uilable根据内容多少自动改变高度

//将获取的问题内容给quest_str
NSString *quest_str = [NSString stringWithFormat:@""];
lab_quests_content.text = quest_str;

[lab_quests_content setNumberOfLines:0];
[lab_quests_content setLineBreakMode:UILineBreakModeWordWrap];

UIFont *font_lab_quest = [UIFont systemFontOfSize:13.0];
lab_quests_content.font = font_lab_quest;

CGSize size = CGSizeMake(lab_quests_content.frame.size.width, 200);
CGSize labSize = [quest_str sizeWithFont:font_lab_quest constrainedToSize:size lineBreakMode:UILineBreakModeWordWrap];

CGRect frame = lab_quests_content.frame;
frame.size.height = labSize.height;
lab_quests_content.frame = frame;

//answer_view
view_quests_answers.frame = CGRectMake(frame.origin.x, frame.origin.y + labSize.height + 20, view_quests_answers.frame.size.width, view_quests_answers.frame.size.height);