监听键盘 限制输入字数
 [self.inputTextField addTarget:self action:@selector(textfieldDidChange:) forControlEvents:UIControlEventEditingChanged];
- (void)textfieldDidChange:(UITextField *)textField{
UITextRange *selectedRange = [textField markedTextRange];
NSString *newText = [textField textInRange:selectedRange];
if (!newText.length) {
    if (self.inputTextField.text.length > 10) {
        self.inputTextField.text = [self.inputTextField.text substringToIndex:10];
        self.characterNumLab.text = @"10/10";
        return;
    }
    self.inputTextField.text = textField.text;
    NSInteger numCount = textField.text.length;
    self.characterNumLab.text = [NSString stringWithFormat:@"%ld/10", (long)numCount];
  }
}