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Chain of Responsibility

Chain of Responsibility in TypeScript

Chain of Responsibility is behavioral design pattern that allows passing request along the chain of potential handlers until one of them handles request.

The pattern allows multiple objects to handle the request without coupling sender class to the concrete classes of the receivers. The chain can be composed dynamically at runtime with any handler that follows a standard handler interface.

Learn more about Chain of Responsibility

Usage of the pattern in TypeScript

Complexity:

Popularity:

Usage examples: The Chain of Responsibility pattern isn’t a frequent guest in a TypeScript program since it’s only relevant when code operates with chains of objects.

Identification: The pattern is recognizable by behavioral methods of one objects indirectly call the same methods in other objects, while all the objects follow the common interface.

Conceptual Example

This example illustrates the structure of the Chain of Responsibility design pattern. It focuses on answering these questions:

  • What classes does it consists of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

index.ts: Conceptual Example

/**
 * The Handler interface declares a method for building the chain of handlers.
 * It also declares a method for executing a request.
 */
interface Handler {
    setNext(handler: Handler): Handler;

    handle(request: string): string;
}

/**
 * The default chaining behavior can be implemented inside a base handler class.
 */
abstract class AbstractHandler implements Handler
{
    private nextHandler: Handler;

    public setNext(handler: Handler): Handler {
        this.nextHandler = handler;
        // Returning a handler from here will let us link handlers in a
        // convenient way like this:
        // monkey.setNext(squirrel).setNext(dog);
        return handler;
    }

    public handle(request: string): string {
        if (this.nextHandler) {
            return this.nextHandler.handle(request);
        }

        return null;
    }
}

/**
 * All Concrete Handlers either handle a request or pass it to the next handler
 * in the chain.
 */
class MonkeyHandler extends AbstractHandler {
    public handle(request: string): string {
        if (request === 'Banana') {
            return `Monkey: I'll eat the ${request}.`;
        }
        return super.handle(request);

    }
}

class SquirrelHandler extends AbstractHandler {
    public handle(request: string): string {
        if (request === 'Nut') {
            return `Squirrel: I'll eat the ${request}.`;
        }
        return super.handle(request);
    }
}

class DogHandler extends AbstractHandler {
    public handle(request: string): string {
        if (request === 'MeatBall') {
            return `Dog: I'll eat the ${request}.`;
        }
        return super.handle(request);
    }
}

/**
 * The client code is usually suited to work with a single handler. In most
 * cases, it is not even aware that the handler is part of a chain.
 */
function clientCode(handler: Handler) {
    const foods = ['Nut', 'Banana', 'Cup of coffee'];

    for (const food of foods) {
        console.log(`Client: Who wants a ${food}?`);

        const result = handler.handle(food);
        if (result) {
            console.log(`  ${result}`);
        } else {
            console.log(`  ${food} was left untouched.`);
        }
    }
}

/**
 * The other part of the client code constructs the actual chain.
 */
const monkey = new MonkeyHandler();
const squirrel = new SquirrelHandler();
const dog = new DogHandler();

monkey.setNext(squirrel).setNext(dog);

/**
 * The client should be able to send a request to any handler, not just the
 * first one in the chain.
 */
console.log('Chain: Monkey > Squirrel > Dog\n');
clientCode(monkey);
console.log('');

console.log('Subchain: Squirrel > Dog\n');
clientCode(squirrel);

Output.txt: Execution result

Chain: Monkey > Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
  Monkey: I'll eat the Banana.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Subchain: Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
  Banana was left untouched.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Chain of Responsibility in Other Languages

Chain of Responsibility in Java Chain of Responsibility in C# Chain of Responsibility in PHP Chain of Responsibility in Python Chain of Responsibility in Ruby Chain of Responsibility in Swift