NSImageRep confusion

Question

I have an NSImage that came from a PDF, so it has one representation, of type NSPDFImageRep. I do an image setDataRetained:YES; to make sure that it remains a NSPDFImageRep. Later, I want to change the page, so I get the rep, and set the current page. This is fine.

The problem is that when I draw the image, only the 1st page comes out.

My impression is that when I draw an NSImage, it picks a representation, and draws that representation. Now, the image only has one rep, so that's the one that is being drawn, and that's the PDFrep. So, why when I draw the image, is it not drawing the correct page?

HOWEVER, when I draw the representation itself, I get the correct page.

What am I missing?

Answer 1

NSImage does a caching of the NSImageRep, when first displayed. In the case of NSPDFImageRep, the "setCacheMode:" message has no effect. Thus, the page that will be displayed will always be the first page. See this guide for more information.

You have then two solutions:

1. Drawing the representation directly.
2. Call the "recache" message on the NSImage to force the rasterization of the selected page.

Answer 2

An alternative mechanism to draw a PDF is to use the CGPDF* functions. To do this, use CGPDFDocumentCreateWithURL to create a CGPDFDocumentRef object. Then, use CGPDFDocumentGetPage to get a CGPDFPageRef object. You can then use CGContextDrawPDFPage to draw the page into your graphics context.

You may have to apply a transform to ensure that the document ends up sized like you want. Use a CGAffineTransform and CGContextConcatCTM to do this.

Here is some sample code pulled out of one of my projects:

// use your own constants here
NSString *path = @"/path/to/my.pdf";
NSUInteger pageNumber = 14;
CGSize size = [self frame].size;

// if we're drawing into an NSView, then we need to get the current graphics context
CGContextRef context = (CGContextRef)([[NSGraphicsContext currentContext] graphicsPort]);

CFURLRef url = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, (CFStringRef)path, kCFURLPOSIXPathStyle, NO);
CGPDFDocumentRef document = CGPDFDocumentCreateWithURL(url);
CGPDFPageRef page = CGPDFDocumentGetPage(document, pageNumber);

// in my case, I wanted the PDF page to fill in the view
// so we apply a scaling transform to fir the page into the view
double ratio = size.width / CGPDFPageGetBoxRect(page, kCGPDFTrimBox).size.width;
CGAffineTransform transform = CGAffineTransformMakeScale(ratio, ratio);
CGContextConcatCTM(context, transform);

// now we draw the PDF into the context
CGContextDrawPDFPage(context, page);

// don't forget memory management!
CGPDFDocumentRelease(document);