State design pattern is a behavioral design pattern that allows an object to alter its behavior when its internal state changes. This pattern is useful when an object needs to change its behavior dynamically based on its state.

In Swift, we can implement the state design pattern using a combination of protocols and classes.

Anatomy of State Design Pattern

The State pattern consists of several key components:

• Context: This is the object whose behavior will change based on its internal state.
• State: This is a protocol that defines a set of methods that represent the different states the context can be in.
• Concrete States: These are the classes that conform to the State protocol and provide the implementation for the methods based on the specific state.

Example Implementation

Let’s say we have a TCPConnection class that can be in one of two states: Connected or Disconnected. We can implement the state pattern as follows:

// State protocol
protocol ConnectionState {
    func connect()
    func disconnect()
}

// Concrete Connected State
class ConnectedState: ConnectionState {
    func connect() {
        print("Already connected")
    }
    
    func disconnect() {
        print("Disconnecting...")
        // Transition to disconnected state
        tcpConnection.state = DisconnectedState()
    }
}

// Concrete Disconnected State
class DisconnectedState: ConnectionState {
    func connect() {
        print("Connecting...")
        // Transition to connected state
        tcpConnection.state = ConnectedState()
    }
    
    func disconnect() {
        print("Already disconnected")
    }
}

// Context class
class TCPConnection {
    var state: ConnectionState
    
    init() {
        state = DisconnectedState()
    }
    
    func connect() {
        state.connect()
    }
    
    func disconnect() {
        state.disconnect()
    }
}

In this example, the TCPConnection class acts as the context, while the ConnectedState and DisconnectedState classes act as concrete states that implement the behavior for the connected and disconnected states, respectively.

Benefits of State Design Pattern

• Simplifies code: State pattern helps in reducing conditional statements by moving the behavior associated with different states into separate classes.
• Easier to maintain: Adding new states and transitions becomes easier as each state is encapsulated in its own class.

State design pattern is a powerful tool for modeling state-based behavior in an object-oriented system, and it can lead to more clean and maintainable code.

References

• Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides