FireChat (http://www.opengarden.com/firechat.html) tried to do mesh network messaging, but security is hard to get right in mesh networks, since the data passes to other clients if needed. For example in Tor, it's great use for anonymizing end users, but it's not great for security - I wouldn't use it to send confidential information such as credit card info.
Bluetooth used to be common until vendors started implementing their own file-sharing or just focused on sharing files in the "cloud", which is really inconvenient when there's no internet connection (not to mention, even when there is internet connection, it's really an inefficient process, bandwidth-wise, to send someone right beside you a single file over the internet).
Maybe we should push a standard for local file-sharing so that all vendors will implement it in their mobile operating systems.
Most of the time, I just clean up my desktop and reorganize files. Also, at least once a month, I simply navigate in the finder and delete old, barely used and unneeded files and apps.
If I find my Mac to be significantly slow, I backup files and apps I only need, and do a clean install of macOS.
Developers can now just upload one set of screenshots for an app. Optionally, they can also upload one app preview per device family. This will definitely make the release process of apps a lot faster.
Concurrency is done with libdispatch library, which is also open source. Right now, it's only compatible with OS X, but int their post, they said that "For Linux, Swift 3 will also be the first release to contain the Swift Core Libraries."
Yes, currently. But in the link, it states that "For Linux, Swift 3 will also be the first release to contain the Swift Core Libraries." And GCD (or libdispatch) is part of the Swift Core Libraries :)