Compare revisions

…

Chain of Responsibility Pattern

Body changes between the two selected stored revisions. Code is not executed.

5 changed sections

− Removed+ Added20 unchanged content blocks omitted.

Content blocks 5 → 5

The text is unchanged, but footnotes, formatting or block attributes have changed.

Content blocks 14 → 14

using System; using System.Collections.Generic; using System.Linq; Handler chain = new BillingHandler(new TechHandler(null)); Reply result = chain.Handle(new Ticket("billing")); Console.WriteLine(result.Handled + ": " + result.Message); sealed record Ticket(string Kind); sealed record Reply(bool Handled, string Message); abstract class Handler(Handler? next) { public Reply Handle(Ticket ticket) { Reply? result = TryHandle(ticket); if (result is not null) return result; return next?.Handle(ticket) ?? new Reply(false, "unhandled: " + ticket.Kind); } protected abstract Reply? TryHandle(Ticket ticket); } sealed class BillingHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "billing" ? new Reply(true, "billing desk") : null; } sealed class TechHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "technical" ? new Reply(true, "technical desk") : null; }

using System; using System.Collections.Generic; using System.Linq; Handler chain = new BillingHandler(new TechHandler(null)); Reply result = chain.Handle(new Ticket("billing")); Console.WriteLine(result.Handled + ": " + result.Message); sealed record Ticket(string Kind); sealed record Reply(bool Handled, string Message); // Keep acceptance rules in handlers and share the forwarding flow in the base class. abstract class Handler(Handler? next) { public Reply Handle(Ticket ticket) { Reply? result = TryHandle(ticket); if (result is not null) return result; // null means this handler declines the ticket, so pass it to the next handler. // If there is no successor, return Handled=false for the whole chain. return next?.Handle(ticket) ?? new Reply(false, "unhandled: " + ticket.Kind); } protected abstract Reply? TryHandle(Ticket ticket); } // Decline with null without choosing the next handler here. sealed class BillingHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "billing" ? new Reply(true, "billing desk") : null; } sealed class TechHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "technical" ? new Reply(true, "technical desk") : null; }

Content blocks 19 → 19

using System; using System.Collections.Generic; using System.Linq; Handler chain = new BillingHandler(new TechHandler(null)); foreach (string kind in new[] { "technical", "other" }) { Reply result = chain.Handle(new Ticket(kind)); Console.WriteLine(result.Handled + ": " + result.Message); } sealed record Ticket(string Kind); sealed record Reply(bool Handled, string Message); abstract class Handler(Handler? next) { public Reply Handle(Ticket ticket) { Reply? result = TryHandle(ticket); if (result is not null) return result; return next?.Handle(ticket) ?? new Reply(false, "unhandled: " + ticket.Kind); } protected abstract Reply? TryHandle(Ticket ticket); } sealed class BillingHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "billing" ? new Reply(true, "billing desk") : null; } sealed class TechHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "technical" ? new Reply(true, "technical desk") : null; }

using System; using System.Collections.Generic; using System.Linq; Handler chain = new BillingHandler(new TechHandler(null)); foreach (string kind in new[] { "technical", "other" }) { Reply result = chain.Handle(new Ticket(kind)); Console.WriteLine(result.Handled + ": " + result.Message); } sealed record Ticket(string Kind); sealed record Reply(bool Handled, string Message); // Keep acceptance rules in handlers and share the forwarding flow in the base class. abstract class Handler(Handler? next) { public Reply Handle(Ticket ticket) { Reply? result = TryHandle(ticket); if (result is not null) return result; // null means this handler declines the ticket, so pass it to the next handler. // If there is no successor, return Handled=false for the whole chain. return next?.Handle(ticket) ?? new Reply(false, "unhandled: " + ticket.Kind); } protected abstract Reply? TryHandle(Ticket ticket); } // Decline with null without choosing the next handler here. sealed class BillingHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "billing" ? new Reply(true, "billing desk") : null; } sealed class TechHandler(Handler? next) : Handler(next) { protected override Reply? TryHandle(Ticket ticket) => ticket.Kind == "technical" ? new Reply(true, "technical desk") : null; }

Content blocks none → 22-26

(No content)

TypeScript example Intake code sends a request to the first handler; each handler decides whether to accept it. This separates handler selection from the intake screen, allowing the chain’s order and composition to change. type Ticket = Readonly<{ kind: string }>; type Reply = Readonly<{ handled: boolean; message: string }>; // Keep acceptance rules in handlers and share the forwarding flow in the base class. abstract class Handler { constructor(private readonly next: Handler | null) {} handle(ticket: Ticket): Reply { const reply = this.tryHandle(ticket); if (reply !== null) return reply; return this.next?.handle(ticket) ?? { handled: false, message: "unhandled: " + ticket.kind }; } protected abstract tryHandle(ticket: Ticket): Reply | null; } // Decline with null without choosing the next handler here. class BillingHandler extends Handler { protected override tryHandle(ticket: Ticket): Reply | null { return ticket.kind === "billing" ? { handled: true, message: "billing desk" } : null; } } class TechHandler extends Handler { protected override tryHandle(ticket: Ticket): Reply | null { return ticket.kind === "technical" ? { handled: true, message: "technical desk" } : null; } } const chain = new BillingHandler(new TechHandler(null)); for (const kind of ["billing", "technical", "other"]) { const result = chain.handle({ kind }); console.log(result.handled + ": " + result.message); } true: billing desk true: technical desk false: unhandled: other null means that the current handler declined; handled=false is the final outcome at the end of the chain. The example stops at the first handler that responds.

Content blocks 24 → 29-30

Each example is a separate .NET 10 console program.

Each C# example is a separate .NET 10 console program. Compile the TypeScript examples for ES2022 or later and run the emitted JavaScript with Node.js.