State in TypeScript
State is a behavioral design pattern that allows an object to change the behavior when its internal state changes.
The pattern extracts state-related behaviors into separate state classes and forces the original object to delegate the work to an instance of these classes, instead of acting on its own.
Complexity:
Popularity:
Usage examples: The State pattern is commonly used in TypeScript to convert massive switch-base state machines into objects.
Identification: State pattern can be recognized by methods that change their behavior depending on the objects’ state, controlled externally.
Conceptual Example
This example illustrates the structure of the State design pattern and focuses on the following questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
index.ts: Conceptual example
/**
* The Context defines the interface of interest to clients. It also maintains a
* reference to an instance of a State subclass, which represents the current
* state of the Context.
*/
class Context {
/**
* @type {State} A reference to the current state of the Context.
*/
private state: State;
constructor(state: State) {
this.transitionTo(state);
}
/**
* The Context allows changing the State object at runtime.
*/
public transitionTo(state: State): void {
console.log(`Context: Transition to ${(<any>state).constructor.name}.`);
this.state = state;
this.state.setContext(this);
}
/**
* The Context delegates part of its behavior to the current State object.
*/
public request1(): void {
this.state.handle1();
}
public request2(): void {
this.state.handle2();
}
}
/**
* The base State class declares methods that all Concrete State should
* implement and also provides a backreference to the Context object, associated
* with the State. This backreference can be used by States to transition the
* Context to another State.
*/
abstract class State {
protected context: Context;
public setContext(context: Context) {
this.context = context;
}
public abstract handle1(): void;
public abstract handle2(): void;
}
/**
* Concrete States implement various behaviors, associated with a state of the
* Context.
*/
class ConcreteStateA extends State {
public handle1(): void {
console.log('ConcreteStateA handles request1.');
console.log('ConcreteStateA wants to change the state of the context.');
this.context.transitionTo(new ConcreteStateB());
}
public handle2(): void {
console.log('ConcreteStateA handles request2.');
}
}
class ConcreteStateB extends State {
public handle1(): void {
console.log('ConcreteStateB handles request1.');
}
public handle2(): void {
console.log('ConcreteStateB handles request2.');
console.log('ConcreteStateB wants to change the state of the context.');
this.context.transitionTo(new ConcreteStateA());
}
}
/**
* The client code.
*/
const context = new Context(new ConcreteStateA());
context.request1();
context.request2();
Output.txt: Execution result
Context: Transition to ConcreteStateA.
ConcreteStateA handles request1.
ConcreteStateA wants to change the state of the context.
Context: Transition to ConcreteStateB.
ConcreteStateB handles request2.
ConcreteStateB wants to change the state of the context.
Context: Transition to ConcreteStateA.
