iOS UILabel根据内容自动调整高度

- (void)viewDidLoad
{
 [super viewDidLoad];
 // Do any additional setup after loading the view.
 
 //根据内容自动调整高度
 NSString *str = @"公元前3000年，印度河流域的居民的数字使用就已经比较普遍，居民们采用了十进位制的计算法。";
 UIFont *font = [UIFont systemFontOfSize:13];
 CGSize size = CGSizeMake(200,2000);
 CGRect labelRect = [str boundingRectWithSize:size options:(NSStringDrawingUsesLineFragmentOrigin|NSStringDrawingUsesFontLeading) attributes:[NSDictionary dictionaryWithObject:font forKey:NSFontAttributeName] context:nil];
 
 //UILabel
 UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(50,100, labelRect.size.width, labelRect.size.height)];
 label.text = str;
 label.backgroundColor=[UIColor redColor];
 label.font = [UIFont systemFontOfSize:13];
 label.numberOfLines=0;
 [self.view addSubview: label];
}