UILabel自适应高度

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 UILabel *instructions=[[UILabel alloc]initWithFrame:CGRectMake(10, 10, 300, 180)];
     NSString *text = @"这是个测试，不信你试试！这是个测试，再不信你copy代码！这是个测试，果然！自适应高度了！";
     instructions.text = text;
     instructions.lineBreakMode = NSLineBreakByWordWrapping; //这个是关键
     instructions.numberOfLines = 0;  //这个是关键
     [instructions setTextColor:[UIColor grayColor]];
     CGSize size = [instructions.text sizeWithFont:[UIFont systemFontOfSize:20.0] constrainedToSize:CGSizeMake(300.0f,CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping];

     CGRect newFrame = instructions.frame;

     newFrame.size.height = size.height;
     instructions.frame = newFrame;
     [instructions sizeToFit];

     [self.view addSubview:instructions];