Non-Transitive R Classes

While upgrading my android project to use AGP 8.0.0, I noticed that android build analyzer had a recommendation to use non-transitive R classes to improve build speed.

I was curious to know what it is and how it works. So, I did some research and here's what I found.

Why

Non-transitive R classes have been around for a while, but it was not enabled by default. With AGP 8.0.0, it is enabled by default. For the curious minds, I have included a blog that talks about the history of R classes and non-transitive R classes at the end of this article.

Non-transitive R classes enable namespacing of each library’s R class so that its R class includes only the resources declared in the library itself and none from the library’s dependencies, thereby reducing the size of the R class for that library.

For example, if the code uses a default animation R.anim.nav_default_enter_anim which comes from the androidx.navigation:navigation-ui-ktx dependency. Without using non-transitive R classes, The 'R' class of module has references to its own resources and transitive references to resources of navigation UI library.

import com.learning.compose.R
...

  .setStartAnimations(activity, R.anim.nav_default_enter_anim, R.anim.nav_default_exit_anim)

With non-transitive R classes enabled, this will force us to use the fully qualified R class name to access the resources in the dependency module, thus reducing the references in the 'R' class of 'compose' module. And also by referencing using fully qualified R class name, the Compose module doesn't have to be recompiled and the build speed is improved.

.setStartAnimations(activity, androidx.navigation.ui.R.anim.nav_default_enter_anim, androidx.navigation.ui.R.anim.nav_default_exit_anim)

Now, if you go to app/build/intermediates/runtime_symbol_list/internalDebug/R.txt, you will see that the references to nav_default_enter_anim and nav_default_exit_anim are removed from the 'R' class of the module.

Since the dependencies become more explicit with non-transitive R classes, the modularity of the code increases. It also decreases complexity, since resources cannot come from transitive dependencies. It also increases the build speed(both clean and incremental) as promised by Android Developers from Android Dev Summit '21.

What to do

If you use a resource from another dependency/module,

• If your android project is 100% Kotlin Only, You are in luck. You can alias imports of R classes of dependency modules to access resources in that module.
• If your project has some Java code references, You have to use fully qualified R class name to access resources in the dependency module

Also, If you use a resource from another module, but you do not declare a dependency on that module, and you do not want to declare a dependency, you could simply duplicate resources in the app module

How to do

Use Android Studio's Refactor -> Migrate to non-transitive R classes to migrate the project to use non-transitive R classes

• This will,

  • Adds the below in gradle.properties

    android.nonTransitiveRClass=true
    

  • Updates fully qualified R class name wherever it finds the non-transitive R class usage

    .setStartAnimations(activity, androidx.navigation.ui.R.anim.nav_default_enter_anim, androidx.navigation.ui.R.anim.nav_default_exit_anim)
    

Additionally, If the code base is in Kotlin, You could alias the fully qualified package name of the R class to access the resources

import androidx.navigation.ui.R as NavUiR

...
  .setStartAnimations(activity, NavUiR.anim.nav_default_enter_anim, NavUiR.anim.nav_default_exit_anim)
  .setExitAnimations(activity, NavUiR.anim.nav_default_pop_enter_anim, NavUiR.anim.nav_default_pop_exit_anim)

Here are some references to the articles that I found useful

• Improve Build speed Recommendation
• History of R classes & non-transitive R classes