After years of iteration, the Facebook codebase does not resemble a typical iOS codebase:

• It’s full of C++, Objective-C(++), and Swift.
• It has dozens of dynamically loaded libraries (dylibs), and so many classes that they can’t be loaded into Xcode at once.
• There is almost zero raw usage of Apple’s SDK — everything has been wrapped or replaced by an in-house abstraction.
• The app makes heavy use of code generation, spurred by Buck, our custom build system.
• Without heavy caching from our build system, engineers would have to spend an entire workday waiting for the app to build.
• FBiOS was never intentionally architected this way. The app’s codebase reflects 10 years of evolution, spurred by technical decisions necessary to support the growing number of engineers working on the app, its stability, and, above all, the user experience.

It must be either brilliant to work on or an absolute pain.