State Design Pattern
Video Lecture
| Section | Video Links |
|---|---|
| State Pattern |   |
| State Use Case | |
Overview
...Refer to Book or Videos for extra content.
Terminology
...Refer to Book or Videos for extra content.
State UML Diagram
Source Code
...Refer to Book or Videos for extra content.
./src/state/state-concept.ts
// The State Pattern Concept
class Context {
// This is the object whose behavior will change
#stateHandles: IState[]
#handle: IState | undefined
constructor() {
this.#stateHandles = [
new ConcreteStateA(),
new ConcreteStateB(),
new ConcreteStateC(),
]
this.#handle = undefined
}
request() {
// A method of the state that dynamically changes which
// class it uses depending on the value of this.#handle
this.#handle = this.#stateHandles[Math.floor(Math.random() * 3)]
return this.#handle
}
}
interface IState {
// A State Interface
toString(): string
}
class ConcreteStateA implements IState {
// A ConcreteState Subclass
toString() {
return 'I am ConcreteStateA'
}
}
class ConcreteStateB implements IState {
// A ConcreteState Subclass
toString() {
return 'I am ConcreteStateB'
}
}
class ConcreteStateC implements IState {
// A ConcreteState Subclass
toString() {
return 'I am ConcreteStateC'
}
}
// The Client
const CONTEXT = new Context()
console.log(CONTEXT.request())
console.log(CONTEXT.request())
console.log(CONTEXT.request())
console.log(CONTEXT.request())
console.log(CONTEXT.request())
Output
node ./dist/state/state-concept.js
ConcreteStateB {}
ConcreteStateA {}
ConcreteStateC {}
ConcreteStateA {}
ConcreteStateC {}
State Use Case
...Refer to Book or Videos for extra content.
State Example Use Case UML Diagram
Source Code
./src/state/client.ts
// The State Use Case Example
enum ExampleState {
Initializing = 'Initializing',
Started = 'Started',
Running = 'Running',
Finished = 'Finished',
}
interface IExampleState {
// A State Interface
request(): void
}
class StateContext implements IExampleState {
#state: ExampleState
constructor() {
this.#state = ExampleState.Initializing
}
public get state() {
return this.#state
}
public set state(value: ExampleState) {
switch (value) {
case ExampleState.Started:
this.request = Started.prototype.request
break
case ExampleState.Running:
this.request = Running.prototype.request
break
case ExampleState.Finished:
this.request = Finished.prototype.request
break
}
this.#state = value
}
request() {
// Does nothing until state changes, when then
// this method handle is reassigned to a different
// concrete states request method
}
}
class Started implements IExampleState {
// A ConcreteState Subclass
request() {
console.log(`I am now Started`)
}
}
class Running implements IExampleState {
// A ConcreteState Subclass
request() {
console.log(`I am now Running`)
}
}
class Finished implements IExampleState {
// A ConcreteState Subclass
request() {
console.log(`I am now Finished`)
}
}
// The Client
const STATE_CONTEXT = new StateContext()
console.log('STATE_CONTEXT = ' + STATE_CONTEXT.state)
STATE_CONTEXT.state = ExampleState.Started
STATE_CONTEXT.request()
STATE_CONTEXT.state = ExampleState.Running
STATE_CONTEXT.request()
STATE_CONTEXT.state = ExampleState.Finished
STATE_CONTEXT.request()
STATE_CONTEXT.state = ExampleState.Started
STATE_CONTEXT.request()
STATE_CONTEXT.state = ExampleState.Running
STATE_CONTEXT.request()
STATE_CONTEXT.state = ExampleState.Finished
STATE_CONTEXT.request()
Output
node ./dist/state/client.js
STATE_CONTEXT = Initializing
I am now Started
I am now Running
I am now Finished
I am now Started
I am now Running
I am now Finished
Summary
...Refer to Book or Videos for extra content.