Swift4 'substring(to:)' is deprecated: Please use String slicing subscript with a 'partial range upt

解决Swift4 substring deprecated: Please use String slicing subscript with a ‘partial range upto’ operator.

extension String {    func substring(from: Int?, to: Int?) -> String {        if let start = from {            guard start < self.characters.count else {                return ""            }        }        if let end = to {            guard end >= 0 else {                return ""            }        }        if let start = from, let end = to {            guard end - start >= 0 else {                return ""            }        }        let startIndex: String.Index        if let start = from, start >= 0 {            startIndex = self.index(self.startIndex, offsetBy: start)        } else {            startIndex = self.startIndex        }        let endIndex: String.Index        if let end = to, end >= 0, end < self.characters.count {            endIndex = self.index(self.startIndex, offsetBy: end + 1)        } else {            endIndex = self.endIndex        }        return String(self[startIndex ..< endIndex])    }    func substring(from: Int) -> String {        return self.substring(from: from, to: nil)    }    func substring(to: Int) -> String {        return self.substring(from: nil, to: to)    }    func substring(from: Int?, length: Int) -> String {        guard length > 0 else {            return ""        }        let end: Int        if let start = from, start > 0 {            end = start + length - 1        } else {            end = length - 1        }        return self.substring(from: from, to: end)    }    func substring(length: Int, to: Int?) -> String {        guard let end = to, end > 0, length > 0 else {            return ""        }        let start: Int        if let end = to, end - length > 0 {            start = end - length + 1        } else {            start = 0        }        return self.substring(from: start, to: to)    }}

let string = "Hello,World!"string.substring(from: 1, to: 7) print: ello,Wostring.substring(to: 7) print: Hello,Wostring.substring(from: 3) print: lo,World!string.substring(from: 1, length: 4) print: ellostring.substring(length: 4, to: 7) print: o,WoUpdated substring(from: Int?, length: Int) to support starting from zero.