iOS UILabel富文本自适应高度

UILabel * chiLabel =[[UILabel alloc]initWithFrame:CGRectMake(10, 100, 200, MAXFLOAT)];

chiLabel.backgroundColor =[UIColor whiteColor];

chiLabel.numberOfLines =0;

[self.view addSubview:chiLabel];

NSString * string =@"Title:需求是这样的，所有的controller上都有一个名字为默认的Btn,点击按钮后，当前显示的界面为下次启动程序时的默认界面；但是原本控制器之间的层次结构是不能改变的例如最初启动程序时根控制器是TabbarController,tabbar下是多个导航控制器，其中一个导航的rootVC是A";

NSMutableParagraphStyle *paraStyle = [[NSMutableParagraphStyle alloc] init];

paraStyle.lineBreakMode = NSLineBreakByCharWrapping;

paraStyle.lineSpacing = 20;

paraStyle.alignment = NSTextAlignmentLeft;

NSDictionary *dic = @{NSFontAttributeName:[UIFont systemFontOfSize:16], NSParagraphStyleAttributeName:paraStyle
};

NSMutableAttributedString *str = [[NSMutableAttributedString alloc] initWithString:string attributes:dic];

[str addAttribute:NSForegroundColorAttributeName value:[UIColor redColor] range:NSMakeRange(0, 6)];

NSLog(@"%lu",(unsigned long)string.length);

chiLabel.attributedText =str;

[chiLabel sizeToFit];