字体自适应label大小

UILabel  *topcontent=[[UILabel alloc] initWithFrame:CGRectMake(80, 728, 800, 40)];
 [topcontent setCenter:CGPointMake(self.view.frame.size.height/2, self.view.frame.size.width-20)];
 topcontent.numberOfLines = 1;
 topcontent.adjustsFontSizeToFitWidth = YES;
 topcontent.minimumFontSize = 1.0f;
// topcontent.font=[UIFont systemFontOfSize:24.0];
 topcontent.textColor=[UIColor whiteColor];
 topcontent.textAlignment=UITextAlignmentCenter;
 [topcontent setBackgroundColor:[UIColor clearColor]];

 [self.view addSubview:topcontent];
 [topcontent release];