不獲取高亮部分的字段
NSString *toBeString = sender.text;
UITextRange *selectedRange = [sender markedTextRange];
//獲取高亮部分
UITextPosition *position = [sender positionFromPosition:selectedRange.start offset:0];
if (!position) {
if (toBeString.length > 50) {
sender.text = [toBeString substringToIndex:50];
[sender resignFirstResponder];
[Loading alertMBProgressHUD:@"超過(guò)50字?jǐn)?shù)的限制！"];
}
}
