Removes the general theory of modularization and links to the official guidance instead.
Clarifies the structure of feature modules, detailing the split into `api` and `impl` submodules and their dependency rules.
Also includes minor heading and formatting updates.
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.
In this learning journey you will learn about the modularization strategy used
to create modules in the Now in Android app. For the theory behind modularization, check out
[the official guidance](https://developer.android.com/topic/modularization)
## Types of modules in Now in Android
@ -142,26 +63,27 @@ 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 `TopLevelDestination`. The `app` module depends on all `feature` modules and
required `core` modules.
### The `app` module
This 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 `TopLevelDestination`. The `app` module depends on all `feature` modules and required `core` modules.
### Feature modules
These are feature-specific modules that handle a single responsibility in the app. For example, the `ForYou` feature handles all content and UI state for the "ForYou" screen. Feature modules aren't gradle modules themselves, they are split into two submodules:
* `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.
* `api` - contains navigation keys
* `impl` - contains everything else
* `core:` modules - common library modules containing auxiliary code and specific dependencies that
This approach allows features to navigate to other features by using the target feature's navigation keys. A feature's `api` and `impl` modules can be used by any app, including test or other flavoured apps. If a class is needed only by one feature module, it should remain within that module. If not, it should be placed into an appropriate `core` module.
A feature's `api` module should not depend on another feature's `api` or `impl` module. A feature's `impl` should only depend on another featur's `api` module. Both submodules should only depend on the `core` modules that they require.
### `core:` modules
These are 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.
### Miscellaneous modules
For example, `sync`, `benchmark` and `test` modules, as well as `app-nia-catalog` - a catalog app for displaying our design system quickly.
Don Turner
2 weeks ago