Chain of Responsibility

Passes a request along a chain of handlers; each handler decides whether to handle it or forward it.

The Problem

Multiple objects may handle a request, but you don't know which one ahead of time. You want to avoid coupling the sender to a specific receiver.

How It Works

›Each handler has a reference to the next handler in the chain
›A handler either handles the request or passes it to the next
›The sender does not need to know which handler will process the request
›Handlers can be added, removed, or reordered at runtime
›Request may go unhandled if no handler processes it — decide on a default

When to Use

✓More than one handler can process a request (middleware pipelines)

✓Which handler to use should be determined at runtime

✓Permission checks, validation chains, event bubbling

✓Logging pipelines, request interceptors

Real-World Examples

🍎

iOS

UIResponder chain (touch events bubble up from subview to window), URLProtocol subclasses

🤖

Android

OkHttp Interceptor chain, View.onTouchEvent propagation, Compose event handling

🌐

Web

Express.js middleware (next()), Koa.js, DOM event bubbling, NestJS guards/interceptors

Code Examples

Swift

protocol RequestHandler: AnyObject {
    var next: RequestHandler? { get set }
    func handle(request: Int) -> String?
}

class LowHandler: RequestHandler {
    var next: RequestHandler?
    func handle(request: Int) -> String? {
        guard request > 10 else { return "Low handled \(request)" }
        return next?.handle(request: request)
    }
}

class MediumHandler: RequestHandler {
    var next: RequestHandler?
    func handle(request: Int) -> String? {
        guard request > 50 else { return "Medium handled \(request)" }
        return next?.handle(request: request)
    }
}

class HighHandler: RequestHandler {
    var next: RequestHandler?
    func handle(request: Int) -> String? { "High handled \(request)" }
}

let low = LowHandler(); let med = MediumHandler(); let high = HighHandler()
low.next = med; med.next = high

print(low.handle(request: 5)!)   // "Low handled 5"
print(low.handle(request: 30)!)  // "Medium handled 30"
print(low.handle(request: 99)!)  // "High handled 99"