iOS 獲取屏幕尺寸 設置背景圖片

CGRect rx = [ UIScreen mainScreen ].bounds;
    CGFloat scale_screen = [UIScreen mainScreen].scale;
    NSString *imageSize=[NSString stringWithFormat:@"%d_%d",[[NSNumber numberWithFloat:(rx.size.height*scale_screen)] intValue] ,[[NSNumber numberWithFloat:(rx.size.width*scale_screen)] intValue]];
    self.view.backgroundColor=[UIColor colorWithPatternImage:[UIImage imageNamed:[NSString stringWithFormat:@"back_%@.png",imageSize]]];