What are the benefits of a multi-project build in Android?

Question

I'm reading the source code of Android : Clean Architecture, mostly to learn how to properly organize an application into layers, and for the MVP pattern, and also to match it with what I've been reading on MVP here.

However, as pretty as I find the structure, I don't really understand the benefit of order a single application into multiple sub-projects or modules. Considering they (data, presentation, domain) depend on one another, and eventually will be part of the same executable, it looks more like configuration hell.

dependencies {
  ...
  compile project(':domain')
  compile project(':data')

What are the benefits of compartmentalizing an Android application into multiple subprojects (modules), rather than keep them in one project but separate them merely by packages?

Answer 1

Actually this question is not Android-specific but more software architecture related as it applies to almost any software you develop (e.g. why does any app consists of several modules and not all in one package).

Splitting the code into modules would provide you at least the following benefits (these are the first 3 that comes to my mind):

1. Clear isolation between the modules. The whole goal of the MVP pattern is to make sure your business logic and presentations layer are not tightly coupled together. By defining these in different modules it makes this separation more clear and encourages you to stick to this pattern.
2. Code reuse - consider the case where you have one application that you'd to sell for several customers, but each of these customers would like to get a different look and feel. If you'll have all of your code in one monolithic project you'll have to either have many forks of your project, one for each customer or otherwise bloat the provided app code with the customization options. Now, if on the other hand you had separated your modules, then you could just prepare a different presentation layer for each customer and bundle it with the other common modules.
3. Splitting your project into sub-project allows more efficient build and testing as you can rebuild and test only the modules that changed rather than recompiling the whole project whenever there's a change in one of the files.

I hope that this makes sense to you.