PRDC12 advanced design patterns

Advanced Design
Patterns &
Anti-Patterns
with Amir Barylko

Amir Barylko Advanced Design Patterns

WHO AM I?

• Software quality expert

• Architect

• Developer

• Mentor

• Great cook

• The one who’s entertaining you for the next hour!

RESOURCES

• Email: amir@barylko.com

• Twitter: @abarylko

• Blog: http://www.orthocoders.com

• Materials: http://www.orthocoders.com/presentations


PATTERNS
What are they?
What are anti-patterns?
Which patterns do you use?


WHAT ARE PATTERNS?

•Software design Recipe
•or Solution
•Should be reusable
•Should be general
•No particular language

ANTI-PATTERNS
• More patterns != better design

• No cookie cutter

• Anti Patterns : Patterns to identify failure

• God Classes

• High Coupling

• Breaking SOLID principles....

• (name some)

WHICH PATTERNS
DO YOU USE?
• Fill here with your patterns:


ADVANCED PATTERNS
Let’s vote!


SOME PATTERNS...

• MVVM • Event Aggregator • Factory

• Chain of resp. • Event Sourcing • Strategy

• Proxy • Test Factory • DTO

• ActiveRecord • Visitor • Page Object

• Repository • Null Object


MODEL VIEW VIEW MODEL

• Separate presentation from business logic (view from model)

• Around the family of MVC, MVP, etc...

• Most useful when binding is available

• Many JS libraries uses it (Knockout.js, Ember.js, etc...)

• WPF it is based on MVVM


THE MODEL

• Represents data

• Could be a one or multiple objects

• Feeds the view model

• However, does not know the view or the view model


THE VIEW

• Represents what is shown to the user

• Desktop window

• HTML Page

• Does not know the model

• Loosely coupled to the the view model, binding does all the
job


THE VIEW MODEL

• Represents all the data that the view needs

• It could be constructed based on on multiple models

• Communicate events to update the view using binding

• Complex UI may require composition of MVVM


WHAT IS BINDING?

• Binding connects a model and a UI that represents that model

• In order to update the UI and the model to match

• Native to the framework or library

• Very related to Subject and Observer

• Also could be an MVC implementation


FOR EACH BINDING
function AppViewModel() {
var self = this;

self.people
{ name:
= ko.observableArray([
'Bert' },
ViewModel
{ name: 'Charles' },
{ name: 'Denise' }
]);

self.addPerson = function() {
self.people.push({ name: "New at " + new Date() });
};

self.removePerson = function() {
self.people.remove(this);
}
}

http://knockoutjs.com/documentation/foreach-binding.html


FOR EACH BINDING
List binding
<h4>People</h4>
<ul data-bind="foreach: people">
<li>
Name at position <span data-bind="text: $index"> </span>:
<span data-bind="text: name"> </span>
<a href="#" data-bind="click: $parent.removePerson">Remove</a>
</li>
</ul>
Data binding
<button data-bind="click: addPerson">Add</button>

Command binding


THE MAGIC
ko.applyBindings(new AppViewModel());

ViewModel View

Adding element Adds <li>

Remove element Removes <li>

Execute function Click <button>


CHAIN OF RESPONSIBILITY

• More than one object may handle a request, and the handler
isn't known a priori.

• The handler should be ascertained automatically.

• You want to issue request to one of several objects without
specifying The receiver explicitly.

• The set of objects that can handle a request should be
speciﬁed dynamically


SnapToX

Next
SnapToY

SnapToPrevious
Next


PROXY

• Avoid creating the object until needed

• Provides a placeholder for additional functionality

• Very useful for mocking

• Many implementations exist (IoC, Dynamic proxies, etc)


GOF


PROXY SEQUENCE


ACTIVERECORD

• Isa Domain Model where classes match very closely the
database structure

• Each table is mapped to class with methods for ﬁnding,
update, delete, etc.

• Each attribute is mapped to a column

• Associations are deduced from the classes


create_table "movies", :force => true do |t|
t.string "title"
t.string "description"
t.datetime "created_at"
t.datetime "updated_at"
end

create_table "reviews", :force => true do |t|
t.string "name"
t.integer "stars"
t.text "comment"
t.integer "movie_id"
t.datetime "created_at"
t.datetime "updated_at"
end

class Movie < ActiveRecord::Base
validates_presence_of :title, :description
has_many :reviews
end
class Review < ActiveRecord::Base
belongs_to :movie
end


movie = Movie.new

movie.all # all records

# filter by title
movie.where(title: 'Spaceballs')

# finds by attribute
movie.find_by_title('Blazing Saddles')

# order, skip some and limit the result
movie.order('title DESC').skip(2).limit(5)

# associations CRUD
movie.reviews.create(name: 'Jay Sherman',
stars: 1,
comment: 'It stinks!')


REPOSITORY

• Mediator between domain and storage

• Acts like a collection of items

• Supports queries

• Abstraction of the storage


http://martinfowler.com/eaaCatalog/repository.html

ANTIPATTERN
CHEAPER BY THE DOZEN


WHAT TO DO?

