Explaining the Chain of Responsibility Design Pattern

The chain of responsibility design pattern is a software design pattern in which an object receives a request and passes it along a chain of other objects until the request is handled. This pattern is used to create a chain of objects that can handle a request, and the objects in the chain are organized in a way that allows the request to be handled by the first object that can handle it.

Here is an example of the chain of responsibility design pattern in Swift:

protocol RequestHandler {
    var nextHandler: RequestHandler? { get set }
    func handleRequest(_ request: String)
}

class RequestHandlerA: RequestHandler {
    var nextHandler: RequestHandler?

    func handleRequest(_ request: String) {
        if request == "request-a" {
            print("Request A handled by RequestHandlerA")
        } else {
            nextHandler?.handleRequest(request)
        }
    }
}

class RequestHandlerB: RequestHandler {
    var nextHandler: RequestHandler?

    func handleRequest(_ request: String) {
        if request == "request-b" {
            print("Request B handled by RequestHandlerB")
        } else {
            nextHandler?.handleRequest(request)
        }
    }
}

class RequestHandlerC: RequestHandler {
    var nextHandler: RequestHandler?

    func handleRequest(_ request: String) {
        if request == "request-c" {
            print("Request C handled by RequestHandlerC")
        } else {
            print("Request not handled")
        }
    }
}

let requestHandlerA = RequestHandlerA()
let requestHandlerB = RequestHandlerB()
let requestHandlerC = RequestHandlerC()

requestHandlerA.nextHandler = requestHandlerB
requestHandlerB.nextHandler = requestHandlerC

requestHandlerA.handleRequest("request-a") // Request A handled by RequestHandlerA
requestHandlerA.handleRequest("request-b") // Request B handled by RequestHandlerB
requestHandlerA.handleRequest("request-c") // Request C handled by RequestHandlerC
requestHandlerA.handleRequest("request-d") // Request not handled