UILabel自动换行，自适应高度

NSString *str = @"那些年错过的大雨";
UILabel *label = [[UILabel alloc] init];
 [self.view addSubview:label];

 [label setBackgroundColor:[UIColor clearColor]];
 [label setFont:[UIFont systemFontOfSize:14]];
 [label setLineBreakMode:UILineBreakModeWordWrap];
 CGSize size = [str sizeWithFont:[UIFont systemFontOfSize:14]    constrainedToSize:CGSizeMake(320,500) lineBreakMode:UILineBreakModeWordWrap];  [label
setText:str];
 [label setFrame:CGRectMake(0.0f, 20.0f, size.width, size.height)];
 [label setNumberOfLines:0];