I’ve been developing my first mac application, Linked Ideas, where I create documents where I write down ideas and connect them easily. For this application, due the presence of a custom document type I wanted to create a QuickLook plugin to generate previous of this documents.

My first direction to learn about this was apple’s quicklook programming guide, where I learned the basics of the different types of plugins. Unfortunately the examples there were in Objective-C, but my application was written in Swift, even more, my application already contained code to render a document so i wanted to reuse that code for creating this plugin.

Googling around on how to write a quicklook plugin I realized the lack of material (or outdated one) to achieve this task. Also, some tutorials I found online started with Create a new Xcode project for the QuickLook plugin, but in my case I needed to share code and embed the final result of the quicklook plugin into my macOS app.

I tried also to get some examples on the internet about How to develop quicklook plugins with Swift, but according to what I found in apple’s dev forum, I realized this was not possible.

After a lot of struggle, here is a list of things I had to learn to achieve my goal:

  1. What are Xcode targets.
  2. How to share code between targets.
  3. Use Swift code in Objective-C land.
  4. How to use Build Phases to embed the quicklook plugin into the macOS app.

First, to learn about what Xcode targets are I recommend the Introduction to Xcode talk from WWDC 2016. In simple terms, Xcode targets allow you to have different executables as a product of each target, and a Xcode project can contain many targets.

Second, for sharing code within different targets and projects I recommend this very good talk from WWDC 2014 about Sharing code between iOS and macOS, here they briefly mention that for small projects, like mine, is better to include the files in both targets, the target for the macOS app and the target for the Quick Look plugin.

Third, given that the code of my application was in Swift and I wanted to reuse it to generate the QuickLook plugin preview I needed to be able to access Swift from Objective-C, to learn about that I recommend the WWDC talk Swift and Objective-C iteroperability. In my case, since my code was part of the two targets of interest, I just had to declare my swift clases that I wanted exposed to Objective-C as public and finally include the TARGETNAME-Swift.h header in my Objective-C file. When trying to compile my quicklook plugin I found a couple problems whose solution are in here and here

Fourth, I needed a way to embed my QuickLook plugin in my macOS app for convenience of shipment, the QuickLook Programming Guide mentions that plugins can be embedded in the macOS app under the MyApp.app/Contents/Library/QuickLook directory. To achieve this I needed to learn how to use build phases, in particular the copy files build phase. That didn’t work entirely, since I had an issue regarding code signing with the QuickLook plugin, luckily Stack-Overflow came to rescue one more time.

Now you can create your own plugin, right?

If you have been following along with my explanations so far you may get the same feeling that I got when googling around to figure out what to do. I found a lot of just do X and it will work, then I was how do I do X in my case?.

For that reason I created a full-length video tutorial where I go step by step creating the pluging and I show how to perform all the previous step I mentioned about.

Also, since my application is Open Source, you can find the Pull Request where I added this plugin to my App.

I hope this information can help you to create your quick look plugin. For suggestions and fixes to this post you can tweet me to @fespinozacast