While Apple has claimed to be a significant contributor to various open-source projects in the past, many of its technologies are still closed source. However, the Cupertino giant has now made a big move by releasing the source code of the XNU kernel's bits for the ARM architecture which powers its flagship mobile operating system, on GitHub.
XNU, which is an abbreviation of 'XNU is Not Unix', is a Unix-like kernel used in macOS, including all the previous versions of the OS, when it was known as OS X, as well as iOS. By giving access to the kernel code, Apple has made it easier for developers to understand how devices and upper layers of software work with the kernel. It is not yet clear how much more the developers can benefit from this release, but it could be one of many steps taken by the company to attract developers.
The source code is supplied with the Apple Public Source License 2.0, which is a rather restrictive license in terms of distribution and code modifications; developers might want to check the details before planning to include parts of the newly released kernel code in their projects. You can find more information about building and testing the XNU kernel on its GitHub repository.