How to decouple your code modules What is dependency injection Put appropriate instances in dont let the object create them Thanks for your atten wait wait its a bit more complicated ID: 1019338
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1. europeanconference
2. Dependency injectionHow to decouple your code modules
3. What is dependency injection?“Put appropriate instances in; don’t let the object create them.”Thanks for your atten… wait, wait, its “a bit” more complicated…
4. What is SOLID?S - Single responsibility principle (SRP)O - Open/closed principle (OCP)L - Liskov substitution principle (LSP)I - Interface segregation principle (ISP)D - Dependency inversion principle (DIP)
5. Single responsibility principleA class should only have one responsibilityhaving problems finding a name is a sign of a violationcreating things is a responsibility -> factoryrelated to separation of concerns (SoC)
6. Dependency inversion principleAbstractions should not depend on details. Details should depend on abstractions"Code against interfaces (i.e. abstractions) not implementations (details)"
7. What is dependency injection"Dependency Injection is a 25-dollar term for a 5-cent concept.""Don't look for things - ask for them!" (Misko Hevery)
8. Why Dependency Injection?Decoupling of software pieces (classes)SRPflexibilitycomposabilityexchangeabilitytestability (mocking)maintainability
9. „To create or not to create“Injectables and creatables – who is who?Example for creatables:All kinds of collections to store data or inner state(lists, dictionaries, TStrings, dynamic arrays, TStream)PODOs (aka data objects) – but sometimes not directly but with a factory depending on other requirementsInjectables are those things that actually do something often called services
10. Reading suggestions
11. More materialNot only purely on dependency injection but in general useful things about software design and architecturehttps://blog.ploeh.dk (Blog of Mark Seemann)http://misko.hevery.com/code-reviewers-guide/(Guide to write testable code)https://www.youtube.com/playlist?list=PL693EFD059797C21E (Google Clean Code Talks)
12. Pure DILearn and understand how DI works, what patterns and practices are helpful before using an automated wayTry putting your dependencies together manually to get a feeling for how to design your classes and interfacesOnce you‘ve written pages and pages of constructor calls, consider using a DI container
13. Service LocatorDI is about passing things (to the constructor basically)Service Locator is about looking for things activelyCode smell and dangerous anti-patternCreates dependencies not only on the service locator but implicitly on types that need to be existing in the service locatorNot easily noticable when creating a class but only when some method gets called
14. Composition rootThe point where DI starts – ideally there is only one and it‘s at the very start of the applicationWhen trying to fit DI into an existing application you might have multiple at firstMove them closer to the start of the application and eventually they will converge
15. DI Container (finally!...)Factory pattern on steroidsAnd a repository, and some other design patterns A central place to register all your injectables and let them be created and composed automatically
16. Registering typesRegisterTypeTell the container what implementing type is available and as what type it can resolve itPossibility to delegate the creation to another place than the container itselfRegisterInstanceTell the container about an existing instance to let it participate in dependency injection
17. Registering types (advanced)RegisterFactoryRegister a type and let the container provide its implemention which then serves as factory to return instances returned by the containerRegisterDecoratorRegister a type and let the container use it as decorator for other classes when it resolves them and automatically apply them
18. Controlling lifetimesNew instance every time or use only one?Transient vs SingletonPossibility to control when new instances are being created or already created ones are returnedSingleton, SingletonPerThread, PerResolve, Pooled
19. Type providersPossibility to extend the container to give it special knowledge about specific types and how to build themExamples of built-in providers are: LazyProvider, ListProvider, DynamicArrayProvider
20. Types of injectionConstructor Injectionconstructor parametersmandatory dependenciesProperty Injectionproperties/setteroptional dependenciesNull-Object patternField InjectionUse only in exceptional cases – this is not possible with pure DI
21. Controlling the compositionTypes can be namedAll injection targets can be marked with the [Inject] attribute to control where and what is being injectedAdditional conditions can be applied to types and injection targets (planned)
22. Let‘s take a look at some example
23. Container.Free