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Visitor Design Pattern
Video Lecture
| Section | Video Links |
|---|---|
| Visitor Pattern |   |
| Visitor Use Case | |
Overview
...Refer to Book or Videos for extra content.
Terminology
...Refer to Book or Videos for extra content.
Visitor UML Diagram
Source Code
...Refer to Book or Videos for extra content.
./src/visitor/visitor-concept.ts
// The Visitor Pattern Concept
interface IVisitor {
// An interface that custom Visitors should implement
visit(part: Part): void
}
interface IVisitable {
// An interface the concrete objects should implement that allows
// the visitor to traverse a hierarchical structure of objects
accept(visitor: IVisitor): void
}
class Part implements IVisitable {
// a.k.a Element. An Object that can be part of any hierarchy
name: string
value: number
parts: Set<Part>
constructor(name: string, value: number, parent?: Part) {
this.name = name
this.value = value
this.parts = new Set()
if (parent) {
parent.parts.add(this)
}
}
accept(visitor: IVisitor) {
// required by the Visitor that will traverse
this.parts.forEach((part) => {
part.accept(visitor)
})
visitor.visit(this)
}
}
// The Client
// Creating an example object hierarchy.
const Part_A = new Part('A', 101)
const Part_B = new Part('B', 305, Part_A)
const Part_C = new Part('C', 185, Part_A)
const Part_D = new Part('D', -30, Part_B)
// Now Rather than changing the Part class to support custom
// operations, we can utilise the accept method that was
// implemented in the Part class because of the addition of
// the IVisitable interface
class PrintPartNamesVisitor implements IVisitor {
// Create a visitor that prints the part names
visit(part: Part) {
console.log(part.name)
}
}
// Using the PrintPartNamesVisitor to traverse the object hierarchy
Part_A.accept(new PrintPartNamesVisitor())
class CalculatePartTotalsVisitor implements IVisitor {
// Create a visitor that totals the part values
totalValue = 0
visit(part: Part) {
this.totalValue += part.value
}
}
// Using the CalculatePartTotalsVisitor to traverse the
// object hierarchy
const CALC_TOTALS_VISITOR = new CalculatePartTotalsVisitor()
Part_A.accept(CALC_TOTALS_VISITOR)
console.log(CALC_TOTALS_VISITOR.totalValue)
Output
node ./dist/visitor/visitor-concept.js
D
B
C
A
561
Visitor Use Case
...Refer to Book or Videos for extra content.
Visitor Example UML Diagram
Source Code
./src/visitor/client.ts
// The Visitor Pattern Use Case Example
import AbstractCarPart from './abstract-car-part'
import IVisitor from './ivisitor'
class CarBody extends AbstractCarPart {
// A part of the car
}
class Engine extends AbstractCarPart {
// A part of the car
}
class Wheel extends AbstractCarPart {
// A part of the car
}
class Car extends AbstractCarPart {
// A Car with parts
#parts: AbstractCarPart[]
constructor(name: string) {
super(name)
this.#parts = [
new CarBody('Utility Body', 'ABC-123-21', 1001),
new Engine('V8 engine', 'DEF-456-21', 2555),
new Wheel('FrontLeft', 'GHI-789FL-21', 136),
new Wheel('FrontRight', 'GHI-789FR-21', 136),
new Wheel('BackLeft', 'GHI-789BL-21', 152),
new Wheel('BackRight', 'GHI-789BR-21', 152),
]
}
accept(visitor: IVisitor) {
this.#parts.forEach((part) => {
part.accept(visitor)
})
visitor.visit(this)
}
}
class PrintPartsVisitor implements IVisitor {
// Print out the part name and sku
visit(abstractCarPart: AbstractCarPart) {
if (abstractCarPart.sku !== undefined) {
console.log(
`${abstractCarPart.name}\t:${abstractCarPart.sku}\t:${abstractCarPart.price}`
)
}
}
}
class TotalPriceVisitor implements IVisitor {
// Print out the total cost of the parts in the car
totalPrice = 0
visit(abstractCarPart: AbstractCarPart) {
if (abstractCarPart.price !== undefined) {
this.totalPrice += abstractCarPart.price as number
}
}
}
// The Client
const CAR = new Car('DeLorean')
// Print out the part name and sku using the PrintPartsVisitor
CAR.accept(new PrintPartsVisitor())
// Calculate the total prince of the parts using the TotalPriceVisitor
const TOTAL_PRICE_VISITOR = new TotalPriceVisitor()
CAR.accept(TOTAL_PRICE_VISITOR)
console.log(`Total Price = ${TOTAL_PRICE_VISITOR.totalPrice}`)
./src/visitor/abstract-car-part.ts
import IVisitable from './ivisitable'
import IVisitor from './ivisitor'
export default abstract class AbstractCarPart implements IVisitable {
// The Abstract Car Part
#name: string
#sku: string | undefined
#price: number | undefined
constructor(name: string, sku?: string, price?: number) {
this.#name = name
this.#sku = sku
this.#price = price
}
public get name(): string {
return this.#name
}
public set name(value: string) {
this.#name = value
}
public get sku(): string | undefined {
return this.#sku
}
public set sku(value: string | undefined) {
this.#sku = value
}
public get price(): number | undefined {
return this.#price
}
public set price(value: number | undefined) {
this.#price = value
}
accept(visitor: IVisitor): void {
visitor.visit(this)
}
}
./src/visitor/ivisitable.ts
import IVisitor from './ivisitor'
export default interface IVisitable {
// An interface the concrete objects should implement that allows
// the visitor to traverse a hierarchical structure of objects
accept(visitor: IVisitor): void
}
./src/visitor/ivisitor.ts
import AbstractCarPart from './abstract-car-part'
export default interface IVisitor {
// An interface that custom Visitors should implement
visit(abstractCarPart: AbstractCarPart): void
}
Output
node ./dist/visitor/client.js
Utility Body :ABC-123-21 :1001
V8 engine :DEF-456-21 :2555
FrontLeft :GHI-789FL-21 :136
FrontRight :GHI-789FR-21 :136
BackLeft :GHI-789BL-21 :152
BackRight :GHI-789BR-21 :152
Total Price = 4132
Summary
...Refer to Book or Videos for extra content.