How would I invert text color on a pdf rendered with CoreGraphics on iOS?

Question

I am using an open-source PDF viewing library (VFR PDF Reader https://github.com/vfr/Reader). I'm trying to implement "night mode" or black background with white text. I can get the background to any color I like but I can't get the text color to change. You can see where you can modify the background color at https://github.com/vfr/Reader/blob/master/Sources/ReaderContentPage.m in the "drawLayer" method. It is simply changing the color of the rectangle the PDF is rendered on.

My question is: is there something I can do to the "context" that would cause the text in the pdf to change color (in my case I want white text)? The line in question looks like this (line 558):

CGContextDrawPDFPage(context, _PDFPageRef); // Render the PDF page into the context

Answer 1

Here is how I solved the night vision:

1)I keep track of user preference (night mode or Day mode) in NSUserDefault

2) in

- (void)drawLayer:(CATiledLayer *)layer inContext:(CGContextRef)context
{

a) retrieve the user preference

NSString *userViewMode = [[NSUserDefaults standardUserDefaults] objectForKey:@"DayOrNightSetting"];

b) set the back ground color of the CGContextRef to white

 CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor);

c) replace CGContextDrawPage(Context,_PDFPageRef) with :

if ([userViewMode isEqualToString:@"Night"]) {
        CGContextSetBlendMode(context, kCGBlendModeDestinationAtop);

        CGContextDrawPDFPage(context, _PDFPageRef);
        CGContextSetBlendMode(context, kCGBlendModeExclusion);
        CGContextDrawPDFPage(context, _PDFPageRef);
    }
    else
    {
       CGContextSetBlendMode(context,kCGBlendModeNormal);
    CGContextDrawPDFPage(context, _PDFPageRef);
    }