Kotlin Design Pattern: Chain of Responsibility

The chain of responsibility pattern is used to process varied requests, each of which may be dealt with by a different handler.

Setup

• Handler: define the interface for handling request, (optional)implements the successor link
• Concrete Handler: handles the request it is responsible for; can access its successor; if the ConcreteHandler can handle this request, then handle it, otherwise it forwards the request to its successor.
• Client: initiates a request to a ConcreteHandler object on the chain.

Example 1

// Step 3: Client code
fun main(args: Array) {
    val arcSoldiers = ArcSoldiers()
    val swordSoldiers = SwordSoldiers()
    val cookSoldiers = CookSoidlers()

    fun orderChain(arcSoldiers: ArcSoldiers,
                   swordSoldiers: SwordSoldiers,
                   cookSoidlers: CookSoidlers): Soldiers{
        arcSoldiers.next = swordSoldiers
        swordSoldiers.next = cookSoidlers
        return arcSoldiers
    }

    val soldiers = orderChain(arcSoldiers, swordSoldiers, cookSoldiers)

    soldiers.onCall("arrows")
    soldiers.onCall("cook")
    soldiers.onCall("swing")
    soldiers.onCall("cannon")
}

// Step 1: Define the request interface

interface Soldiers{
    fun onCall(order: String)
}

// Step 2: ConcreteHandlers to deal with different orders

class ArcSoldiers(var next: Soldiers? = null): Soldiers{
    override fun onCall(order: String) {
        if (order == "arrows")
            println("Aye! Loose the arrows!")
        else
            next?.onCall(order)?: println("General, we are short-handed.")
    }
}

class SwordSoldiers(var next: Soldiers? = null): Soldiers{
    override fun onCall(order: String) {
        if(order == "swing")
            println("Aye! Swing the swords!")
        else
            next?.onCall(order)?: println("General, we are short-handed.")
    }
}


class CookSoidlers(var next: Soldiers? =null): Soldiers{
    override fun onCall(order: String) {
        if (order == "cook")
            println("Aye! Cook the dinners!")
        else
            next?.onCall(order)?: println("General, we are short-handed.")
    }
}

/**
        prints

        Aye! Loose the arrows!
        Aye! Cook the dinners!
        Aye! Swing the swords!
        General, we are short-handed.
 **/

Example 2

// Step 3: Client code

fun main(args: Array) {
    val authenticationHeader = AuthenticationHeader("123456")
    val contentTypeHeader = ContentTypeHeader("json")
    val messageBody = BodyPayload("{\"username\"=\"dbacinski\"}")

    // Here in 'fun messageChainWithAuthorization()', client could define the successors.

    fun messageChainWithAuthorization(authenticationHeader: AuthenticationHeader,
                                      contentTypeHeader: ContentTypeHeader,
                                      messageBody: BodyPayload): MessageChain {
        authenticationHeader.next = contentTypeHeader
        contentTypeHeader.next = messageBody
        return authenticationHeader
    }

    val messageChainWithAuthorization = messageChainWithAuthorization(authenticationHeader,
            contentTypeHeader,
            messageBody)
    val messageWithAuthentication = messageChainWithAuthorization.addLines(
            "Message with Authentication:\n")
    println(messageWithAuthentication)
}

// Step 1: Define the interface for handling request.

interface MessageChain {
    fun addLines(inputHeader: String): String
}

// Step 2: ConcreteHandler to handle specific request.

class AuthenticationHeader(val token: String?, var next: MessageChain? = null) : MessageChain {

    override fun addLines(inputHeader: String): String {
        token ?: throw IllegalStateException("Token should be not null")
        return "$inputHeader Authorization: Bearer $token\n".let { next?.addLines(it) ?: it }
    }
}

class ContentTypeHeader(val contentType: String, var next: MessageChain? = null) : MessageChain {

    override fun addLines(inputHeader: String): String
            = "$inputHeader ContentType: $contentType\n".let { next?.addLines(it) ?: it }
}

class BodyPayload(val body: String, var next: MessageChain? = null) : MessageChain {

    override fun addLines(inputHeader: String): String
            = "$inputHeader $body\n".let { next?.addLines(it) ?: it }
}

/**
        prints

        Message with Authentication:
        Authorization: Bearer 123456
        ContentType: json
        {"username"="dbacinski"}
        **/

NOTE
Safe call operator '?.' & Elvis operator '?:'

// Safe call operator  ?.

/**
        foo?.bar()
        in Kotlin is same as

        if(foo == null)
            return null;
        else
            return foo.bar();
        in Java

        In plain words, if foo is null then returns null, otherwise returns foo.bar()
 **/

// Elvis operator  ?:

/**
        foo?:bar
        in Kotlin is same as

        if(foo != null)
            return foo;
        else
            return bar;
        in Java

        In plain words, if foo is NOT null, return itself, otherwise it returns the
        value after the elvis operator.
 **/


// Combination of safe call operator & elvis operator

/**
        For example

        a == b is same as:

        a?.equals(b)?:(b === null)

        in plain words:
        if a === null, return (b === null), if (b === null), return true;
        if a !== null, return a.equals(b), which is true or false.

 **/