In Objective-C, selectors, methods, and implementations refer to particular aspects of the runtime, although in normal conversation, these terms are often used interchangeably to generally refer to the process of message sending.
Here is how each is described in Apple’s Objective-C Runtime Reference:
- Selector (
typedef struct objc_selector *SEL): Selectors are used to represent the name of a method at runtime. A method selector is a C string that has been registered (or “mapped”) with the Objective-C runtime. Selectors generated by the compiler are automatically mapped by the runtime when the class is loaded . - Method (
typedef struct objc_method *Method): An opaque type that represents a method in a class definition. - Implementation (
typedef id (*IMP)(id, SEL, ...)): This data type is a pointer to the start of the function that implements the method. This function uses standard C calling conventions as implemented for the current CPU architecture. The first argument is a pointer to self (that is, the memory for the particular instance of this class, or, for a class method, a pointer to the metaclass). The second argument is the method selector. The method arguments follow.
The best way to understand the relationship between these concepts is as follows: a class (Class) maintains a dispatch table to resolve messages sent at runtime; each entry in the table is a method (Method), which keys a particular name, the selector (SEL), to an implementation (IMP), which is a pointer to an underlying C function.
To swizzle a method is to change a class’s dispatch table in order to resolve messages from an existing selector to a different implementation, while aliasing the original method implementation to a new selector.
Read More : http://nshipster.com/method-swizzling
