These slides gives guides you through what the Publish-Subscribe pattern is, how to create an Event Aggregator, how you can use it in the UI and in other layers and 2 code samples that refactors from a non pub-sub architecture to a pub-sub architecture.

1. Publish- and Subscribe to
events using an
Event Aggregator
Lars-Erik Kindblad
Senior Consultant
Blog: kindblad.com

2. 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-subscribe
Summary

3. What is the Publish-Subscribe Pattern?
Publish-Subscribe = Pub/Sub
Message/event based pattern
The 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

4. What is a Message?
A simple .NET POCO class with or without properties

5. What is a Publisher?
Any class that wants to create and publish a message

6. What is a Subscriber?
Any class that wants to do something when a specific message or event
occurs

7. What is an Event Aggregator?
A lightweight message bus
Accepts a message and publishes it out to only those who subscribes for it
Can be synchronous or asynchronous
Must have
 Publish(message) method
Can have
 Subscribe(subscriber) method
 Unsubscribe(subscriber) method

8. Event Aggregator for the UI

9. Event Aggregator for non-UI

10. Code Sample #1

11. Name: MainForm
Name: NewCustomerControl
Name: ToolbarControl

12. New requirement
Save button should be disabled when the first name or lastname is empty

14. NewCustomerControl
ToolbarControl
BtnSave.Enabled = true/false
Dependency graph
Strongly Coupled Code

15. Control Control
Control
Control
Control
Control
Control Control
Dependency graph for a more complex scenario
Strongly Coupled Code

16. Dependency graph for an enterprise application
Even more strongly coupled code

17. Event Aggregator
(Pub-Sub)
Control Control
Control
Control
Control
Control
Control Control
A better solution - Loosely coupled code

18. 1. Define the messages

19. 2. Publish the messages

20. 3. Subscribe to- and handle the messages

21. NewCustomerControl
ToolbarControl Event Aggregator
ToolbarControl
NewCustomerControl
Publish:
• DisableSave
• EnableSave
BtnSave.Enabled
= true/false
Subscribe to & handle:
• DisableSave
• EnableSave
From strong- to loose coupling

22. Code Sample #2

23. New Requirement
The customer should receive an e-mail when the customer has been
registered

24. 1. Create a New Class

25. 2. Call EmailCustomer From RegisterCustomer

26. New Requirement
Send a SMS to the customer when the customer has been registered

27. 1. Create a New Class

28. 2. Call SmsCustomer From RegisterCustomer

29. Open Closed Principle
“Software entities (classes, modules, functions, etc.) should be open for
extension, but closed for modification”
The current implementation breaks this principle
Publish-Subscribe would not break the principle
RegisterCustomer
EmailCustomer SmsCustomer
Event Aggregator
EmailCustomer SmsCustomer
RegisterCustomer
Publish message:
CustomerRegistered
Subscribe to &
handle the
message
Invoke

30. 1. Define the Message

31. 2. Create and Publish the Message

32. 3. Subscribe to the Message

33. RegisterCustomer
EmailCustomer SmsCustomer
Event Aggregator
EmailCustomer SmsCustomer
RegisterCustomer
Publish message:
CustomerRegistered
Subscribe to &
handle the
message
Invoke
Overview

34. 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 layer
2. Other layers when needed
How to use it?
1. Download or create your own event aggregator
2. Define a message
3. Create and publish a message
4. Subscribe to the message

35. QUESTIONS?

36. The information contained in this presentation is proprietary.
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