johnnystefan/pattern-state

Skill: Implement State Pattern

Purpose

Allows an object to alter its behavior when its internal state changes. It appears as if the object changed its class.

Procedural Instructions

1. Map Finite States: Identify the possible states and transitions.
2. Declare State Interface: Define methods for all state-dependent actions.
3. Create Concrete State Classes: Extract logic from the original class into these new classes, one per state.
4. Define Context: The original class (Context) stores a reference to a state object and delegates work to it.
5. Manage Transitions: Either the Context or the states themselves can trigger a state change.

TypeScript Example

interface State {
  handle(context: Context): void;
}

class Context {
  constructor(private state: State) {}
  transitionTo(state: State) {
    this.state = state;
  }
  request() {
    this.state.handle(this);
  }
}

class ConcreteStateA implements State {
  handle(context: Context) {
    context.transitionTo(new ConcreteStateB());
  }
}

Payoff

• SRP: Organizes state-specific code into separate classes.
• Simplified Logic: Eliminates massive, complex conditionals based on state flags.