What is the structure of dispatching?

There are three main types of method dispatching in Swift:

1. Direct Dispatch: The fastest. The compiler knows exactly which method to call at compile time. Used by default for structs, classes with final methods, private methods.

   struct MyStruct {
       func directMethod() { // Direct dispatch
           print("Direct")
       }
   }
   

2. Table Dispatch: Used for class methods (not final). Each class has a virtual method table (vtable), where each method has a unique index. The compiler accesses the vtable at runtime to find the correct implementation. Allows polymorphism.

   class MyClass {
       func tableMethod() { // Table dispatch
           print("Table")
       }
   }
   
   class MySubClass: MyClass {
       override func tableMethod() { // Overridden method also uses the table
           print("Table override")
       }
   }
   

3. Message Dispatch: The most flexible but slowest. Used for objc methods (marked @objc dynamic). The Objective-C system looks up the method implementation by its name (selector) at runtime.

   @objc class MyObjcClass: NSObject {
       @objc dynamic func messageMethod() { // Message dispatch (for ObjC, dynamic)
           print("Message")
       }
   }
   

Dispatch Type	Speed	Flexibility (polymorphism, dynamism)	Default application
Direct	Fast	Low	Structs, final classes/methods, private
Table	Medium	Medium	Class methods
Message	Slow	High	@objc dynamic methods (Objective-C)