Cannot create PDF document with 400+ pages on iOS

Question

I am using the following pseudocode to generate a PDF document:

CGContextRef context = CGPDFContextCreateWithURL(url, &rect, NULL);

for (int i = 1; i <= N; i++)
{
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
  CGContextBeginPage(context, &mediaBox);

  // drawing code

  CGContextEndPage(context);
  [pool release];
}

CGContextRelease(context);

It works very well with small documents (N < 100 pages), but it uses too much memory and crashes if the document has more than about 400 pages (it received two memory warnings before crashing.) I have made sure there were no leaks using Instruments. What's your advice on creating large PDF documents on iOS? Thanks a lot.

edit: The pdf creation is done in a background thread.

Answer 1

Since you're creating a single document via CGPDFContextCreateWithURL the entire thing has to be held in memory and appended to, something that commonly (though I can't say for certain with iOS and CGPDFContextCreateWithURL) requires a full before and after copy of the document to be kept. No need for a leak to create a problem, even without the before-and-after issue.

If you aren't trying to capture a bunch of existing UIKit-drawn stuff -- and in your sample it seems that you're not -- use the OS's printing methods instead, which offer built-in support for printing to a PDF. UIGraphicsBeginPDFContextToFile writes the pages out to disk as they're added so the whole thing doesn't have to be held in memory at once. You should be able to generate a huge PDF that way.