之前在cocoachina看到一篇文章http://www.cocoachina.com/ios/20160106/14889.html，挺实用的，非常好用，肯定以后可以遇到，登录的时候肯定能遇到
- (void)textFieldTextDidChanged:(UITextField *)sender
{
NSString * tempString = sender.text;
if (sender.markedTextRange == nil && tempString.length > kMaxLength)
{
sender.text = [tempString substringToIndex:kMaxLength];
[sender.undoManager removeAllActions];
}
}