Decorator

Wraps an object to add new behavior dynamically, without subclassing.

The Problem

You need to add responsibilities to individual objects, not the whole class. Subclassing leads to a combinatorial explosion when features are mixed (e.g. bold + italic + underline text).

How It Works

›Each decorator wraps the component and delegates the core call to it
›Decorators can be stacked in any order — highly composable
›Follows Open/Closed Principle: extend behavior without modifying existing code
›More flexible than inheritance — applied at runtime per object, not per class
›Watch for deep nesting — too many layers can be hard to debug

When to Use

✓Adding behavior to individual objects at runtime without affecting others

✓When subclassing would produce an explosion of classes for every combination

✓When extending with inheritance isn't possible (sealed/final classes)

✓Logging, validation, caching, and compression wrappers

Real-World Examples

🍎

iOS

Combine operators (.map, .filter, .debounce chain), SwiftUI view modifiers

🤖

Android

Compose Modifier chaining, Kotlin extension functions, OkHttp interceptors

🌐

Web

Express middleware, React Higher-Order Components, Redux enhancers

Code Examples

Swift

protocol DataSource {
    func fetchData() -> String
}

class RemoteDataSource: DataSource {
    func fetchData() -> String { "raw data" }
}

class CachingDecorator: DataSource {
    private let wrapped: DataSource
    private var cache: String?
    init(_ ds: DataSource) { wrapped = ds }

    func fetchData() -> String {
        if let cached = cache { print("Cache hit"); return cached }
        let data = wrapped.fetchData()
        cache = data; return data
    }
}

class LoggingDecorator: DataSource {
    private let wrapped: DataSource
    init(_ ds: DataSource) { wrapped = ds }

    func fetchData() -> String {
        print("Fetching data...")
        let data = wrapped.fetchData()
        print("Got: \(data)")
        return data
    }
}

// Stack decorators — order matters
let source: DataSource = LoggingDecorator(CachingDecorator(RemoteDataSource()))
_ = source.fetchData()  // "Fetching..." → cache miss → "Got: raw data"
_ = source.fetchData()  // "Fetching..." → Cache hit → "Got: raw data"