Introduction Method dispatch is an algorithm used to select the appropriate method that needs to be invoked upon a call. The primary goal of method dispatch is to provide the program with information on where it can find the executable code for a specific method in memory. Types of Method Dispatch Compiled languages have three types of method dispatch: Static or direct dispatch Table or virtual dispatch Message dispatch Static or direct dispatch Table or virtual dispatch Message dispatch Static Dispatch Static dispatch is the fastest dispatch method in Swift. Since there is no method overriding available, there is only one implementation of the method, and it resides at a single location in memory. We can use static dispatch using keywords such as static ,final, private. static final private Static dispatch is a default method dispatch for the value types since the value types can’t be overridden. Let’s look at some examples: Final Keyword Once we add the final keyword to a class, its methods do not support overriding, and this is when static dispatch comes into play. final // MARK: Final class final class ClassExample { // MARK: Static dispatch func method() { // implementation ... } } // MARK: Final class final class ClassExample { // MARK: Static dispatch func method() { // implementation ... } } Protocol Extension Once you add a default implementation of a protocol using an extension, its dispatch method switches to static dispatch instead of using a Witness Table. // MARK: Prorocol Extension extension ProtocolExample { // MARK: Direct Dispatch func method() { // implementation ... } } class ClassExample2: ProtocolExample {} let classExample2 = ClassExample2() classExample2.method() // MARK: Prorocol Extension extension ProtocolExample { // MARK: Direct Dispatch func method() { // implementation ... } } class ClassExample2: ProtocolExample {} let classExample2 = ClassExample2() classExample2.method() Class Extension When a method is implemented in an extension, it means it can’t be overridden by subclasses. In this case, there is room for static dispatch. // MARK: Example Class Extension class ClassExample3 {} extension ClassExample3 { // MARK: Direct Dispatch func method() { // implementation ... } } let classExample3 = ClassExample3() classExample3.method() // MARK: Example Class Extension class ClassExample3 {} extension ClassExample3 { // MARK: Direct Dispatch func method() { // implementation ... } } let classExample3 = ClassExample3() classExample3.method() Access Control We can’t access a private method outside of the class body. This means that the method can’t be overridden and uses static dispatch. // MARK: Access Control class ClassExample4 { // MARK: Direct Dispatch private func method() { // implementation ... } } // MARK: Access Control class ClassExample4 { // MARK: Direct Dispatch private func method() { // implementation ... } } Table Dispatch Table dispatch is used when we have to deal with inheritance. This is a default type of dispatch used in Swift. Virtual Table For each instance of a class or subclass, a virtual table is created that contains information about implemented methods for each class and stores a reference to the appropriate implementation. The main disadvantage of virtual table dispatch is that it has a lower speed than static dispatch. Let’s look at an example: // MARK: Virtual Table class ParentClass { func method1() {} func methdod2() {} } class ChildClass: ParentClass { override func method1() {} func method3() {} } // MARK: Virtual Table class ParentClass { func method1() {} func methdod2() {} } class ChildClass: ParentClass { override func method1() {} func method3() {} } For each instance, its own virtual table is created as follows: Witness Table A Witness Table is used by protocols and is created for each class that conforms to the protocol. The CPU uses this table to determine where it should look for an appropriate implementation. Each type (value and reference) that conforms to a protocol has its own Protocol Witness Table, which contains pointers to the methods of the type required by the protocol. Let’s look at an example: // MARK: Witness Table Dispatch protocol ProtocolExample { func method1() func method2() } class ClassExample1: ProtocolExample { func method1() {} func method2() {} } class ClassExample2: ProtocolExample { func method1() {} func method2() {} } // MARK: Witness Table Dispatch protocol ProtocolExample { func method1() func method2() } class ClassExample1: ProtocolExample { func method1() {} func method2() {} } class ClassExample2: ProtocolExample { func method1() {} func method2() {} } In this case, a witness table is created for each class: Message Dispatch Message Dispatch is the most dynamic method dispatch style. It looks for an appropriate implementation during runtime. Because it operates during runtime, we can use Method Swizzling to change method implementations. If you want to use message dispatch, you need to add @objc dynamic before a method implementation. @objc dynamic // MARK: Message Dispatch class ClassExample: NSObject { @objc dynamic func method() {} } class SubClassExample: ClassExample { @objc dynamic override func method() {} } let subclass = SubClassExample() subclass.method() // MARK: Message Dispatch class ClassExample: NSObject { @objc dynamic func method() {} } class SubClassExample: ClassExample { @objc dynamic override func method() {} } let subclass = SubClassExample() subclass.method() The implementation of the method is searched for within SubClassExample. If there is no implementation of this method in that class, the search continues in the parent class, and so on until it reaches NSObject. SubClassExample NSObject Let’s combine all the types into a single table: Conclusion In summary, method dispatch in Swift is a critical aspect of code execution, impacting performance and flexibility. By choosing the right dispatch method, developers can optimize their code, ensure adaptability, and leverage Swift’s dynamic features effectively. Understanding and mastering method dispatch is essential for building efficient and adaptable Swift applications.
