In my last article about the good software architecture I wrote that in order to have a good architecture we should apply Object Oriented programming principles (some of them are known as SOLID principles) and design patterns. Let’s discuss today some of these principles, and let’s start with Inversion of control and see how it shapes the design and contributes to good architecture. According to Martin, Robert C principle states:
A. High level modules should not depend upon low level modules. Both should depend upon abstractions.
B. Abstractions should not depend upon details. Details should depend upon abstractions.
With other words a class dependencies should be expressed as interfaces and not implementations. This simple principle leads to significant benefit in design – loose coupling. And loose coupling in turn leads us to
- extensible code. Do you remember the traits of poor design from my previous article? When you come to the point when you don’t know where to put your new code, because there is no place where it would fit well, that could be the sign that code is not extensible. Think about this principle next time, when you will be puzzled. Maybe Inversion of control is the solution how to make your architecture extendable.
- testability – dependencies can be easily swaped with mocks thus unit tested
- parallel development – the only thing you have to take care now is the interface definitions between modules and work on modules can be done in parallel.
As you can see even by applying this one OO principle, we can improve the design in several areas. But don’t rush to provide an interface for every object. So when is it better to not abstract an implementation behind an interface? Most of the time we are writing a code by extending existing framework (in our case it is Android) using the classes and the methods of the framework and calling the third party libraries. In order to abstract away these kind of classes we would need to take another extra step of wrapping them in our own classes and only then abstract our class behind the interface. In this case, the only benefit we would gain is testability, but in case of Android with the help of Mockito we already can easily mock framework classes. Another case are models: ViewModels, DTO, Database Models and static methods (once you made them static, you must be counted on the fact that they won’t change the signature). The one thing in common for all those cases is that you are not planning or you can’t swap the existing method implementation. And so there is no need for abstraction.
Once we make necessary dependencies abstract, we still need to provide an implementation for them but in the way that the module is not aware of the implementation. Remember module sees only abstractions! And here at hand comes another design principle Inversion of Control (IoC) sometimes known as Hollywood Principle. The general philosophy is that control is handed over. For example in a plugin framework, you’re expected to override some call back methods. Let’s take the Android’s Activity class and it’s lifecycle methods. The class doesn’t have the control over when the methods get called, Android framework decides when to call them. The control is inverted. Dependency Injection (DI) is another example of IoC – the class doesn’t create its dependencies but instead gets them from someone else. By using DI we can follow one more principle – Single responsibility principle (SRP):
1) object creation and its lifetime management is delegated to DI container and thus
these responsibilities are taken away from the module and we are one step closer to SRP
2) if too many arguments are in the constructor it could be signal that we are violating the SRP. In this case, we should consider grouping dependencies and hiding them behind a Facade.