UILabel自适应字体宽度

UILabel* myLable=[[UILabel alloc] initWithFrame:CGRectMake(0, 23, 175, 33)];
                    [myLable setFont:[UIFont fontWithName:@"Helvetica" size:10.0]];
                    [myLable setNumberOfLines:0];
                    [myLable setBackgroundColor:[UIColor clearColor]];
                    [myAdView addSubview:myLable];

    UIFont *font = [UIFont fontWithName:@"Helvetica" size:10.0];
       CGSize size = [text sizeWithFont:font constrainedToSize:CGSizeMake(175.0f, 2000.0f) lineBreakMode:UILineBreakModeWordWrap];
       CGRect rect=myLable.frame;
       rect.size=size;
       [myLable setFrame:rect];
       [myLable setText:text];