保存全屏为image

    CGSize imageSize = [[UIScreen mainScreen] bounds].size;

    UIGraphicsBeginImageContextWithOptions(imageSize, NO,
0);

    CGContextRef context =
UIGraphicsGetCurrentContext();

    for (UIWindow * window
in [[UIApplication sharedApplication] windows]) {

    if (![window respondsToSelector:@selector(screen)] || [window screen] == [UIScreen mainScreen]) {

    CGContextSaveGState(context);

    CGContextTranslateCTM(context, [window center].x, [window center].y);

    CGContextConcatCTM(context, [window transform]);

    CGContextTranslateCTM(context, -[window bounds].size.width*[[window layer] anchorPoint].x, -[window bounds].size.height*[[window layer] anchorPoint].y);

    [[window layer] renderInContext:context];

    CGContextRestoreGState(context);

    }

    }

    UIImage *image =
UIGraphicsGetImageFromCurrentImageContext();