1. 1 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson How to Develop Frameworks Ralph E. Johnson Dept. of Computer Science 1304 W. Springfield Ave. Urbana, IL 61801 http://st-www.cs.uiuc.edu/users/johnson/ johnson@cs.uiuc.edu (217) 244-0093

2. 2 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson What is a Framework? Framework: is reusable design of an application or subsystem is represented by a set of abstract classes and the way objects in those classes collaborate. is overly simple program that can be extended to make real program. tells how to decompose a problem into objects. is not just the classes, but the way instances of the classes collaborate.

3. 3 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson What is a Framework? Just a program - but one that can be easily extended. Complex pattern - made up of several simpler design patterns. Inversion of control -- "don't call us, we'll call you".

4. 4 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson How to Use a Framework Use framework to build application by: Creating new subclasses Configuring objects together Modifying working examples (editing scripts).

5. 5 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Parts of a system Framework Framework (abstract classes) Library Application Concrete subclasses Other classes Script (in programming language or in XML)

6. 6 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Reasons to Use Frameworks Framework used to simplify existing code to build applications quickly to make applications more consistent to meet standard to make maintenance easier

7. 7 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Disadvantages of Frameworks More complex (powerful) than necessary. Slower and larger than necessary. Takes time to learn. Difficult to develop

8. 8 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson How to Develop Frameworks Start with applications. Refactor to simplify designs and share code. Build more applications. Refactor to simplify designs and share code. Build more applications. … When new applications no longer require refactoring shared code, the framework is done.

9. 9 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson First Rule of Framework Design Don't. You don’t really need it. Don't. Use an existing one. or

10. 10 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Frameworks and XP Is framework the simplest thing that could possibly work? Yes - if you already know it. No - if you have to design it. Yes - if you are building 20 similar applications. No - if it doesn’t quite do what you want and it is not clear how to change it. Only use a framework when it is cost effective. Never know for sure Include learning, documentation

11. 11 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Theme for Designing Frameworks Framework design is derived from application design. Design frameworks to be overly simple and concrete, and refactor to be more reusable. Refactoring: Improving the Design of Existing Code, Martin Fowler, Addison-Wesley 1999. Design Patterns: Elements of Reusable Object-Oriented Software, Gamma, Helms, Johnson, Vlissides, Addison- Wesley 1995.

12. 12 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Relevant Principles Frameworks are abstractions: people generalize from concrete examples Designing reusable code requires iteration Frameworks encode domain knowledge Customer of framework is application programmer

13. 13 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Generalize from Concrete Cases People think concretely, not abstractly. Abstractions are found bottom-up, by examining concrete examples. Generalization proceeds by finding things that are given different names but are really the same, parameterizing to eliminate differences, breaking large things into small things so that similar components can be found, and categorizing things that are similar.

14. 14 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Frameworks are Generalizations Frameworks are OO programs generalized to fit many examples. Common ideas become abstract classes. Objects parameterized to make them reusable. Ad-hoc systems broken down into smaller parts. Use inheritance to organize class library.

15. 15 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Finding Abstract Classes Abstract classes are discovered by generalizing from concrete classes. To give two classes a common superclass: give them common interface + rename operations so classes use same names + reorder arguments, change types of arguments, etc. + refactor (split or combine) operations if operations have same interface but different implementation, make them abstract if operations have same implementation, move to superclass

16. 16 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Eliminating Refused Bequest S doB X doA doB doC Clients never use doC X doA doC Y doA doD Y doA doD

17. 17 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Applying the Composite Pattern WholePart * WholePart * Component W PP P W PP P W PP

18. 18 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Frameworks Require Iteration Reusable code requires many iterations. Basic law of software engineering If it hasn't been tested, it doesn't work. Corollary: software that hasn't been reused is not reusable.

19. 19 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Frameworks Encode Domain Knowledge Frameworks solve a particular set of problems. Not necessarily application-domain specific, but domain specific. (GUI, distribution, structured drawing editor, business transaction processing, workflow) If you aren’t a domain expert when you start to build a framework, you will be when you are done!

20. 20 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Customers are Application Programmers Purpose of framework is to make it easier to build applications. Applications are user stories/test cases of frameworks. Framework design driven by application design.

