Creating a Quick Look plugin

27 Jan 2012

I’ve been searching for some decent documentation on how to create a Quick Look plugin. It took me several months before I finally found a way to develop the plugin. With many thanks to a soon to be released book by The Big Nerd Ranch.

For those wondering on what Quick Look does, here is Apple’s explanation.

In this blog post I’ll show you how to…

create a simple Quick Look plugin
support a custom file type
debug your plugin with Xcode

Setup your Xcode project

Now let’s start developing!

Create a new Quick Look project
Change the extension for both GenerateThumbnailForURL.c & GeneratePreviewForURL.c to .m. (This allows you to use Objective-C code and frameworks in these files)

Support an extension

In this example I want to support the .10to1 extension so that I can preview this file as a text file.

Below is an example on what you have to add to the Info.plist to support the .10to1 extension.

<key>UTImportedTypeDeclarations</key>
<array>
  <dict>
    <key>UTTypeIdentifier</key>
    <string>be.10to1.quicklook</string>
    <key>UTTypeDescription</key>
    <string>10to1 document</string>
    <key>UTTypeConformsTo</key>
    <array>
      <string>public.data</string>
    </array>
    <key>UTTypeTagSpecification</key>
    <dict>
      <key>public.filename-extension</key>
      <array>
        <string>10to1</string>
      </array>
    </dict>
  </dict>
</array>	
<key>CFBundleDocumentTypes</key>
<array>
  <dict>
    <key>CFBundleTypeRole</key>
    <string>QLGenerator</string>
    <key>LSItemContentTypes</key>
    <array>
      <string>be.10to1.quicklook</string>
    </array>
  </dict>
</array>

Be aware, the default project template for a Quick Look plugin already has the following line of code included in the plist.

<key>CFBundleDocumentTypes</key>
<array>
  <dict>
    <key>CFBundleTypeRole</key>
    <string>QLGenerator</string>
    <key>LSItemContentTypes</key>
    <array>
      <string>SUPPORTED_UTI_TYPE</string>
    </array>
  </dict>
</array>

If you want to create a plugin for an existing UTI type, you just have to change SUPPORTED_UTI_TYPE string with for example public.png for a PNG image.

But if your plugin supports a non existing UTI type you have to define it in the plist under the UTImportedTypeDeclarations key. If you want to find out to what UTI type your file conforms to, just use the mdsl executable and you’ll find the UTI type(s) in key kMDItemContentTypeTree.

mdls FILENAME

More information can be found here.

Render the preview

This is the view shown after the space button is pressed. This will display the content of the .10to1 file.

Add the Cococa framework
Import <Cocoa/Cocoa.h> in the GeneratePreviewForURL.m file
Add some generation code inside the GeneratePreviewForURL function as done in this file

Render the thumbnail

This is the icon with a generated content that is shown in your Finder window.

Add the WebKit framework
Import <Cocoa/Cocoa.h> & <WebKit/WebKit> in the GenerateThumbnailForURL.m file
Add some generation code inside the GenerateThumbnailForURL function as done in this file

Debugging

But how do we debug a Quick Look plugin? You can’t just run it as an executable. Therefore we use qlmanage provided by Apple.

We first have to copy the plugin to ~/Library/QuickLook after a successfull build, otherwise your plugin will not run correctly with Xcode.

Add a New Copy Files build phase to your target
Select Absolute Path and enter ~/Library/QuickLook as the Subpath
Add the generated file to the files list

Run the plugin using qlmanage.

Because you can’t select qlmanage in the Finder you have to copy the binary to your project root

cp /usr/bin/qlmanage PROJECT_ROOT/.

Edit your scheme and select the Run/Info tab
Select the qlmanage executable in your project root
In the Run/Arguments tab you can add the folling Arguments Passed On Launch to render the preview: