Ever since the first version of OS X, Apple has regularly released the kernel source code for Macs. In theory, you could learn from it or even build your own projects from it. For iOS, though? Not so simple. Even if you had source code, it wouldn't matter much unless it was optimized for the ARM-based chips that you see in most phones and mobile tablets. However, Apple is shaking things up a bit. The company has quietly posted ARM-friendly source code for the XNU kernels used in iOS and macOS. That's particularly relevant if you're interested in iOS, since you now have code that would theoretically run on an iPhone or iPad. However, it's not quite the breakthrough move it seems at first blush.