iosswiftios-pdfkitapple-pdfkit

iOS PDFKit adjust pdf frame in PDFView

I have a PDF that has pages of different heights that I want to display in a horizontally paged fashion, one page at a time, with no gaps between pages, and the top of each page being aligned with the top of the view. For example:

Desired rendering

I would like to use PDFKit's PDFView, especially since these settings can be achieved by:

let pdfView = PDFView()
pdfView.displayDirection = .horizontal
pdfView.displaysPageBreaks = false
pdfView.usePageViewController(true, withViewOptions: nil)

However, this produces the following:

Actual rendering

This is due to the varying heights of the pages. Each page is centered vertically in the PDFView, and if its height of the page is greater than the height of the view (e.g. Page 2), it is aspect-scaled to fit.

To attempt to solve the first issue, the vertical offset, I have attempted several things. First, and this is gross and I know it's wrong, after inspecting the view hierarchy and finding the private view (a view of class named PDFTextInputView) that the page is rendered within I've tried adjusting its frame's origin.y to 0 whenever the outer-most scrollview (another private view) scrolls. This works in that whenever the next or previous page is scrolling into view, the page is rendered at the top of the view. However, once the user pinches to zoom on that page, its frame is automatically jumps back to being centered. This approach, aside from relying on private view hierarchy, just won't work.

To attempt to solve the "tall page problem" (Page 2) I've tried adjusting the scale factor of the PDFView whenever a new page appears. Sadly, the .PDFViewPageChanged notification only fires after the scroll view ends decelerating, so the page is zoomed out as it's scrolling in and then when I adjust the scale factor it jumps to fit the width of the screen. Then I turned to the .PDFViewVisiblePagesChanged which does fire as the next/previous pages are coming into view (as well as a few more times, which is fine) and this has the same issue as with .PDFViewPageChanged.

Other things I've tried:

Does anyone have any ideas of how I can achieve what I'm after?

Solution

  • New Hack:

    So it turns out, after digging through some more private APIs (yes, this solution is still gross) that there is this magical property that I had completely overlooked on PDFScrollView (a private, internal view) called... 🥁

    🎉 forcesTopAlignment 🎉

    To enable this, find the PDFScrollView in your PDFView and:

    pdfScrollView.setValue(true, forKey: "forcesTopAlignment")

    That does the trick!

    Old Hack:

    After a few days up against the wall I finally figured out a way to get PDFView to behave how I want.

    Disclosure: This solution uses method swizzling and relies on private view hierarchy and could break at any time. That said, it works for now and I'm happy with the solution.

    The full solution is available in this gist. (The meat is in overrideCenterAlign.)

    There is a private method aptly named _centerAlign which vertically centers and scales the pdf pages as they come onto the screen and as they're scaled with the pinch gesture. Swizzling that method allows me to inject my own logic and apply my own transforms to position the pdf view how I'd like (with the top of the page aligned to the top of the view.)

    There are two cases to consider. The "short page" case (pages 1, 3, 4 in the example) and the "tall page" case (page 2 in the example.) In both cases I start by invoking the original implementation of _centerAlign so that it can apply its transforms and manage updating the internal scroll view.

    All of that said, I'm open to cleaner solutions that don't rely on private methods and view hierarchy. If you know of a better way to accomplish this, please post an answer!