Publish- and Subscribe to events using an

Event Aggregator

Lars-Erik KindbladSenior Consultant

Blog: kindblad.com

Agenda

What is the Publish-Subscribe Pattern? Message Publisher Subscriber Event Aggregator and how to create one

Code Sample #1 and #2 without and with publish-subscribeSummary

What is the Publish-Subscribe Pattern?

Publish-Subscribe = Pub/SubMessage/event based patternThe publisher creates and publishes messages through an event

aggregator (message bus) – but don’t program the messages to be sent directly to any subscribers

The subscribers define what messages they want to subscribe to

Event Aggregator

Subscriber Subscriber

Publisher

Message

MessageMessage

Subscribe

What is a Message?

A simple .NET POCO class with or without properties

What is a Publisher?

Any class that wants to create and publish a message

What is a Subscriber?

Any class that wants to do something when a specific message or event occurs

What is an Event Aggregator?

A lightweight message busAccepts a message and publishes it out to only those who subscribes for itCan be synchronous or asynchronousMust have

Publish(message) method

Can have Subscribe(subscriber) method Unsubscribe(subscriber) method

Event Aggregator for the UI

Event Aggregator for non-UI

Code Sample #1

Name: MainForm

Name: NewCustomerControl

Name: ToolbarControl

New requirement

Save button should be disabled when the first name or lastname is empty

NewCustomerControl

ToolbarControl

BtnSave.Enabled = true/false

Dependency graph

Strongly Coupled Code

Control Control

Control

Control

Control

Control

Control Control

Dependency graph for a more complex scenario

Strongly Coupled Code

Dependency graph for an enterprise application

Even more strongly coupled code

Event Aggregator (Pub-Sub)

Control Control

Control

Control

Control

Control

Control Control

A better solution - Loosely coupled code

1. Define the messages

2. Publish the messages

3. Subscribe to- and handle the messages

NewCustomerControl

ToolbarControl Event Aggregator

ToolbarControl

NewCustomerControl

Publish:• DisableSave• EnableSave

BtnSave.Enabled= true/false

Subscribe to & handle:• DisableSave• EnableSave

From strong- to loose coupling

Code Sample #2

New Requirement

The customer should receive an e-mail when the customer has been registered

1. Create a New Class

2. Call EmailCustomer From RegisterCustomer

New Requirement

Send a SMS to the customer when the customer has been registered

1. Create a New Class

2. Call SmsCustomer From RegisterCustomer

Open Closed Principle

“Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”

The current implementation breaks this principlePublish-Subscribe would not break the principle

RegisterCustomer

EmailCustomer SmsCustomer

Event Aggregator

EmailCustomer SmsCustomer

RegisterCustomer

Publish message:CustomerRegistered

Subscribe to & handle the message

Invoke

1. Define the Message

2. Create and Publish the Message

3. Subscribe to the Message

RegisterCustomer

EmailCustomer SmsCustomer

Event Aggregator

EmailCustomer SmsCustomer

RegisterCustomer

Publish message:CustomerRegistered

Subscribe to & handle the message

Invoke

Overview

Summary

Publish & Subscribe + Event Aggregator gives: Code that is loosely coupled Less complex code that is easier to maintain and unit-test

Where to use it?1. UI/Presentation layer2. Other layers when needed

How to use it?1. Download or create your own event aggregator2. Define a message3. Create and publish a message4. Subscribe to the message

QUESTIONS?

The information contained in this presentation is proprietary.© 2012 Capgemini. All rights reserved.

www.capgemini.com

About Capgemini

With more than 120,000 people in 40 countries, Capgemini is one of the world's foremost providers of consulting, technology and outsourcing services. The Group reported 2011 global revenues of EUR 9.7 billion.Together with its clients, Capgemini creates and delivers business and technology solutions that fit their needs and drive the results they want. A deeply multicultural organization, Capgemini has developed its own way of working, the Collaborative Business ExperienceTM, and draws on Rightshore ®, its worldwide delivery model.

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