sizeWithFont在ios7.0之后失效

cell.myCourse.text=cell_info.course;

cell.myCourse.font=FONT;

CGSize myCourseSize=[cell.myCourse.textsizeWithAttributes:[NSDictionarydictionaryWithObjectsAndKeys:cell.myCourse.font,NSFontAttributeName,nil]];

cell.myCourse.frame=CGRectMake(cell.myStatusLabel.frame.origin.x, cell.myStatusLabel.frame.origin.y+cell.myStatusLabel.frame.size.height+3,

 myCourseSize.width, myCourseSize.height);

iOS动态修改UILabel的宽度