The specified length of the string fills the entire TextField（指定长度的字符串占满整个TextField）
Swift 4.2+ Xcode 10.1+
Cocoapods is developing, you can drag the LLimitTextField folder into your project.
//if auto insert space
limitTextField.isAutoInsertSpace = true
//auto insert space step
limitTextField.insertSpaceStep = 4
//Color of text
limitTextField.textColor = UIColor.black
limitTextField.font = UIFont.systemFont(ofSize: 25)
//the color of cursor
limitTextField.cursorColor = UIColor.black
limitTextField.keyboardType = .numberPad
//LLimitTextFieldDelegate
limitTextField.delegate = self
//under line type
limitTextField.underlineType = .one
//under line color
limitTextField.underlineColor = UIColor.cyan
//Max limit
limitTextField.limitLength = 16
enum TextFieldUnderlineType{
//no underline
case none
//one line
case one
//Underline each character below
case spaced
//Gridlines
case grid
}