swift3.0 对UITextField，输入框输入的内容进行监控

首先需要继承 UITextFieldDelegate

class TestViewController: UIViewController,UITextFieldDelegate{

}

添加事件委托

textField.delegate = self

点击输入框时触发以下事件：

func textFieldDidBeginEditing(_ textField: UITextField){

}

我的需求为输入一百以内最多为两位小数，使用以下方式对输入的值进行实时监控

 func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        var newString = (textField.text! as NSString).replacingCharacters(in: range, with: string)
        let expression = "^[0-9]*(?:\\.[0-9]{0,2})?$"
            
        do {
            let regex: NSRegularExpression = try NSRegularExpression(pattern: expression, options: NSRegularExpression.Options.caseInsensitive)
            let numberOfMatches = regex.matches(in: newString, options: NSRegularExpression.MatchingOptions.reportProgress, range: NSMakeRange(0, newString.characters.count))
            
            if numberOfMatches.count == 0{
                return false
            }else{
                    
                if newString != "" {
                    if newString[newString.startIndex] == "." {
                        newString = "0" + newString
                    }
                        
                    if newString[newString.index(before: newString.endIndex)] == "." {
                        newString = newString + "0"
                    }
                        
                    if (Double(newString)! > 100.0){
                        account_textfield.text = "100"
                        return false
                    }
                }
                return true
            }      
        }
        catch {
            return false
        }
    }

通过return值对输入框的值进行控制，return false则放弃编辑，输入框内容不会变更