Ask HN: What are some must know xcode / obj-c topics / libraries?

I'm trying to step up my iOS game to be competent + have the ability to use the appropriate libraries / tools when building any type of iOS app.

Some topics + tools that I've already used w/ obj-c + xcode:

-auto-layouts

-cocoapods

-afnetworking

  -post / get 

-json handling

-location manager

-mantle

-RNCryptor

-singletons

-blocks

-Segues

-kiwi testing

-spritebuilder

What are some other must know topics / libraries / tools (e.g. webview optimization tips, running an app in the background etc..)?