Dependency injection on Android

Pedro V. Gómez Sánchez - pedro@karumi.com - @pedro_g_s - github.com/pedrovgs
Dependency Injection on Android
Pedro Vicente Gómez Sánchez
Senior Android Developer at Karumi
pedro@karumi.com
@pedro_g_s
github.com/pedrovgs

Alberto Gragera
Technical Director
Jorge Barroso
Senior Android Developer
Davide Mendolia
Senior Full Stack Engineer
Sergio Gutierrez
Senior Full Stack Engineer

Agenda
● Introduction.
● What can I do using Dependency Injection?
● Scoped Graphs.
● Dagger on Android.
● Dagger 2.

Introduction
Inversion of Control
vs
Dependency Inversion
vs
Dependency Injection

Inversion of Control
The natural evolution of the structured and imperative
programming and the usage of reusable frameworks.
The execution point where you software takes the control
is not defined by your software.

Dependency Inversion
One of the most important S.O.L.I.D principles.
Depend on abstractions, forget about implementation
details.
Provide implementation details depending on
abstractions can be tedious.

Dependency Injection
Software Design Pattern used to facilitate the usage of
Dependency Inversion.
Provides dependencies given some previous
configuration where abstractions are linked with
concretions.

Why?
The usage of IoC, Dependency Inversion and Dependency
Injection provides give us the following benefits:
● Testability.
● Decoupling.
● Modularity.

Show me the code!

Dependency Injection Example
The presenter inside this fragment has to be instantiated
with all the dependency tree. Without a dependency
injector this can be tedious.

All the dependencies are passed in construction.

The implementation details are part of the Dependency
Injector configuration.

Now the Presenter is injected into the Fragment.

What can I do with a
Dependency Injector?

What can I do?
● Do not link the class to the runtime lifecycle or frameworks.
● Improve testability.
● Decouple your code and improve our software design.
● Detect code smells related to class dependencies and SRP violations.
● Change the implementation details in build time.
● Extract implementation details out of the business logic.

What can I do?
Do not link the class to the runtime lifecycle.

What can I do?
Improve testability.

What can I do?
Detect code smells related to class dependencies and single
responsibility principle violations.

What can I do?
Change the implementation details in build time.

What can I do?
Decouple your code and improve our software design.
Using a Dependency Injector combined with the correct usage of
Dependency Inversion we can easily develop the same feature at the
same time in different teams and layers.

What can I do?
Extract implementation details out of the business logic.
Develop software being agnostic of the Framework or the Runtime used.
Implementation details are provided using the dependency injector and
your business logic does not depend on it.

What can I do?
The dependency container provide us different configurations to provide
dependencies:
● Singletons.
● Lazy Initialization.
● Provided dependencies.
● Named dependencies.
● Override dependencies already configured.
● Custom qualifiers.

What can I do?
Singletons

What can I do?
Lazy Initialization

What can I do?
Provided dependencies

What can I do?
Named Dependencies

What can I do?
Custom Qualifiers

Scoped Graphs

Scoped Graphs
Dependencies provided by the Dependency Container or Object Graph
inside the Application class are linked to the Application lifecycle.
We need more than one graph to improve the Dependency Injector usage
and performance.
At the same time, some dependencies are needed just when some
components like Activities or Custom Views are initialized.

Scoped Graphs
Application Graph
Activity A
Graph
Activity B
Graph
Create Activity A
Create Activity B

Dagger on Android

Dagger on Android
Step 1. Add Dagger and Dagger Compiler as dependencies:

Dagger on Android
Step 2. Create an Application extension and configure the Object Graph:

Dagger on Android
Step 3. Configure Dagger Modules:

Dagger on Android
Step 4. Initialize your Application Object Graph:

Dagger on Android
Step 5. Create a method to extend the Application Object Graph:

Dagger on Android
Step 6. Create a BaseActivity with the code needed to extend the
Application Graph.

Step 7. Create an abstract method requesting the Activity modules:
Dagger on Android

Step 8. Extend the Application Object Graph using the Activity modules:
Dagger on Android

https://www.github.com/pedrovgs/EffectiveAndroidUI

Can you repeat, please?
Dependency Injection on Android Workshop

Dagger 2

Dagger 2
● Configuration based on components not in graphs.
● Generated code is now something you can read.
● Reflection free.
● 13 % faster.
● Full graph validated in build time.

Questions?

Dependency injection on Android

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