Dependency injection is an important technique for building software systems that are maintainable and testable. You’re likely already doing dependency injection, maybe without even realizing it. Dependency injection is nowhere near as complex as its name implies, and this book will guide you through everything you need to know!
This book will guide you through Dagger, Google’s framework for Java, Kotlin, and Android. Dagger will help you solve many of the development and performance issues that have plagued reflection-based solutions.
This book is for intermediate Kotlin or Android developers who want to know how to implement the dependency injection pattern with Dagger and Hilt libraries.
Before You Begin
This section tells you a few things you need to know before you get started, such as what you’ll need for hardware and software, where to find the project files for this book, and more.
In this section, understand why you should use a dependency injection (DI) library like Dagger by learning all about the problem you need to solve: dependency.
You’ll understand what dependencies are and why you need to control them to create successful apps. You’ll get to know the Busso App, which you’ll work on, and improve throughout this book. And you’ll take your next step toward implementing better apps that are easier to test and modify.
In this chapter, you’ll learn how Scope and dependency are related to each other. You’ll start by refactoring how SplashActivity uses Navigator.
Next, you’ll define multiple ServiceLocator implementations, helping you understand how they depend on each other.
In this chapter, you’ll use an architectural pattern — Model View Presenter — along
with what you learned in the previous chapters to create a fully-testable app.
You’ll use techniques that would work in a world without frameworks like Dagger or Hilt.
Section II: Introducing Dagger
In this section, you’ll learn what Dagger is, how it works, and how it slashes the amount of code you need to write by hand when you implement dependency injection in your app.
You’ll learn how to deal with constructor, field and method injection with Dagger, how to simplify the implementation of @Module by using @Binds in cases when you have abstraction and its implementation, and how to use @Singleton to solve a very common problem.
In this chapter, you’ll learn how to use different Dagger
@Modules in the same app, how to optimize start-up performances using
Dagger’s Lazy interface, and how to avoid cycled dependencies using the
In this chapter, you’ll learn more about modules. You'll see the benefits of
using the @Binds annotation in a @Module, how to provide existing objects,
like Android Context, when optional bindings can help, and more.
Section III: Components & Scope Management
In this section, you’ll migrate the Busso App from the homemade framework to Dagger. In the process, you’ll learn how to migrate the existing ServiceLocators and Injectors to the equivalent Dagger @Modules and @Components, how to provide existing objects with a customized Builder for the @Component using @Component.Builder, and how to use @Component.Factory as a valid alternative to @Component.Builder.
The first migration will not be optimal — there will still be some fundamental aspects you will improve.
In this chapter, you'll go back to working on the Busso App. You'll learn how
to migrate the existing ServiceLocators and Injectors to the equivalent Dagger
Modules and @Components, how to provide existing objects with a
customized Builder for the @Component using @Component.Builder, and how to use
@Component.Factory as a valid alternative to @Component.Builder.
In this chapter, you'll learn You’ll also learn about the definition of
a component and how it relates to containers. You'll see what a lifecycle is,
why it’s important, and what its relationship to the scope is. You'll also
learn more about @Singletons, and What a @Scope is, and how it improves your
In this chapter, you'll learn even more about @Components and dependencies. In
particular, you'll learn why @Singleton is not so different from the other
@Scopes, why you might need a different approach to managing component
dependencies, and more.
Section IV: Advanced Dagger
In this section, you’ll dive deeper into the advanced features of Dagger like multi-binding. Multibinding is a very interesting feature of Dagger because it simplifies the integration of new features using a plugin pattern you’ll learn in this section.
You’ll implement a simple framework that allows you to integrate new services in the Busso app in a very simple and declarative way. You’ll learn all you need to know about multi-binding with Set and Map.
In this chapter, you'll learn how to use multibinding with Map. You'll learn
how to configure multibinding with Map and how to use fundamental type keys
with @StringKey, @ClassKey, @IntKey, and @LongKey. You'll also create a simple
custom key and use @KeyMap to build complex custom keys.
In this chapter, you'll learn how to configure and use Dagger in multi-module
Section V: Introducing Hilt
In the last section, you’ll learn everything you need to know about Hilt. Hilt is a dependency injection library for Android that reduces the boilerplate of doing manual dependency injection in your project.
Hilt is built on top of the DI library Dagger to benefit from the compile-time correctness, runtime performance, scalability, and Android Studio support that Dagger provides.
Learn dependency injection — an important technique for building software systems that are maintainable and testable. Dependency injection is nowhere near as complex as its name implies, and it becomes even easier by using Dagger — Google's framework that will help you solve development and performance issues that have plagued reflection-based solutions.