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About the Book
In Principles of Package Design Matthias Noback tells you everything about designing software components, also known as packages. In the first part you'll revisit the SOLID principles of class design. They will help you prepare your classes for use in packages. The second part covers the important, yet lesser known package design principles. When you've finished this book, you'll be ready to design packages that have high cohesion, low coupling and are at the same time user- and maintainer-friendly.
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Table of Contents
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Introduction
- The situation
- Overview of the contents
- Notes about the code samples
- Thank you
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Class design
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Introduction
- SOLID principles
- Why follow the principles?
- Prepare for change
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The Single responsibility principle
- A class with too many responsibilities
- Responsibilities are reasons to change
- Refactoring: using collaborator classes
- Advantages of having a single responsibility
- A small peek ahead: common closure
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The Open/closed principle
- A class that is closed for extension
- Refactoring: abstract factory
- Refactoring: making the abstract factory open for extension
- Refactoring: polymorphism
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Conclusion
- Packages need classes that are open for extension
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The Liskov substitution principle
- Violation: a derived class does not have an implementation for all methods
- Violation: different substitutes return things of different types
- Violation: a derived class is less permissive with regard to method arguments
- Violation: secretly programming a more specific type
- Conclusion
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The Interface segregation principle
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Violation: leaky abstractions
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Refactoring: create separate interfaces and use multiple inheritance
- What did we do?
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Refactoring: create separate interfaces and use multiple inheritance
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Violation: multiple use cases
- Refactoring: separate interfaces and multiple inheritance
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Violation: no interface, just a class
- Implicit changes in the implicit interface
- Refactoring: add header and role interfaces
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Violation: leaky abstractions
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The Dependency inversion principle
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Example of dependency inversion: the FizzBuzz generator
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Making the
FizzBuzzclass open for extension -
Removing the specificness from the
FizzBuzzclass
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Making the
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Violation: a high-level class depends upon a low-level class
- Refactoring: abstractions and concretions both depend on abstractions
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Violation: a class depends upon a class from another package
- Solution: add an abstraction and remove the dependency using composition
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Conclusion
- Peeking ahead: abstract dependencies
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Example of dependency inversion: the FizzBuzz generator
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Introduction
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Package design
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Principles of cohesion
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- Becoming a programmer
- The hardest part
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Cohesion
- Class design principles benefit cohesion
- Package design principles, part I: Cohesion
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The Release/reuse equivalence principle
- Keep your package under version control
- Add a package definition file
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Use semantic versioning
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Design for backward compatibility
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Rules of thumb
- Don’t throw anything away
- When you rename something, add a proxy
- Only add parameters at the end and with a default value
- Methods should not have side effects
- Dependency versions should be permissive
- Use objects instead of primitive values
- Use private object properties and methods for information hiding
- Use object factories
- And so on
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Rules of thumb
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Design for backward compatibility
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Add meta files
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READMEand documentation- Installation and configuration
- Usage
- Extension points
- Limitations (optional)
- License
- Change log
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Quality control
- Quality from the user’s point of view
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What the package maintainer needs to do
- Add tests
- Conclusion
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The Common reuse principle
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Signs that the principle is being violated
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Feature strata
- Obvious stratification
- Obfuscated stratification
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Classes that can only be used when … is installed
- Suggested refactoring
- A package should be “linkable”
- Cleaner releases
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Bonus features
- Suggested refactoring
- Sub-packages
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Feature strata
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Conclusion
- Guiding questions
- When to apply the principle
- When to violate the principle
- Why not to violate the principle
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Signs that the principle is being violated
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The Common closure principle
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A change in one of the dependencies
- Assetic
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A change in an application layer
- FOSUserBundle
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A change in the business
- Sylius
- The tension triangle of cohesion principles
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A change in one of the dependencies
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Principles of coupling
- Coupling
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The Acyclic dependencies principle
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Coupling: discovering dependencies
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Different ways of package coupling
- Composition
- Inheritance
- Implementation
- Usage
- Creation
- Functions
- Not to be considered: global state
- Visualizing dependencies
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Different ways of package coupling
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The Acyclic dependencies principle
- Nasty cycles
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Cycles in a package dependency graph
- Dependency resolution
- Release management
- Is it all that bad?
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Solutions for breaking the cycles
- Some pseudo-cycles and their dissolution
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Some real cycles and their dissolution
- Dependency inversion
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Inversion of control
- Mediator
- Chain of responsibility
- Mediator and chain of responsibility combined: an event system
- Conclusion
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Coupling: discovering dependencies
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The Stable dependencies principle
- Stability
- Not every package can be highly stable
- Unstable packages should only depend on more stable packages
- Measuring stability
- Decreasing instability, increasing stability
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Violation: your stable package depends on a third-party unstable package
- Solution: use dependency inversion
- A package can be both responsible and irresponsible
- Conclusion
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The Stable abstractions principle
- Stability and abstractness
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How to determine if a package is abstract
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The
Ametric
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The
- Abstract things belong in stable packages
- Abstractness increases with stability
- The main sequence
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Types of packages
- Concrete, instable packages
- Abstract, stable packages
- Strange packages
- Conclusion
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Conclusion
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Creating packages is hard
- Reuse-in-the-small
- Reuse-in-the-large
- Embracing software diversity
- Component reuse is possible, but requires more work
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Creating packages is doable
- Reducing the impact of the first rule of three
- Reducing the impact of the second rule of three
- Creating packages is easy?
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Creating packages is hard
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Principles of cohesion
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Appendices
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Appendix I: The full
Pageclass
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Appendix I: The full
About the Author
Matthias Noback started his professional career 12 years ago as a freelance web developer. After working as a PHP developer for various companies, he decided to try something else; though programming will always be his passion, he has now taken this further and is also teaching, writing and consulting. He is the author of A Year With Symfony and Principles of PHP Package Design and regularly posts on his blog PHP & Symfony.
