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README.md

Welcome to SKIMEnabler!

Summary

Skim does not work on Sierra, because Apple seriously fucked up its PDFKit framework, on which Skim relies. A large number of users is affected, and for several months there was no easy way out. Apple, on their side, have not fixed the problem and appear not to care.

Like other people, I have created multiple workarounds for this problem, which all had basically involved somehow using an older version of PDFKit. This has side-effects mostly on the Preview App, which is why we needed to switch back and forth between the versions of PDFKit.

This work here finally solves this problem: It provides for a preconfigured Framework directory structure that contains the working PDFKit, and otherwise only symbolic links, and more importantly it contains an installer script that deploys the workaround and applies the required patches with one simple line.

So if you have your version of Skim in your /Applications folder, and have cloned / downloaded this solution, you can also ignore the rest of this description, open to a Terminal window, go to whereever you have downloaded the solution (where you find a file install.sh), and just execute this command:

./install.sh

My prior work

This is a little patch script (and documentation) that I came up with motivated by Andrea Alberti who contacted me because of the article I had written here, trying to work around the completely fucked up PDFKit implementation that Apple is torturing its customers with - and that, most importantly, prevents the awesome application Skim from working correctly. You can read more about the problem on the Skim forum.

My solution was to create a wrapper script that would, before starting Skim, replace the PDFKit by an older version (of Mavericks), then start Skim, and then replace back the PDFKit by its current version, doing this process:

  1. Swap the default PDFKit with the Mavericks one
  2. Launch Skim
  3. Wait some 5 seconds
  4. Swap the PDFKit back in favor of the default one

The biggest drawback of this approach was that, besides having to switch around the PDFKit all the time, you also needed to leave System Integrity Protection off, all the time.

Andreas Work

Andrea Alberti had the idea of replacing the whole Quartz framework, of which PDFKit is a part, by a patched version which would include the older, working, version of PDFKit - and then to patch Skim to refer to the older version of the Quartz framework, as well as making sure that that older version of the Quartz framework does not suddenly still refer to the newer version.

The drawback of this approach is, basically, that you need to have a full copy of the Quartz framework, while what you really want is to just replace the PDFKit inside it.

My Contribution

This is exactly what I did: I revisited what was needed: Essentially, you have

/System/Library/Frameworks/Quartz.framework

That directory contains the PDFKit Framework:

/System/Library/Frameworks/Quarks.framework/Versions/Current/Frameworks/PDFKit.framework

Which is what we want to replace by the Mavericks version. It also contains the binary

/System/Library/Frameworks/Quarks.framework/Versions/Current/Quartz

which has an absolute reference to

/System/Library/Frameworks/Quarts.framework

Similarly, inside the Skim.app, there are four files which contains an absolute reference to that same location:

Skim.app/Contents/Frameworks/SkimNotes.framework/Versions/A/SkimNotes
Skim.app/Contents/Library/Spotlight/SkimImporter.mdimporter/Contents/MacOS/SkimImporter
Skim.app/Contents/MacOS/Skim
Skim.app/Contents/SharedSupport/skimpdf

What I hence did was to first of all create a Quarks.framework (mind the spelling!) that consists of mostly relative symbolic links to the real Quartz.framework, except for the PDFKit, where it contains the actual Maverics version.

This directory structure is meant to be put next to the original Quartz.framework.

I then shamelessly recycled a little Perl script, patch.pl, which I had found somewhere on the Internet over 10 years ago and have, unfortunately, no reference of the author - and I use that script to patch binary files - i.e. the four files from Skim, as well as the Quartz binary mentioned above, replacing

/System/Library/Frameworks/Quartz.framework

by

/Applications/Skim.app/Contents/Q.framework

Note:

User Kevin L on the Skim forum tried successfully to deploy the Quarks.framework in a location that is not protected by SIP. The only requirement is that the location is of the same string length as the original Quartz.framework. I have hence adapted the install script and am deploying now, unless you configure otherwise, at the top of the script, into the Skim.app directory. I.e., instead of the "Quarks" location I had previously used:

/System/Library/Frameworks/Quarks.framework

I now use

/Applications/Skim.app/Contents/Q.framework

You can configure that at the top of the script, and the script checks for the identical length of the strings.

That binary patch removes the need to use install_name_tool, which Andrea used, and which I did find to not work in all cases: Essentially, we know that the reference to our library is available as plain text in our binaries, so we can replace that string by a different string of exactly the same length (Quarks instead of Quartz). Andrea also saw the need to use a binary patch, and provided a Python script for that purpose; I preferred my Perl script as I know that Perl is going to be available, and also, have seen that script working in all sorts of situations over a decade.

Out of interest, if you want to verify which libraries a binary, e.g., Skim, refers to, you can use the command otool -L Skim.

Finally, I wrote a litte installer, install.sh, which takes away from you the whole deployment process.

Open Points

For some reason, on my system, I have observed that I don't even need to patch the four files from Skim - which appears to make no sense, as they contain absolute references to the original (broken) Quartz.framework. In other words, it was sufficient for me to just deploy Quarks.framework next to Quartz.framework, and that was it. This is something I don't yet have enough information to make sense of, so feel free to investigate. I chose to anyway patch the binaries of Skim, to be on the safe side.

Installation

Once you have downloaded the distribution from the git repository, either using git clone https://github.com/mnott/skimenabler or using the Clone or Download - Download ZIP menu option on the GitHub web page, you should end up with a directory that contains, among other things, a file install.sh.

If you do not have your Skim.app under /Applications, you need to open install.sh in an editor and modify, towards the top of the file, the location of Skim:

#
# Your Location of Skim.app
#
SKIM=/Applications/Skim.app

#
# To work, needs to be the exact same length as
#
#      /System/Library/Frameworks/Quartz.framework
#
QUARKS=/Applications/Skim.app/Contents/Q.framework

#
# Whether to check SIP
#
SIPCHECK=false

#
# Whether to check for being root
#
ROOTCHECK=false

You need to check for SIP only if you deploy beneath a directory structure beneath /System; in that case, and also if your location of Skim.app is not owned by you, you also need to check for being root, and run the script with sudo ./install.sh instead of just ./install.sh.

Then, in a Terminal window, go to the directory where you have install.sh and just run this command (though not strictly necessary, you might want to close Skim before doing this, should you have Skim running):

./install.sh

The program will attempt a large number of verifications, and it will also create backups of any file it is going to patch.

Final Thoughts

Remember to redo the process if you apply a system update to MacOS, or get a new version of Skim. Also, please do check out very much in detail before you attempt to do this on "the next version of MacOS" - as we have no idea whether this process will make any sense there.

Before I patch a new version of Skim, or MacOS, I of course first of all check whether the patch is still needed at all.

Also, please make sure to let Apple know how much annoyed you are.