Visitor Pattern By Steve Hayes

•

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melbournepatterns

Technology Business

Visitor
Steve Hayes
Cogent Consulting

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

The Problem

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Show me everything in a consistent manner...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Show me everything in a consistent manner...

... and send it to my friend via email

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Show me everything in a consistent manner...

... and send it to my friend via email

oh, and send it to her phone as well...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Show me everything in a consistent manner...

... and send it to my friend via email

oh, and send it to her phone as well...

and print it.....

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Some analysis music please...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Commonality is in the eye of the beholder

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

With a little casual application of
polymorphism...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Does this look bad to you?

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

The solution

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Visitors

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

“Represent an operation to be performed
on the elements of an object structure.
Visitor lets you deﬁne a new operation
without changing the classes of the
elements on which it operates”

-- GOF

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

A solution to a problem in a context

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Headings, Details, ...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

and the Iterator

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Let’s show it in code!

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Time for visitors...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Melbourne Pushes Boundaries

The Wolves in the Wall

Redhill Consulting Pty Ltd - Ruby on Rails Plugins

Let's talk about tests, baby...

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Melbourne Pushes Boundaries
Tim Colebatch
Melbourne is experiencing its biggest growth surge
since the 1960's

The Wolves in the Wall
12.06

Redhill Consulting Pty Ltd - Ruby on Rails Plugins
Here are some Ruby on Rails plugins we've developed...

Let's talk about tests, baby...
Some long babbling about test naming heuristics
Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Cool!

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

When?

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

“an object structure contains many classes
of objects with differing interfaces, and
you want to perform operations on these
objects that depend on their concrete
classes”

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

“many distinct and unrelated operations
need to be performed on objects in an
object structure, and you want to avoid
‘polluting’ their classes with these
operations.Visitor lets you keep unrelated
operations together by deﬁning them in
one class. When the object structure is
shared by many applications, use Visitor to
put operations in just those applications
that need them”
Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

“the classes deﬁning the object structure
rarely change, but you often want to
deﬁne new operations over the structure.
Changing the object structure classes
requires redeﬁning the interface to all
visitors, which is potentially costly. If the
object structure classes change often, then
it’s probably better to deﬁne the
operations in those classes”

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Consequences

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Visitor makes adding new operations easy

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

A visitor gathers related operations and
separates unrelated ones

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Adding new ConcreteElement classes is
hard

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Visitors can cross object hierarchies

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Visitors can accumulate state

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Visitors may compromise encapsulation

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Discussion

Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au)
Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

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Visitor Pattern By Steve Hayes

1. Visitor Steve Hayes Cogent Consulting Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

2. The Problem Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

3. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

4. Show me everything in a consistent manner... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

5. Show me everything in a consistent manner... ... and send it to my friend via email Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

6. Show me everything in a consistent manner... ... and send it to my friend via email oh, and send it to her phone as well... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

7. Show me everything in a consistent manner... ... and send it to my friend via email oh, and send it to her phone as well... and print it..... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

8. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

9. Some analysis music please... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

10. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

11. Commonality is in the eye of the beholder Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

12. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

13. With a little casual application of polymorphism... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

14. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

15. Does this look bad to you? Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

16. The solution Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

17. Visitors Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

18. “Represent an operation to be performed on the elements of an object structure. Visitor lets you deﬁne a new operation without changing the classes of the elements on which it operates” -- GOF Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

19. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

20. A solution to a problem in a context Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

21. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

22. Headings, Details, ... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

23. and the Iterator Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

24. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

25. Let’s show it in code! Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

26. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

27. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

28. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

29. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

30. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

31. Time for visitors... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

32. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

33. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

34. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

35. Melbourne Pushes Boundaries The Wolves in the Wall Redhill Consulting Pty Ltd - Ruby on Rails Plugins Let's talk about tests, baby... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

36. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

37. Melbourne Pushes Boundaries Tim Colebatch Melbourne is experiencing its biggest growth surge since the 1960's The Wolves in the Wall 12.06 Redhill Consulting Pty Ltd - Ruby on Rails Plugins Here are some Ruby on Rails plugins we've developed... Let's talk about tests, baby... Some long babbling about test naming heuristics Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

38. Cool! Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

39. When? Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

40. “an object structure contains many classes of objects with differing interfaces, and you want to perform operations on these objects that depend on their concrete classes” Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

41. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

42. “many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid ‘polluting’ their classes with these operations.Visitor lets you keep unrelated operations together by deﬁning them in one class. When the object structure is shared by many applications, use Visitor to put operations in just those applications that need them” Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

43. Show me everything in a consistent manner... ... and send it to my friend via email oh, and send it to her phone as well... and print it..... Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

44. “the classes defining the object structure rarely change, but you often want to define new operations over the structure. Changing the object structure classes requires redefining the interface to all visitors, which is potentially costly. If the object structure classes change often, then it’s probably better to define the operations in those classes” Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

45. Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

46. Consequences Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

47. Visitor makes adding new operations easy Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

48. A visitor gathers related operations and separates unrelated ones Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

49. Adding new ConcreteElement classes is hard Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

50. Visitors can cross object hierarchies Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

51. Visitors can accumulate state Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

52. Visitors may compromise encapsulation Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

53. Discussion Copyright Cogent Consulting Pty Ltd (http://www.cogentconsulting.com.au) Licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

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