Apple's relationship with open source is pretty average. In the end, they use open source strategically. If there's no benefit to open-sourcing something, they won't.
Clang, WebKit and Swift are "proper" open source projects, done in the open. But there aren't many of those. They have no open source apps, and most frameworks such as Cocoa, AppKit etc. are completely proprietary.
Some of their open source efforts have been half-hearted; Darwin was enthusiastically embraced by the community, then essentially shut down by Apple. The XNU sources are APL, but there's nothing open about its development. I/O Kit is closed, I believe.
Clang, WebKit and Swift are "proper" open source projects, done in the open. But there aren't many of those. They have no open source apps, and most frameworks such as Cocoa, AppKit etc. are completely proprietary.
Some of their open source efforts have been half-hearted; Darwin was enthusiastically embraced by the community, then essentially shut down by Apple. The XNU sources are APL, but there's nothing open about its development. I/O Kit is closed, I believe.