frame 和bounds
CGRect bounds = [[UIScreen mainScreen] bounds];
//bounds是uiview自身的坐标,frame是父uiview的坐标,center是uiview中心在父uiview(superview)的坐标
int screenWidth = bounds.size.width;
int screenHeight = bounds.size.height;
CGRect frame = CGRectMake((screenWidth-312)/2, (screenHeight-312)/2, 312, 312);//从第一个到第四个分别是: x坐标,y坐标,宽度width,高度height;CGRectMake的参考点是左上角