Android Architecture Patterns

Overview of architecture patterns

Organisation is key when it comes to any software project. Good architecture will help us in lots of areas as a project grows and we add more classes:

• It's easier to debug small well-defined classes, than large unweildy ones.
• Code will be easier to maintain when everyone knows where everything should be.
• There should be less risk of duplicating code, ensuring DRY (Don't Repeat Yourself) is adhered to.
• Changes to code should be easier - we can more easily implement new features.

Simply put, the architecture is the foundation of the app. We decide how to split the code up into different sections, ideally before we've written the first line of code.

MVVM

MVVM (Model-View-ViewModel) is an architectural pattern often used in Android projects. I've personally been using it in all my new projects and use it during my 9-5 as well. It aims to separate the user interface (View) from the business logic (ViewModel) and data (Model). The ViewModel acts as an intermediary between the View and the Model, exposing data and commands that the View can bind to. As mentioned above, this separation allows for better testability, maintainability, and flexibility.

• The View managers (Activities, Fragments) should not contain any logic, other than for working with the views, such as showing or hiding elements. They include things such as click listeners, and general UI set up. They observe data from the ViewModel and update components accordingly.
• The ViewModel contains all the logic, for example calculating the price based on the user inputting that they want 10 medium sized widgets in in blue. Calls from the View manager to request data from the Model come directly through the ViewModel, which in turn sends data back from the model to the View manager to be displayed.
• The Model is any class relating to data access. Having a repository here is good practise to ensure one source of truth in cases where we obtain data from both a local source such as a SQLite DB, and also from a remote server via API calls.

Clean Architecture

MVVM architecture can cause a project to become unweildy after it gets to a certain point. Very large projects can benefit from something called Clean Architecture, which focuses on the separation of concerns. The image below shows an example of this, although not all the layers need to be included as it will vary depending on the project.

A crucial point to note is that outer layers can refer to inner layers, but inner layers cannot refer to outer layers. 'The outer layers should depend on the higher order of ideas which is contained in the inner layer.' Let's look at