How Flashlight Works Internally

Note: this is slightly out-of-date, but more or less accurate

The Flashlight.app Xcode target is a fork of EasySIMBL (which is designed to allow loading runtime injection of plugins into arbitrary apps) that's been modified to load a single plugin (stored inside its own bundle, rather than an external directory) into the Spotlight process. It should be able to coexist with EasySIMBL if you use it.

The SIMBL plugin that's loaded into Spotlight, SpotlightSIMBL.bundle, patches Spotlight to add a new subclass of SPQuery, the internal class used to fetch results from different sources. It runs a bundled Python script, which uses commanding to parse queries and determine their intents and parameters, then invokes the appropriate plugin's plugin.py script and presents the results using a custom subclass of SPResult.

Since I'm not sure how to subclass classes that aren't available at link time, subclasses of Spotlight internal classes are made at runtime using Mike Ash's instructions and helper code.

The Spotlight plugin is gated to run only on versions 911-916 (Yosemite GM through 10.10.2 seed). If a new version of Spotlight comes out, you can manually edit SpotlightSIMBL/SpotlightSIMBL/Info.plist key SIMBLTargetApplications.MaxBundleVersion, restarts Spotlight, verify everything works, and then submit a pull request.