Knowledge is of two kinds. We know a subject ourselves, or we know where we can find information upon it.
Samuel Johnson, Boswell’s Life of Johnson
No aspect of iOS programming is more important than a fluid and nimble relationship with the documentation. There is a huge number of built-in Cocoa classes, with many methods and properties and other details. Apple’s documentation, whatever its flaws, is the definitive official word on how you can expect Cocoa to behave, and on the contractual rules incumbent upon you in working with this massive framework whose inner workings you cannot see directly.
Your primary access to the documentation is in Xcode, through the documentation window. But there are other forms of documentation and assistance. Quick Help popovers and the Quick Help inspector provide documentation without leaving the code editor. You can examine the code headers, which provide a useful overview and often contain valuable comments; and you can jump quickly to a symbol declaration. Apple provides sample code, and there are lots of additional online resources.
There are two main categories of documentation provided by Apple:
The primary documentation (reference documentation) for Cocoa classes and other symbols is included entirely within Xcode, and is displayed in the documentation window (Window → Documentation and API Reference, or Help → Documentation and API Reference, Command-Shift-0). ...