UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(10, 10, 0, 0)];
[label setBackgroundColor:[UIColor grayColor]];
[label setFont:[UIFont systemFontOfSize:20]];
label.text = @"北京欢迎您！！！";

//sizeToFit:直接改变了这个label的宽和高，使它根据上面字符串的大小做合适的改变
[label sizeToFit];

NSLog(@"width=%.1f  height=%.1f ", label.frame.size.width, label.frame.size.height);
[self.view addSubview:label];

UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(10, 10, 0, 0)];
[label setBackgroundColor:[UIColor grayColor]];
[label setFont:[UIFont systemFontOfSize:20]];
label.text = @"北京欢迎您！！！";

//sizeThatFits并没有改变原始label的大小
CGSize sizeThatFits = [label sizeThatFits:CGSizeZero];
NSLog(@"sizeThatFits: width=%.1f  height=%.1f", sizeThatFits.width, sizeThatFits.height);

NSLog(@"width=%.1f  height=%.1f", label.frame.size.width, label.frame.size.height);

[self.view addSubview:label];