处理UILabel根据文字长度和大小自动适应宽度和高度

   //这个frame是初设的，没关系，后面还会重新设置其size。
    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0,0,0,0)];
    label.numberOfLines = 0;
    label.backgroundColor = [UIColor clearColor];
     
    NSDictionary *attributes = @{NSFontAttributeName:[UIFont systemFontOfSize:20],};
     
    NSString *str = @"abcdefg你上课可是你的拿到了";
    CGSize textSize = [str boundingRectWithSize:CGSizeMake(100, 100) options:NSStringDrawingTruncatesLastVisibleLine attributes:attributes context:nil].size;;
     
    [label setFrame:CGRectMake(100, 100, textSize.width, textSize.height)];
    label.textColor = [UIColor greenColor];
    label.text = str;
    [self.view addSubview:label];