# Modularization learning journey In this learning journey you will learn about modularization, and the modularization strategy used to create the modules in the Now in Android app. ## Overview Modularization is the practice of breaking the concept of a monolithic, one-module codebase into loosely coupled, self contained modules. ### Benefits of modularization This offers many benefits, including: **Scalability** - In a tightly coupled codebase, a single change can trigger a cascade of alterations. A properly modularized project will embrace the [separation of concerns](https://en.wikipedia.org/wiki/Separation_of_concerns) principle. This in turn empowers the contributors with more autonomy while also enforcing architectural patterns. **Enabling work in parallel** - Modularization helps decrease version control conflicts and enables more efficient work in parallel for developers in larger teams. **Ownership** - A module can have a dedicated owner who is responsible for maintaining the code and tests, fixing bugs, and reviewing changes. **Encapsulation** - Isolated code is easier to read, understand, test and maintain. **Reduced build time** - Leveraging Gradle’s parallel and incremental build can reduce build times. **Dynamic delivery** - Modularization is a requirement for [Play Feature Delivery](https://developer.android.com/guide/playcore/feature-delivery) which allows certain features of your app to be delivered conditionally or downloaded on demand. **Reusability** - Proper modularization enables opportunities for code sharing and building multiple apps, across different platforms, from the same foundation. ### Modularization pitfalls However, modularization is a pattern that can be misused, and there are some gotchas to be aware of when modularizing an app: **Too many modules** - each module has an overhead that comes in the form of increased complexity of the build configuration. This can cause Gradle sync times to increase, and incurs an ongoing maintenance cost. In addition, adding more modules increases the complexity of the project’s Gradle setup, when compared to a single monolithic module. This can be mitigated by making use of convention plugins, to extract reusable and composable build configuration into type-safe Kotlin code. In the Now in Android app, these convention plugins can be found in the [`build-logic` folder](https://github.com/android/nowinandroid/tree/main/build-logic). **Not enough modules** - conversely if your modules are few, large and tightly coupled, you end up with yet another monolith. This means you lose some benefits of modularization. If your module is bloated and has no single, well defined purpose, you should consider splitting it. **Too complex** - there is no silver bullet here. In fact it doesn’t always make sense to modularize your project. A dominating factor is the size and relative complexity of the codebase. If your project is not expected to grow beyond a certain threshold, the scalability and build time gains won’t apply. ## Modularization strategy It’s important to note that there is no single modularization strategy that fits all projects. However, there are general guidelines that can be followed to ensure you maximize its benefits and minimize its downsides. A barebone module is simply a directory with a Gradle build script inside. Usually though, a module will consist of one or more source sets and possibly a collection of resources or assets. Modules can be built and tested independently. Due to Gradle's flexibility there are few constraints as to how you can organize your project. In general, you should strive for low coupling and high cohesion. * **Low coupling** - Modules should be as independent as possible from one another, so that changes to one module have zero or minimal impact on other modules. They should not possess knowledge of the inner workings of other modules. * **High cohesion** - A module should comprise a collection of code that acts as a system. It should have clearly defined responsibilities and stay within boundaries of certain domain knowledge. For example, the [`core:network` module](https://github.com/android/nowinandroid/tree/main/core/network) in Now in Android is responsible for making network requests, handling responses from a remote data source, and supplying data to other modules. ## Types of modules in Now in Android ![Diagram showing types of modules and their dependencies in Now in Android](images/modularization-graph.png "Diagram showing types of modules and their dependencies in Now in Android") **Top tip**: A module graph (shown above) can be useful during modularization planning for visualizing dependencies between modules. The Now in Android app contains the following types of modules: * The `app` module - contains app level and scaffolding classes that bind the rest of the codebase, such as `MainActivity`, `NiaApp` and app-level controlled navigation. A good example of this is the navigation setup through `NiaNavHost` and the bottom navigation bar setup through `NiaTopLevelNavigation`. The `app` module depends on all `feature` modules and required `core` modules. * `feature:` modules - feature specific modules which are scoped to handle a single responsibility in the app. These modules can be reused by any app, including test or other flavoured apps, when needed, while still keeping it separated and isolated. If a class is needed only by one `feature` module, it should remain within that module. If not, it should be extracted into an appropriate `core` module. A `feature` module should have no dependencies on other feature modules. They only depend on the `core` modules that they require. * `core:` modules - common library modules containing auxiliary code and specific dependencies that need to be shared between other modules in the app. These modules can depend on other core modules, but they shouldn’t depend on feature nor app modules. * Miscellaneous modules - such as `sync`, `benchmark` and `test` modules, as well as `app-nia-catalog` - a catalog app for displaying our design system quickly. ## Modules Using the above modularization strategy, the Now in Android app has the following modules:
Name | Responsibilities | Key classes and good examples |
app
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Brings everything together required for the app to function correctly. This includes UI scaffolding and navigation. | NiaApp, MainActivity App-level controlled navigation via NiaNavHost, NiaTopLevelNavigation
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feature:1, feature:2 ... |
Functionality associated with a specific feature or user journey. Typically contains UI components and ViewModels which read data from other modules. Examples include:
|
AuthorScreen AuthorViewModel
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core:data
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Fetching app data from multiple sources, shared by different features. | TopicsRepository AuthorsRepository
|
core:ui
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UI components, composables and resources, such as icons, used by different features. | NiaIcons NewsResourceCardExpanded
|
core:common
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Common classes shared between modules. | NiaDispatchers Result
|
core:network
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Making network requests and handling responses from a remote data source. | RetrofitNiaNetworkApi
|
core:testing
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Testing dependencies, repositories and util classes. | NiaTestRunner TestDispatcherRule
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core:datastore
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Storing persistent data using DataStore. | NiaPreferences UserPreferencesSerializer
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core:database
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Local database storage using Room. | NiaDatabase DatabaseMigrations Dao classes
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core:model
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Model classes used throughout the app. | Author Episode NewsResource
|
core:navigation
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Navigation dependencies and shared navigation classes. | NiaNavigationDestination
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