限制输入6位数字密码

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

{

    if (range.length + range.location > textField.text.length) {

    return NO;

    }NSUInteger length = textField.text.length + string.length - range.length;

    return length <= 6;//这里可以更改为需要的位数

    return YES;

}