android-dagger | Dependency injection is a technique | Dependency Injection library

You can add the compiler option to your app/build.gradle file in the android -> defaultConfig block like this:

HasActivityInjector is a part of dagger.android. You can but not have to use it. You can easily live without this thing and still use everything that dagger offers. That's why it is in some guides but not all of them.

You should read this guide to undestand the purpose of it. It will help you decide if you want it or not. Here's the most important part.

Why Dagger on Android is hard

One of the central difficulties of writing an Android application using Dagger is that many Android framework classes are instantiated by the OS itself, like Activity and Fragment, but Dagger works best if it can create all the injected objects. Instead, you have to perform members injection in a lifecycle method. [...]

This has a few problems:

1. Copy-pasting code makes it hard to refactor later on. As more and more developers copy-paste that block, fewer will know what it actually does.
2. More fundamentally, it requires the type requesting injection (FrombulationActivity) to know about its injector. Even if this is done through interfaces instead of concrete types, it breaks a core principle of dependency injection: a class shouldn’t know anything about how it is injected.

dagger.android

The classes in dagger.android offer one approach to simplify the above problems. This requires learning some extra APIs and concepts but gives you reduced boilerplate and injection in your Android classes at the right place in the lifecycle.