Ios – Fast and Lean PDF Viewer for iPhone / iPad / iOS – tips and hints

apple-pdfkitcalayeriospdfswift

There has been many Questions recently about drawing PDF's.

Yes, you can render PDF's very easily with a UIWebView but this cant give the performance and functionality that you would expect from a good PDF viewer.

You can draw a PDF page to a CALayer or to a UIImage. Apple even have sample code to show how draw a large PDF in a Zoomable UIScrollview

But the same issues keep cropping up.

UIImage Method:

  1. PDF's in a UIImage don't optically
    scale as well as a Layer approach.
  2. The CPU and memory hit on generating
    the UIImages from a PDFcontext
    limits/prevents using it to create a
    real-time render of new zoom-levels.

CATiledLayer Method:

  1. Theres a significant Overhead (time)
    drawing a full PDF page to a CALayer: individual tiles can be seen rendering (even with a tileSize tweak)
  2. CALayers cant be prepared ahead of
    time (rendered off-screen).

Generally PDF viewers are pretty heavy on memory too. Even monitor the memory usage of apple's zoomable PDF example.

In my current project, I'm developing a PDF viewer and am rendering a UIImage of a page in a separate thread (issues here too!) and presenting it while the scale is x1. CATiledLayer rendering kicks in once the scale is >1. iBooks takes a similar double take approach as if you scroll the pages you can see a lower res version of the page for just less than a second before a crisp version appears.

Im rendering 2 pages each side of the page in focus so that the PDF image is ready to mask the layer before it starts drawing.Pages are destroyed again when they are +2 pages away from the focused page.

Does anyone have any insights, no matter how small or obvious to improve the performance/ memory handling of Drawing PDF's? or any other issues discussed here?

EDIT: Some Tips (Credit- Luke Mcneice,VdesmedT,Matt Gallagher,Johann):

  • Save any media to disk when you can.

  • Use larger tileSizes if rendering on TiledLayers

  • init frequently used arrays with placeholder objects, alternitively another design approach is this one

  • Note that images will render faster than a CGPDFPageRef

  • Use NSOperations or GCD & Blocks to prepare pages ahead
    of time.

  • call CGContextSetInterpolationQuality(ctx, kCGInterpolationHigh); CGContextSetRenderingIntent(ctx, kCGRenderingIntentDefault); before CGContextDrawPDFPage to reduce memory usage while drawing

  • init'ing your NSOperations with a docRef is a bad idea (memory), wrap the docRef into a singleton.

  • Cancel needless NSOperations When you can, especially if they will be using memory, beware of leaving contexts open though!

  • Recycle page objects and destroy unused views

  • Close any open Contexts as soon as you don't need them

  • on receiving memory warnings release and reload the DocRef and any page Caches

Other PDF Features:

Documentation

Example projects

Best Solution

I have build such kind of application using approximatively the same approach except :

  • I cache the generated image on the disk and always generate two to three images in advance in a separate thread.
  • I don't overlay with a UIImage but instead draw the image in the layer when zooming is 1. Those tiles will be released automatically when memory warnings are issued.

Whenever the user start zooming, I acquire the CGPDFPage and render it using the appropriate CTM. The code in - (void)drawLayer: (CALayer*)layer inContext: (CGContextRef) context is like :

CGAffineTransform currentCTM = CGContextGetCTM(context);    
if (currentCTM.a == 1.0 && baseImage) {
    //Calculate ideal scale
    CGFloat scaleForWidth = baseImage.size.width/self.bounds.size.width;
    CGFloat scaleForHeight = baseImage.size.height/self.bounds.size.height; 
    CGFloat imageScaleFactor = MAX(scaleForWidth, scaleForHeight);

    CGSize imageSize = CGSizeMake(baseImage.size.width/imageScaleFactor, baseImage.size.height/imageScaleFactor);
    CGRect imageRect = CGRectMake((self.bounds.size.width-imageSize.width)/2, (self.bounds.size.height-imageSize.height)/2, imageSize.width, imageSize.height);
    CGContextDrawImage(context, imageRect, [baseImage CGImage]);
} else {
    @synchronized(issue) { 
        CGPDFPageRef pdfPage = CGPDFDocumentGetPage(issue.pdfDoc, pageIndex+1);
        pdfToPageTransform = CGPDFPageGetDrawingTransform(pdfPage, kCGPDFMediaBox, layer.bounds, 0, true);
        CGContextConcatCTM(context, pdfToPageTransform);    
        CGContextDrawPDFPage(context, pdfPage);
    }
}

issue is the object containg the CGPDFDocumentRef. I synchronize the part where I access the pdfDoc property because I release it and recreate it when receiving memoryWarnings. It seems that the CGPDFDocumentRef object do some internal caching that I did not find how to get rid of.

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