Road to iOS Series 9

November 11, 2014

ImageIO - could not find ColorSync function [Solved]

Recently I found an undocumented bug in iOS 8 when I tried to generate pdf under UIkit framework. The bug blocks me from generating pdf in a preferred way using UIPrint, so I had to use a more primitve method to generate the pdf in iOS8.

For better understanding this issue, first I will introduce ways of generating PDF in iOS, and why it may become a problem in iOS8 or above.

  • The first way of generating PDF is using UiKit and core graphics. It provides a set of functions for generating PDF content using native drawing code. However, there are some classes like UIBezierPath provide more high level API for drawing. Truth is, those drawing classes wrap Core Graphics code into their methods to ease drawing for the programmer.
  • Notice, the core graphics is a 2D drawing C API, it deals with lower level of drawing compared to the latter one. And you need to provide frame, margins, translate matrix parameters, number of pages etc in order to generate a user-friendly pdf.

  • For a more detailed examples of generating PDF, visit apple doc at generating PDF content

  • The task I involved is to generate PDF from html string. So I decide to use UIWebView’s viewPrintFormatter and other UIPrint* classes to achieve the goal.
  • However, there is a key method in the UIPrintPageRenderer class causes drawing thread hang up, and never gets returned, here is the code and error message:
// the sixth line causes the problem
for ( int i = 0 ; i < self.numberOfPages ; i++ )
    {
        UIGraphicsBeginPDFPage();

        [self drawPageAtIndex: i inRect: bounds];
    }

// the console output
// *** ImageIO - could not find ColorSync function 'ColorSyncProfileCreateSanitizedCopy'
  • When you start to trace the source of the method call, you will find it’s something worked behind the scence, you can’t even see the source code. So what I end up with doing is using a core graphics approach, avoiding using drawPageAtIndex: call, and figure out a way how to draw multiple pdf pages in pdf.
for (int i = 0 ;i < pages ;i++)
    {
        if (maxHeight * (i + 1) > height)
        {
            // Check to see if page draws more than the height of the UIWebView
            CGRect scrollViewFrame = [scrollView frame];
            scrollViewFrame.size.height -= (((i + 1) * maxHeight) - height);
            [scrollView setFrame:scrollViewFrame];
        }
        // Specify the size of the pdf page
        UIGraphicsBeginPDFPageWithInfo(CGRectMake(0, 0, kPaperWidthA4, kPaperHeightA4), nil);
        CGContextRef currentContext = UIGraphicsGetCurrentContext();

        CGContextTranslateCTM(currentContext, kMargin, -(maxHeight * i) + kMargin);

        [scrollView setContentOffset:CGPointMake(0, maxHeight * i) animated:NO];

        [scrollView.layer renderInContext:currentContext];
    }

Reference

  1. Stanford iOS 7 development
  2. Mac Developer Library