Law of Demeter

Transcript

1. Proposal: TheScore Design Talks
2. Design Principles: • Constrain degrees of freedom. • Feedback from Code(e.g. Unit Tests)
3. Law of Demeter
4. What is the Law of Demeter? From an object, one can only call methods of: - The object itself. - An argument of the method. - Any object created within the method. - Any direct properties/fields of the object. - **Global Instances accessible by the object
5. Purpose: Hide Internals to hide change class PushNotifier ... def article_primary_tag_uris push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri) end end
6. What I like about LoD: Code Decomposition
7. The One Dot Rule
9. Case Study class PushNotifier ... def article_primary_tag_uris push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri) end end
10. push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri)
11. push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri) push_alert.article.resource_tags.primary_uris
12. push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri) push_alert.article.resource_tags.primary_uris push_alert.article.primary_tag_uris
13. push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri) push_alert.article.resource_tags.primary_uris push_alert.article.primary_tag_uris push_alert.article_primary_tag_uris
14. push_alert.article.resource_tags.select { |tag| tag.isPrimary? }.map(&:uri) push_alert.article.resource_tags.primary_uris push_alert.article.primary_tag_uris push_alert.article_primary_tag_uris What I Did: push_alert.article_primary_tags.map(&:uri)
15. push_alert.article_primary_tags -> article.primary_tags -> resource_tags.select(&:primary_tag?) -> primary_tag? in ResourceTag* * Actually ResourceTaggable
16. LoD Smells • Private Methods hiding the chain • Delegate Explosion • Or Use Rails Delegate? OR ...
17. Is this giving me design feedback? • Maybe this class has too many associations?(violation of the Single Responsibility Principle) • Pass in data via constructor/method arguments? So someone else above now does what this class is doing?
18. Variations of LoD • Minimalist LoD: Hide structure only? push_alert.article_primary_tags.map(&:uri) • Extended LoD • Hide the structure of your base class
19. References • http://devblog.avdi.org/2011/07/05/demeter-its-not-just-a- good-idea-its-the-law/ • How to Design Programs, 2e
20. The End Let's pool our knowledge.