How do blocks work and why is __block needed?
Essence
A block (^{ ... }) is a closure in Objective-C: an object (its isa is one of the block classes) that contains a function pointer and copies of the captured variables. A block can be stored, passed around and called later.
The rest of this answer, with code and interview follow-ups, is in the DevPrep app.
Read the full answer in DevPrep