21. 21 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Up-front Design of Framework 1) Analyze problem domain Learn well-known abstractions. Collect examples of programs to be built from framework. (Minimum of 4 or 5). 2) Design abstraction that covers examples. 3) Test framework by using it to solve the examples. Each example is a separate application. Performing a test means writing software. Analysis Design Test

22. 22 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Typical Way to Develop Framework Notice that many applications are similar. Develop next application in that domain in an OO language. Divide software into reusable and nonreusable parts. Develop next application reusing as much software as possible. Surprise! Framework is not very reusable. Fix it.

23. 23 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson My Way to Develop Framework Start with applications. Refactor to simplify designs and share code. Build more applications. Refactor to simplify designs and share code. Build more applications. … When new applications no longer require refactoring shared code, the framework is done.

24. 24 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Nine Steps in Developing Frameworks 1) Three Examples 2) White-box Framework 3) Component Library Build applications and add components to library 4) Hot Spots Separate Changeable from Stable Code Reduce the amount of code that is written. Eliminate duplication, split classes/methods into parts that vary and parts that don’t. Design Patterns

25. 25 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Nine Steps in Developing Frameworks 5) Pluggable Objects 6) Fine-grained Objects 7) Black-box Framework 8) Visual Builder 9) Language Tools http://st-www.cs.uiuc.edu/users/droberts/evolve.html

26. 26 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson White-box vs. Black-box Black-box Customize by configuring Emphasize polymorphism Must know interfaces Complex, harder to design Easier to learn, requires less programming. White-box Customize by subclassing Emphasize inheritance Must know internals Simpler, easier to design Harder to learn, requires more programming.

27. 27 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Examples Good examples are crucial: for finding framework for teaching framework Examples define scope of the framework. Concrete examples abstract framework

28. 28 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Finding Examples Start with similar examples. should illustrate features that framework must support provides a base for generalization Eventually examples should cover wide range. minimize number of examples Look for examples that will break the framework. perhaps they will define limits of framework perhaps they will lead to generalization avoid lots of special cases

29. 29 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Advantages of Black-box Frameworks Replace reuse by inheritance with reuse by object composition -> change behavior without defining new classes. Ultimate black-box framework lets you plug objects together with visual language. Application development doesn't require programming.

30. 30 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Disadvantages of Black-box Framework Black-box frameworks harder to design and harder to make powerful. Black-box framework tends to have more kinds of objects more artificial kinds of objects more complex relationships between objects more objects Not-quite-ultimate framework forces you to debug more complex system.

31. 31 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Visual Builder It is easy to draw pictures of an application made from a black-box framework. It is easy to generate code from a picture for a black-box framework. It is easy to make special-purpose visual programming languages for black-box frameworks. Visual programming languages let non-programmers build applications.

32. 32 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Language Tools Warning! New languages require new tools. Do you really want to support debuggers, testing tools, performance analyzers, configuration management? Is this new environment any better than the old one?

33. 33 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Hints for Framework Design Use object composition instead of inheritance Incrementally apply patterns / lazy generalization Framework should work out of the box

34. 34 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Size Matters Must decide which frameworks to build. Must schedule new releases. Documentation. Training Small Large When you spot an abstraction that looks useful, generalize. Periodically take time to clean up your design. Explain design to your peers when they need to understand it.

35. 35 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Strategic Concerns Developing a framework is expensive, so look before you leap. Framework development requires long term commitment. Pick frameworks that will give you competitive advantage. Start small and work up. - get experience with OOP - select and train good abstractors - build small frameworks first - generalize existing systems

36. 36 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Customers are Crucial Framework development is easiest if all the developers are working on the same team. If framework users are on a different team, make sure the initial set succeed. First set of customers are really part of the development team. their input is critical they will suffer from redesign of framework they must feel that their contribution to framework is important

37. 37 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Reuse Scenarios Ideal: Successive versions of your framework meet the needs of a growing customer base. Actual: Projects may customize the initial framework, and even start competing streams of development.

38. 38 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Dealing with Iteration Don't claim framework is useful until your customers say it is. This will take two or three versions. Keep customer base small while framework is evolving. A successful framework is a living framework, evolve it to meet new customer needs. Don't constantly tinker. Plan releases and coordinate with customers.

39. 39 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Documentation and Training Documentation for framework costs several times usual how to use how to extend / how it works Software must be understandable to be usable. Improving documentation can make software more reusable.

40. 40 How to Design Frameworks -- Copyright 2005 by Ralph E. Johnson Documentation and Training Base documentation on examples. Must debug documentation and training. Documenting system shows how to change it. Framework developers must be intimately involved.