This document discusses the visitor pattern and presents the Visitors Framework Generator (VFG). The VFG automatically generates model-specific visitor frameworks from Ecore metamodels. It creates visitor and visitable interfaces as well as default abstract visitor implementations. This avoids duplicating code and separates structure from behavior. The VFG was used to generate visitors for the Eclipse OCL and QVTd modeling projects, creating frameworks for 7 and 10 Ecore metamodels respectively. The visitor pattern and VFG are useful when behavior is unclear or evolving, and allows clients to add specific behavior algorithms without changing structure.

1. Visitors Framework
Generator
Computer Science Department
University of York
Adolfo Sánchez-Barbudo Herrera (asbh500@york.ac.uk)

2. Agenda
● Visitor Pattern
● Visitors Framework Generator
○ Overview
○ Model traversals
○ Visitors framework
○ Example
● Visitors in Eclipse OCL/QVTd
● Conclusions
2

3. Visitor Pattern
● OOP design pattern
● Separate structure from behaviour
2

4. Visitor Pattern II
● Add/Remove/Modify/Replace behaviour
without changing structure.
○ Ideal when working with third party structures.
3

5. Visitor Pattern III
● Main disadvantage
○ It’s not the ideal pattern with evolving structures.
4

6. Visitor Pattern IV
● Visitors applied to modeling languages
5

7. Visitors Framework Generator
● VFG Overview
VFG
Ecore
Model
Visitable
Model
Implementation
Visitor/
Visitable
Interfaces
Abstract
Visitors
Impl.
Model-Specific Visitors
Framework
Manual
Visitors
Generated
Visitors
More
MDE-based
generators
Manual Generated
6

8. Visitor Generation Framework II
● Visitable implementation from Ecore MM
7

9. Visitor Generation Framework III
● Ecore MM specific visitors framework
1. Visitor/Visitable
Interfaces
2. Default Abstract
Implementations
● Contextful Visitor
● Null Visitor
● Extending Visitor
● Delegating Visitor
● ...
8

10. Model traversals
9

11. Model Traverals II
● Traversal in accept method
○ Doomed to the same traversal algorithm.
10
● Traversal in client code
○ Ok, but traversal is part of the algorithm
● Traversal in visitors
○ Best, but more patterns for reuse sake
Traversal
Visitor
Action

12. Visitors Framework
11

13. Visitors Framework II
12

14. Visitors Framework III
13

15. Example
14

16. Visitors Usage in Eclipse OCL/QVTd
● Eclipse OCL
○ 7 Ecore metamodels
○ 48 generated abstract visitors
○ 67 manual visitors
● Eclipse QVTd
○ 10 Ecore metamodels
○ 89 generated abstract visitors
○ 76 manual visitors
15

17. Conclusions
● VFG:
○ Automatically brings the benefits of the visitor
pattern to Ecore-based models
○ Automatically creates a MM-specific framework to
leverage visitors creation
○ It´s a convenient practice when your models have
unclear behaviour or an evolving one
○ It´s a key practice to let clients add their specific
behaviour algorithms
○ A key tool in the development of Eclipse OCL/QVTd
16

18. Thank you very much
Questions ?
doubts, feedback, enquiries, etc. to asbh500@york.ac.uk