• Use a criteria or better a queryable result (LINQ)

• Use a factory to return repositories

• Use a UnitOfWork with a factory


EVENT AGGREGATOR
• Manage events using a subscribe / publish mechanism

• Isolates subscribers from publishers

• Decouple events from actual models

• Events can be distributed

• Centralize event registration logic

• No need to track multiple objects


Channel events
from multiple
objects into a
single object to
s i m p l i f y
registration for
clients


MT COMMONS


COUPLED


DECOUPLED


EVENT SOURCING

• Register all changes in the application using events

• Event should be persisted

• Complete Rebuild

• Temporal Query

• Event Replay


http://martinfowler.com/eaaDev/EventSourcing.html

LIST COMPREHENSION

• Syntax Construct in languages

• Describe properties for the list (sequence)

• Filter

• Mapping

• Same idea for Set or Dictionary comprehension


LANGUAGE COMPARISON
• Scala
for (x <- Stream.from(0); if x*x > 3) yield 2*x

• LINQ
var range = Enumerable.Range(0..20);
from num in range where num * num > 3 select num * 2;

• Clojure
(take 20 (for [x (iterate inc 0) :when (> (* x x) 3)] (* 2 x)))

• Ruby
(1..20).select { |x| x * x > 3 }.map { |x| x * 2 }


TEST FACTORY / BUILDER

• Creates an object for testing (or other) purposes

• Assumes defaults

• Easy to conﬁgure

• Fluent interface

• Usually has methods to to easily manipulate the domain


public class When_adding_a_an_invalid_extra_frame
{
[Test]
public void Should_throw_an_exception()
{
// arrange
10.Times(() => this.GameBuilder.AddFrame(5, 4));

var game = this.GameBuilder.Build();

// act & assert
new Action(() => game.Roll(8)).Should().Throw();
}
}

http://orthocoders.com/2011/09/05/the-bowling-game-kata-ﬁrst-attempt/


VISITOR

• Ability to traverse (visit) a object structure

• Different visitors may produce different results

• Avoid littering the classes with particular operations


LINQ EXPRESSION TREE


EXPRESSION VISITOR


AND EXPR EVALUATION


NULL OBJECT

• Represent “null” with an actual instance

• Provides default functionality

• Clear semantics of “null” for that domain


class animal {
public:
virtual void make_sound() = 0;
};

class dog : public animal {
void make_sound() { cout << "woof!" << endl; }
};

class null_animal : public animal {
void make_sound() { }
};

http://en.wikipedia.org/wiki/Null_Object_pattern


FACTORY

• Creates instances by request

• More ﬂexible than Singleton

• Can be conﬁgured to create different families of objects

• IoC containers are closely related

• Can be implemented dynamic based on interfaces

• Can be used also to release “resource” when not needed

interface GUIFactory {
public Button createButton();
}

class WinFactory implements GUIFactory {
public Button createButton() {
return new WinButton();
}
}
class OSXFactory implements GUIFactory {
public Button createButton() {
return new OSXButton();
}
}

interface Button {
public void paint();
}
http://en.wikipedia.org/wiki/Abstract_factory_pattern


STRATEGY

• Abstracts the algorithm to solve a particular problem

• Can be conﬁgured dynamically

• Are interchangeable


http://orthocoders.com/2010/04/

DATA TRANSFER OBJECT

• Simpliﬁes information transfer across services

• Can be optimized

• Easy to understand


http://martinfowler.com/eaaCatalog/dataTransferObject.html


PAGE OBJECT

• Abstract web pages functionality to be used usually in testing

• Each page can be reused

• Changes in the page impact only the implementation, not the
clients


class ProjectListPage
include PageObject

def navigate
visit('/projects')
self
end

def edit(project)
row = find(:xpath, "//td[.='#{project.name}']")
row.parent.click_link('Edit')
ProjectEditPage.new
end

def projects
all(:css, "#projects tr")......
end
end


When /^I go to the projects page$/ do
project_list_page.navigate
end

When /^I activate the project$/ do
project_list_page.
navigate.
edit(current_project).
activate.
save
end


QUESTIONS?


RESOURCES

• Email: amir@barylko.com, @abarylko

• Slides: http://www.orthocoders.com/presentations

• Patterns: Each pattern example has a link


RESOURCES II


RESOURCES III


PRDC12 advanced design patterns

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