Protocols primarily provide a group of behaviors unrelated to a specific class, and should be named to describe the group of behaviors. Typically protocol names are in the gerund form, ending in “ing”.
If a protocol will be used in a delegate or data source pattern, append the word Delegate or DataSource to the class name of the object type that calls objects conforming to the protocol.
The Code Naming Basics section of Apple’s Coding Guidelines for Cocoa describes how to name protocols. In general, following Apple’s guidelines is recommended.
// Group of behaviors unrelated to a class
@protocol NSLock; // bad: sounds like a class name
@protocol NSLocking; // good
@protocol XYZViewHidingCoordinator; // bad: sounds like a class name
@protocol XYZViewHiding; // good
@protocol XYZHeaderProvider; // bad: sounds like a class name
@protocol XYZHeading; // good
// Delegate and DataSource patterns
@protocol UITableViewCallbackHandler; // bad: delegate patterns should append Delegate
@protocol UITableViewDelegate; // good: follows delegate pattern and appends Delegate to UITableView
@protocol UITableViewDataProvider; // bad: follows data source pattern but doesn't append DataSource
@protocol UITableViewDataSource; // good: follows data source pattern and appends DataSource