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

//注册通知，textfield内容改变调用
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textFieldDidChange:) name:UITextFieldTextDidChangeNotification object:self.testTextField];

- (void)textFieldDidChange:(NSNotification *)note{
UITextField *textField = note.object;
//获取文本框内容的字节数
int bytes = [self stringConvertToInt:self.testTextField.text];
//设置不能超过32个字节，因为不能有半个汉字，所以以字符串长度为单位。
if (bytes > 16)
{
//超出字节数，还是原来的内容
self.testTextField.text = self.lastTextContent;
}
else
{
self.lastTextContent = self.testTextField.text;
}
}
//得到字节数函数
- (int)stringConvertToInt:(NSString*)strtemp
{ 
int strlength = 0; 
char* p = (char*)[strtemp cStringUsingEncoding:NSUnicodeStringEncoding];
for (int i=0 ; i<[strtemp lengthOfBytesUsingEncoding:NSUnicodeStringEncoding] ;i++)
{ 
if (*p) { 
p++; 
strlength++; 
} 
else { 
p++; 
}
}
return (strlength+1)/2;
}

- (BOOL)textFieldShouldBeginEditing:(UITextField *)textField
{
self.lastTextContent = textField.text;
return YES;
}