使用相机预览在iphone中拍摄特定视图的屏幕

Question

CGRect screenRect = [[UIScreen mainScreen] bounds];
    UIGraphicsBeginImageContext(screenRect.size);
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    [[UIColor whiteColor] set];
    CGContextFillRect(ctx, screenRect);
    [self.view.layer renderInContext:ctx];
    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return img;

现在我用它来拍摄屏幕截图。它工作正常，但相机预览显示黑色。我需要为IOS6和IOS7拍摄相机预览的屏幕截图。任何建议都非常难以接受..