Composite Design Pattern
Video Lecture
Overview
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Terminology
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Composite UML Diagram
Source Code
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./src/composite/composite-concept.ts
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101 | // The Composite pattern concept
interface ICompositeComponent {
// A component interface describing the common
// fields and methods of leaves and composites
name: string // A name for this component
referenceToParent?: Composite
method(): void // A method each Leaf and composite container should implement
detach(): void // Called before a leaf is attached to a composite
}
class Leaf implements ICompositeComponent {
// A Leaf can be added to a composite, but not a leaf
referenceToParent?: Composite = undefined
name: string
constructor(name: string) {
this.name = name
}
method(): void {
const parent = this.referenceToParent
? this.referenceToParent.name
: 'none'
console.log(`<Leaf>\t\tname:${this.name}\tParent:\t${parent}`)
}
detach(): void {
'Detaching this leaf from its parent composite'
if (this.referenceToParent) {
this.referenceToParent.delete(this)
}
}
}
class Composite implements ICompositeComponent {
// A composite can contain leaves and composites
referenceToParent?: Composite
components: ICompositeComponent[]
name: string
constructor(name: string) {
this.name = name
this.components = []
}
method(): void {
const parent = this.referenceToParent
? this.referenceToParent.name
: 'none'
console.log(
`<Composite>\tname:${this.name}\tParent:\t${parent}\tComponents:${this.components.length}`
)
this.components.forEach((component) => {
component.method()
})
}
attach(component: ICompositeComponent): void {
// Detach leaf / composite from any current parent reference and
// then set the parent reference to this composite
component.detach()
component.referenceToParent = this
this.components.push(component)
}
delete(component: ICompositeComponent): void {
// Removes leaf/composite from this composite this.components
const index = this.components.indexOf(component)
if (index > -1) {
this.components.splice(index, 1)
}
}
detach(): void {
// Detaching this composite from its parent composite
if (this.referenceToParent) {
this.referenceToParent.delete(this)
this.referenceToParent = undefined
}
}
}
// The Client
const LEAF_A = new Leaf('leaf-a')
const LEAF_B = new Leaf('leaf-b')
const COMPOSITE_1 = new Composite('comp-1')
const COMPOSITE_2 = new Composite('comp-2')
// Attach LEAF_A to COMPOSITE_1
COMPOSITE_1.attach(LEAF_A)
// Instead, attach LEAF_A to COMPOSITE_2
COMPOSITE_2.attach(LEAF_A)
// Attach COMPOSITE1 to COMPOSITE_2
COMPOSITE_2.attach(COMPOSITE_1)
// Run the methods that
LEAF_B.method() // not in any composites
COMPOSITE_2.method() // COMPOSITE_2 contains both COMPOSITE_1 and LEAF_A
|
Output
node ./dist/composite/composite-concept.js
<Leaf> name:leaf-b Parent: none
<Composite> name:comp-2 Parent: none Components:2
<Leaf> name:leaf-a Parent: comp-2
<Composite> name:comp-1 Parent: comp-2 Components:0
Composite Use Case
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Composite Example UML Diagram
Source Code
./src/composite/client.ts
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24 | // A use case of the composite pattern.
import File from './file'
import Folder from './folder'
const FILESYSTEM = new Folder('root')
const FILE_1 = new File('abc.txt')
const FILE_2 = new File('123.txt')
FILESYSTEM.attach(FILE_1)
FILESYSTEM.attach(FILE_2)
const FOLDER_A = new Folder('folder_a')
FILESYSTEM.attach(FOLDER_A)
const FILE_3 = new File('xyz.txt')
FOLDER_A.attach(FILE_3)
const FOLDER_B = new Folder('folder_b')
const FILE_4 = new File('456.txt')
FOLDER_B.attach(FILE_4)
FILESYSTEM.attach(FOLDER_B)
FILESYSTEM.dir('')
// now move FOLDER_A and its contents to FOLDER_B
console.log()
FOLDER_B.attach(FOLDER_A)
FILESYSTEM.dir('')
|
./src/composite/file.ts
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24 | import IComponent from './icomponent'
import Folder from './folder'
export default class File implements IComponent {
// The File Class. The files are the leaves
name: string
referenceToParent?: Folder = undefined
constructor(name: string) {
this.name = name
}
dir(indent: string): void {
console.log(`${indent}<FILE> ${this.name}`)
}
detach(): void {
'Detaching this leaf from its parent composite'
if (this.referenceToParent) {
this.referenceToParent.delete(this)
}
}
}
|
./src/composite/folder.ts
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46 | import IComponent from './icomponent'
export default class Folder implements IComponent {
// A composite can contain leaves and composites
referenceToParent?: Folder
name: string
components: IComponent[]
constructor(name: string) {
this.name = name
this.components = []
}
dir(indent: string): void {
console.log(`${indent}<DIR> ${this.name}`)
this.components.forEach((component) => {
component.dir(indent + '..')
})
}
attach(component: IComponent): void {
// Detach leaf / composite from any current parent reference and
// then set the parent reference to this composite
component.detach()
component.referenceToParent = this
this.components.push(component)
}
delete(component: IComponent): void {
// Removes leaf/composite from this composite this.components
const index = this.components.indexOf(component)
if (index > -1) {
this.components.splice(index, 1)
}
}
detach(): void {
// Detaching this composite from its parent composite
if (this.referenceToParent) {
this.referenceToParent.delete(this)
this.referenceToParent = undefined
}
}
}
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./src/composite/icomponent.ts
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14 | import Folder from './folder'
export default interface IComponent {
// A component interface describing the common
// fields and methods of leaves and composites
referenceToParent?: Folder
dir(indent: string): void
// A method each Leaf and composite container should implement
detach(): void
// Called before a leaf is attached to a composite
}
|
Output
node ./dist/composite/client.js
<DIR> root
..<FILE> abc.txt
..<FILE> 123.txt
..<DIR> folder_a
....<FILE> xyz.txt
..<DIR> folder_b
....<FILE> 456.txt
<DIR> root
..<FILE> abc.txt
..<FILE> 123.txt
..<DIR> folder_b
....<FILE> 456.txt
....<DIR> folder_a
......<FILE> xyz.txt
Summary
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