swift 字符串截取

获取末尾两个字符：

let sessionId = "this is a test"

let index = sessionId.index(sessionId.endIndex, offsetBy: -2)

let suffix = sessionId.substring(from: index)

最后结果为：“st”

获取开头字符两个：

let sessionId = "this is a test"

let index = sessionId.index(sessionId.startIndex, offsetBy: 2)

let prefix = sessionId.substring(to: index)

获取中间字符两个：

a,笨方法

let sessionId = "this is a test"

let index = sessionId.index(sessionId.startIndex, offsetBy: 5)

let prefix = sessionId.substring(to: index)

let index = sessionId.index(prefix.startIndex, offsetBy: -2)

b,直接用range截取

let subString1 = (swiftStringasNSString).substring(with: NSMakeRange(1,3))