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I'm using unity and I'm creating a class that wrapps it and I dont' know how to call it, service locator or factory, both encapsulate the creation of the objects, so.... what's the difference?
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A Factory Design Pattern is used when the entire object can be easily created and object is not very complex. Whereas Builder Pattern is used when the construction process of a complete object is very complex.
The purpose of the Service Locator pattern is to return the service instances on demand. This is useful for decoupling service consumers from concrete classes. An implementation will consist of the following components: Client – the client object is a service consumer.
The real difference between factory and dependency injection lies in the fact that in the case of a factory, your dependent class is still reliant on a factory, which is a new form of dependency while DI takes out the dependency completely.
All components need to have a reference to the service locator, which is a singleton. The service locator makes the application hard to test. A service locator makes it easier to introduce breaking changes in interface implementations.
A factory creates objects for you, when requested.
Service locator returns objects that may already exist, that is services that may already exist somewhere for you.
Just think about the meaning of the names:
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Actually there is a clear separation between this both pattern. It's common known that both pattern are used for avoid dependencies from concrete types.
However after read
Some severe contradictions arises:
Seemann said: "An Abstract Factory is a generic type, and the return type of the Create method is determined by the type of the factory itself. In other words, a constructed type can only return instances of a single type."
While Rober C. Martin didn't mention anything about generic types and furthermore, factory example in his book allow to create instance of more than one type of objects distinguish between them using a key string as parameter in the Factory.Make().
Gamma said that intent of Abstract Factory is to "Provide an interface for creating families of related or dependent objects without specifying their concrete classes". Is worth to mention that Gamma Abstract Factory example violate Interface Segregation Principle (ISP) stated by Martin. ISP and SOLID in general are more moderns principles or maybe for simplicity where omitted.
Gamma and Martin's works precede Seemann's, so I think he should follow definition already made.
While Fowler propose Service Locator as a way to implement Dependency Inversion, Seemann consider it as an anti-pattern. Neither Gamma or Martin mention Service Locator.
However, Seemann and Fowler agreed in that Service Locator needs a configuration step to register an instance of a concretes class, that instance is what will be later returned when an object of that kind be requested. This configuration step is not mentioned by Martin or Gamma in their definition of Abstract Factory. Abstract Factory pattern suppose a new object to be instantiated every time an object of that kind be requested.
Conclusion
The main difference between Service Locator and Abstract Factory is that Abstract Factory suppose a new object be instantiated an returned at each requested and Service Locator needs to be configured with an object instance and every time the same instance will be returned.
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