Template Method Design Pattern
Overview
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Terminology
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Template Method UML Diagram
Source Code
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./src/template/template-method-concept.ts
// The Template Method Pattern Concept"
abstract class AbstractClass {
// A template class containing a template method and primitive methods
stepOne(): void {
// Hooks are normally empty in the abstract class. The
// implementing class can optionally override providing a custom
// implementation
}
abstract stepTwo(): void
// An abstract method that must be overridden in the implementing
// class. It has been given `@abstractmethod` decorator so that
// pylint shows the error
stepThree(): void {
// Hooks can also contain default behavior and can be optionally
// overridden
console.log(
'Step Three is a hook that prints this line by default.'
)
}
templateMethod() {
// This is the template method that the subclass will call.
// The subclass(implementing class) doesn't need to override this
// method since it has would have already optionally overridden
// the following methods with its own implementations
this.stepOne()
this.stepTwo()
this.stepThree()
}
}
class ConcreteClassA extends AbstractClass {
// A concrete class that only overrides step two"
stepTwo() {
console.log('Class_A : Step Two (overridden)')
}
}
class ConcreteClassB extends AbstractClass {
// A concrete class that only overrides steps one, two and three"
stepOne() {
console.log('Class_B : Step One (overridden)')
}
stepTwo() {
console.log('Class_B : Step Two. (overridden)')
}
stepThree() {
console.log('Class_B : Step Three. (overridden)')
}
}
// The Client
const CLASS_A = new ConcreteClassA()
CLASS_A.templateMethod()
const CLASS_B = new ConcreteClassB()
CLASS_B.templateMethod()
Output
node ./dist/template-method/template-method-concept.js
Class_A : Step Two (overridden)
Step Three is a hook that prints this line by default.
Class_B : Step One (overridden)
Class_B : Step Two. (overridden)
Class_B : Step Three. (overridden)
Template Method Use Case
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Template Method Use Case UML Diagram
Source Code
./src/template/client.ts
// The Template Pattern Use Case Example
import TextDocument from './text-document'
import HTMLDocument from './html-document'
const TEXT_DOCUMENT = new TextDocument()
TEXT_DOCUMENT.createDocument('Some Text')
const HTML_DOCUMENT = new HTMLDocument()
HTML_DOCUMENT.createDocument('Line 1\nLine 2')
./src/template/abstract-document.ts
// An abstract document containing a combination of hooks and abstract methods
export interface Document {
[id: string]: string
}
export abstract class AbstractDocument {
// A template class containing a template method and primitive methods
document: Document = {}
abstract title(document: Document): void
// Must implement
description?(document: Document): void
// Optional
author?(document: Document): void
// Optional
backgroundColour(document: Document): void {
// Optional with a default behavior
document['bg-col'] = 'white'
}
abstract text(document: Document, text: string): void
// Must implement
footer?(document: Document): void
// Optional
print(document: Document): void {
// Optional with a default behavior"
console.log('----------------------')
Object.keys(document).forEach((attribute: string) => {
console.log(`${attribute}\t: ${document[attribute]}`)
})
console.log()
}
createDocument(text: string): void {
// The template method
this.title(this.document)
if (this.description) this.description(this.document)
if (this.author) this.author(this.document)
this.backgroundColour(this.document)
this.text(this.document, text)
if (this.footer) this.footer(this.document)
this.print(this.document)
}
}
./src/template/text-document.ts
import { Document, AbstractDocument } from './abstract-document'
export default class TextDocument extends AbstractDocument {
title(document: Document): void {
document['title'] = 'New Text Document'
}
text(document: Document, text: string): void {
document['text'] = text
}
footer(document: Document): void {
document['footer'] = '-- Page 1 --'
}
}
./src/template/html-document.ts
// A HTML document concrete class of AbstractDocument
import { Document, AbstractDocument } from './abstract-document'
export default class HTMLDocument extends AbstractDocument {
title(document: Document): void {
document['title'] = 'New HTML Document'
}
text(document: Document, text: string): void {
// Putting multiple lines into there own p tags
const lines = text.split('\n')
let markup = ''
lines.forEach((line) => {
markup = markup + ' <p>' + line + '</p>\n'
document['text'] = markup.substring(0, markup.length - 1)
})
}
print(document: Document): void {
// overriding print to output with html tags
console.log('<html>')
console.log(' <head>')
Object.keys(document).forEach((attribute: string) => {
if (
['title', 'description', 'author'].indexOf(attribute) > -1
) {
console.log(
` <${attribute}>${document[attribute]}</${attribute}>`
)
}
if (attribute === 'bg-col') {
console.log(' <style>')
console.log(' body {')
console.log(
` background-color: ${document[attribute]};`
)
console.log(' }')
console.log(' </style>')
}
})
console.log(' </head>')
console.log(' <body>')
console.log(`${document['text']}`)
console.log(' </body>')
console.log('</html>')
}
}
Output
node ./dist/template-method/client.js
----------------------
title : New Text Document
bg-col : white
text : Some Text
footer : -- Page 1 --
<html>
<head>
<title>New HTML Document</title>
<style>
body {
background-color: white;
}
</style>
</head>
<body>
<p>Line 1</p>
<p>Line 2</p>
</body>
</html>
Summary
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