRangeThrough<Int> (ex. ...5)
    public subscript(range: PartialRangeThrough<Int>) -> String {
        let end = index(startIndex, offsetBy: range.upperBound)
        return String(self[...end])
    }
}