Seven Habits of Effective Pattern Writers Facade Pattern PH pp GoF pp John Klacsmann

Other Famous Gang of Four’s Chinese Communist Politicians (1976) British Post-Punk Band ( )

Seven Habits of Effective Pattern Writers 1. Take Time to Reflect 2. Adhere to a Structure 3. Be Concrete Early and Often 4. Keep Patterns Distinct and Complementary 5. Present Effectively 6. Iterate Tirelessly 7. Collect and Incorporate Feedback

1. Take Time To Reflect Think about past problems and solutions Record Your Experiences Describe the problem Describe the problem Why is it difficult? Why is it difficult? Write down new approaches Write down new approaches Why did it fail? Why did it succeed? Why did it fail? Why did it succeed? Forms raw material of patterns Forms raw material of patterns

1. Take Time To Reflect Identify Patterns You Already Know If find something new & unique make sure you have at least two existing examples of a problem & solution

2. Adhere to a Structure Pattern: “A Structured Exposition of a solution to a problem in context” Include: Name Name Statement of Problem Statement of Problem Context and Justification of Solution Context and Justification of Solution Solution itself Solution itself Settle on structure & keep it consistent

3. Be Concrete Early and Often People understand better when presented in concrete first then abstract Use examples and counterexamples “Tell the whole truth” Warn reader of potential pitfalls Warn reader of potential pitfalls Extra Cost Extra Cost Ill-behavior under certain circumstances Ill-behavior under certain circumstances Other patterns Other patterns Etc. Etc.

4. Keep patterns distinct and complementary Insure patterns are distinct Might not be obvious Ask: “How is pattern X different from pattern Y?” Let intents of patterns be guide to differences, not class structures Spend time comparing and contrasting your patterns

5. Present Effectively Typesetting and Writing Style are important Use drawings liberally to illustrate key points Make pattern approachable Use down-to-earth writing style Use down-to-earth writing style Write conversationally Write conversationally

6. Iterate Tirelessly Pattern writing is an ongoing process Expect to write and rewrite your pattern many times Don’t perfect before moving on

7. Collect and Incorporate Feedback “No pattern can be trusted until it is used by someone other than its author” Make pattern understandable to people who have never had the problem Discuss with colleagues Look for opportunities to use Get comments & be prepared to hear the worst Give reviewers benefit of doubt Give reviewers benefit of doubt Bend over backwards to make them happy Bend over backwards to make them happy

Seven Habits of Effective Pattern Writers Follow these steps to make your own patterns better The better your patterns are, the more impact they’ll have

GoF’s Facade Pattern Make it simpler Defines a higher-level interface that makes the subsystem easier to use From:

Motivation Structuring into subsystems helps reduce complexity Provides a single, simplified interface to the more general (complex) facilities of a subsytem

Compiler Example Contains subsystem of classes Most clients don’t care about details – just want to compile Low-level only complicates task

Compiler Example Compiler class acts as Façade Provides higher level interface that shields clients from lower-level classes Makes life easier for programmer without hiding the low- level functionality completely Puts all pieces of compile operation together

When to use Facade When you want a simple interface to a complex subsystem Provides simple default view good enough for most clients Provides simple default view good enough for most clients

When To Use Facade When there are many dependencies between clients and implementation classes Decouples subsystem from clients Decouples subsystem from clients When you want to layer your subsystems Simplify dependencies between subsystems by making them communicate only through their facades Simplify dependencies between subsystems by making them communicate only through their facades

Structure Façade Delegates client requests to appropriate subsystem objects Delegates client requests to appropriate subsystem objects Subsystem Classes Implement subsystem Implement subsystem Handle work assigned by Façade Handle work assigned by Façade Have no knowledge of façade (keep no references) Have no knowledge of façade (keep no references)

Benefits Shields client from subsystem Reduces number of objects the client deals with Reduces number of objects the client deals with Makes subsystem easier to use Makes subsystem easier to use Promotes weak coupling between subsystem and its clients Helps eliminate complex or circular dependencies Helps eliminate complex or circular dependencies Minimizes recompilation time needed for small subsystem change Minimizes recompilation time needed for small subsystem change Applications can still use subsystem classes if needed Provides both ease of use & generality Provides both ease of use & generality

How is this different then Mediator? Mediator abstracts communication between objects, centralizing functionality Mediator’s colleagues are aware of it and communicate with it instead of with each other Façade abstracts the interface to a subsystem to make it easier It doesn’t provide any new functionality and subsystem classes don’t know about it

Questions?