iOS 截图保存相册最佳实现，游戏中使用

        UIGraphicsBeginImageContext([[UIApplication sharedApplication] keyWindow].rootViewController.view.frame.size);
        CGContextRef context = UIGraphicsGetCurrentContext();
        [[[UIApplication sharedApplication] keyWindow].layer renderInContext:context];
        UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        UIImageWriteToSavedPhotosAlbum(theImage, self, nil, nil);