@russolsen

Design Patterns in

Ruby

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2

FORTRAN

BASIC

Pascal

Python

C++C

Java

RubyClojureGo

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4

Design Patterns What are they?

Some Examples

The Good & Bad

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What is a

Design Pattern?

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7

Prepackaged Solution

Common Problem

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9Main Street

10Main Street??

11Main Street??

12

h

Main Street

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2hh

Main Street

Name: Main Street

Problem: Open Feeling

Solution: Width = 2 * Ht

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1) Prepackaged Solutions

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Why Bother?

1) Prepackaged Solutions

2) Examples of Good Design

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Why Bother?

1) Prepackaged Solutions

2) Examples of Good Design

3) Big Ideas

!

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Why Bother?

Some Examples

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Name: Template Method

Problem:

Solution:

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Name: Template Method

Problem:Varying details

Solution:

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Documents &

Rendering 21

template*.rb

22Template Method

Document!def render render_title(title) text.each_line do|l| render_line(l) end ...

PlainDocumentdef render_title(t) end !def render_line(l) end

HtmlDocumentdef render_title(t) end !def render_line(l) end

23Abstract Animals

Mammal

Lion Squirrel

Name: Template Method

Problem:Varying details

Solution: Push the details into a subclass

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Big Ideas 1) Abstractions!

2) Separate out change

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The Problem is

!

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The Problem is

Commitment 27

# I start off fine!

doc1 = HtmlDocument.new(…)

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# I start off fine!

doc1 = HtmlDocument.new(…)

# But then I need to render doc1 in plain text…

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# I start off fine!

doc1 = HtmlDocument.new(…)

# But then I need to render doc1 in plain text…

doc2 = PlainDocument.new(doc1.title, doc1.text)

doc2.render

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Try Again

31strategy.rb

Strategy Pattern

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33Strategy

Document

def render @renderer.render(self) end

HtmlRendererdef render(doc) puts “<title>” puts @doc.title puts “</title>” … end

PlainRendererdef render(doc) puts doc.title ... end

34Strategy

Document

def render @renderer.render(self) end

HtmlRendererdef render(doc) puts “<title>” puts @doc.title puts “</title>” … end

PlainRenderdef render(doc) puts doc.title ... end

35Helpers

Homeowner

Electrician

Plumber

Name: Strategy

Problem:Varying details

Solution: Push the whole job into a separate object

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Big Ideas 1) Abstractions!

2) Separate out change

3) Composition over inheritance

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A Different Problem

38Obs*.rb

Observer Pattern

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40Observer Pattern

Document

def text=(t) @observers.each {…} end

Viewdef on_doc_change(doc) puts “<title>” puts @doc.title puts “</title>” … end

Databasedef on_doc_change(doc) # Save the doc ... end

41Observer Pattern

42Observer Pattern

43Observer Pattern

Amazon

Clinton

Russ

44Observer Pattern

Document

def text=(t) @observers.each {…} end

45Black Boxes

on_doc_change(doc) on_doc_change(doc)

Big Ideas 1) Abstractions!

2) Separate out change

3) Composition not inheritance

4) Interface not implementation

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Good Bad

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Good Orange Beatles

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Bad Rampaging Patterns

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52Strategy

Document

def render @renderer.render(self) end

HtmlRendererdef render(doc) puts “<title>” puts @doc.title puts “</title>” … end

PlainRenderdef render(doc) puts doc.title ... end

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Renderer

Document

HtmlRenderer PlainRenderer

Strategy + Template

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RendererDocument

HtmlRenderer PlainRenderer

Observer

Observer

Observer

Observer

Strategy + Insanity

Pain First

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Big Ideas 1) Abstractions!

2) Separate out change

3) Composition not inheritance

4) Interface not implementation

5) Pain first, then the pattern56

Summary

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Prepackaged Solution

Common Problem

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59Template Method

Document!def render render_title(title) text.each_line do|l| render_line(l) end ...

PlainDocumentdef render_title(t) end !def render_line(l) end

HtmlDocumentdef render_title(t) end !def render_line(l) end

60Strategy

Document

def render @renderer.render(self) end

HtmlRendererdef render(doc) puts “<title>” puts @doc.title puts “</title>” … end

PlainRenderdef render(doc) puts doc.title ... end

61Observer Pattern

Document

def text=(t) @observers.each {…} end

Viewdef on_doc_change(doc) puts “<title>” puts @doc.title puts “</title>” … end

Databasedef on_doc_change(doc) # Save the doc ... end

Big Ideas 1) Abstractions!

2) Separate out change

3) Composition not inheritance

4) Interface not implementation

5) Pain first, then the pattern62

Questions?

63

russ@russolsen.com @russolsen

!

cognitect.com

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