What are the arrow brackets "<…>" in an Obj-C class interface for?

Answer 1

The angle brackets indicate a protocol. They're analogous to interfaces in other languages.

Answer 2

Apple documentation reports the use of brackets; see The Objective-C Programming Language on the chapter 4, on "Adopting a Protocol".

Answer 3

You can also use them in code like a cast to tell the complier to expect an object that conforms to a particular protocol.

id <NSFetchedResultsSectionInfo> sectionInfo = [[self.noteFetcher sections] objectAtIndex:section];

Answer 4

The angle brackets in a class interface definition indicates the protocols that you class is conforming to.

A protocol is almost like an interface in Java or C#, with the addition that methods in an Objective-C protocol can be optional.

Additionaly in Objective-C you can declare a variable, argument or instance variable to conform to several protocols as well. Example

NSObject<NSCoding, UITableViewDelegate> *myVariable;

In this case the class must be NSObject or a subclass (only NSProxy and its subclasses would fail), and it must also conform to both NSCoding and UITableViewDelegate protocols.

In Java or C# this would only be possible by actually declaring said class.