State

Allows an object to change its behavior when its internal state changes — appears to change class.

The Problem

An object's behavior depends on its current state, leading to large if/else or switch statements scattered throughout methods that all check the current state.

How It Works

›Each state is a separate class that encapsulates behavior for that state
›State transitions happen by replacing the current state object
›Eliminates complex conditional logic scattered across methods
›New states can be added without modifying existing state classes (Open/Closed)
›Context delegates all state-dependent behavior to the current state object

When to Use

✓Object's behavior must change based on its state at runtime

✓Finite state machines: order lifecycle, connection states, player states

✓When state-specific code is duplicated across many conditional branches

✓When transitions between states need to be explicit and well-defined

Real-World Examples

🍎

iOS

UIViewController lifecycle (viewDidLoad → viewWillAppear → viewDidAppear), AVPlayer state

🤖

Android

Fragment backstack, MediaPlayer states (Idle→Initialized→Prepared→Started), ViewModel state flows

🌐

Web

Promise states (pending/fulfilled/rejected), XState FSM, React component lifecycle

Code Examples

Swift

protocol OrderState {
    var label: String { get }
    func next(order: Order)
}

class Order {
    var state: OrderState = PendingState()
    func advance() { state.next(order: self) }
    func status()  { print("Order: \(state.label)") }
}

class PendingState: OrderState {
    var label = "Pending"
    func next(order: Order) { order.state = ProcessingState() }
}
class ProcessingState: OrderState {
    var label = "Processing"
    func next(order: Order) { order.state = ShippedState() }
}
class ShippedState: OrderState {
    var label = "Shipped"
    func next(order: Order) { order.state = DeliveredState() }
}
class DeliveredState: OrderState {
    var label = "Delivered"
    func next(order: Order) { print("Already delivered") }
}

let order = Order()
order.status()   // "Order: Pending"
order.advance(); order.status()   // "Order: Processing"
order.advance(); order.status()   // "Order: Shipped"
order.advance(); order.status()   // "Order: Delivered"