CGRect frame = [[UIScreen mainScreen] bounds];
    NSLog(@"frame :%@",frame); 
这样输入是null

NSLog(@"frame :%f",frame.size.width);  这样有值

[[UIScreen mainScreen] bounds].size.width,  [[UIScreen mainScreen] bounds].size.height  

屏幕的宽和高

IOS 获取屏幕尺寸