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Dagger 2
Manage dependencies in Google style
Royce Mars
Tech Lead
@ DataArt
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What is dependency injection?
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Composition != inheritance
Member variables need to be initialized
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Creating objects with “new” keyword
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Initializing new objects
Via constructor parameters
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Composition. The simplest example
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Applying inheritance and composition
When creating new objects
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See dependencies in code
● Car depends on Engine.
● Engines may vary.
● Need different engines for
testing and production
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Dagger and JSR-330
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Evolution of Dagger
● Dagger 2 - Google, Greg Kick
● Dagger - Square, Jake Wharton
● Guice - Google, Jesse Wilson
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Inversion of control
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Reflection vs Compile time
Dagger 2 vs Dagger 1
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JSR-330
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Simple piece of magic...
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Default constructor injection
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Injection through parameters
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So, how to make it work?
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How does injection work?
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Components & Modules
Pic. by Miroslaw Stanek from Azimo
http://frogermcs.github.io/dagger-graph-creation-performance/
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Module declaration
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Module annotations
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Component declaration
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Component annotations
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Component initialization
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Injection
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Adding modules
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Custom scopes
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Objects lifecycle
Pic. by Miroslaw Stanek from Azimo
http://frogermcs.github.io/dagger-graph-creation-performance/
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And again - Components & Modules
Pic. by Miroslaw Stanek from Azimo
http://frogermcs.github.io/dagger-graph-creation-performance/
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This mysterious “plus”...
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Declaring Subcomponent
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Adding Module instance
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Adding subcomponent
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Managing subcomponents lifetime
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Scope
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@ActivityScope
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@ActivityScope
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Custom scope
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Custom scope
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Custom scope
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So… what should we inject?
● Demo data sources / modules
● Presenters
● Singletons
● Test implementations
● All other dependencies :)
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Best practices
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New Android Injector
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New Android Injector
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Favor constructor injection
over field injection
● Field injection requires non final and non private fields
● Forget an @Inject - get NullPointerException on a field
● Constructor injection = immutable and thread safe
objects. Protect from partially constructed state
● For objects, constructed by the system (Activities,
Fragments) - still use field injection
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Favor constructor injection
over field injection
BAD:
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Favor constructor injection
over field injection
BETTER:
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Favor @Inject over @Provides
● @Provides methods should not duplicate the
constructor boilerplate
● Code is easier to understand when coupled concerns are
in one place
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Favor @Inject over @Provides - no need in
provides method in Module
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Singletons should be extremely rare
● Singletons are useful when we need a centralized access
to a mutable state
● Immutable objects doesn’t need to be a singleton
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Singletons should be extremely rare
BAD:
GOOD:
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Favor static @Provides methods
● The generated code can directly invoke the method
instead of having to create a module instance. That
method call can be inlined by the compiler
● All bindings being static will result in a sizable
performance increase
● Make your modules abstract and Dagger will fail at
compile time if one of the @Provides methods isn’t static
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Favor static @Provides methods
BAD:
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Favor static @Provides methods
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Use Kotlin, insert Koin :)
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Home readings for Java
● Py “Keeping the Daggers Sharp”
● Mert Simsek “New Android Injector with Dagger 2”
● James Shvarts “Demystifying the new Dagger Android
Injection API”
● Fernando Cejas “Tasting Dagger 2 on Android”
● Miroslaw Stanek “Dagger2 - graph creation
performance”
● CodePath’s guide to Dagger2
● Xoxol_89 @ Habrahabr “Dagger 2. Часть третья.
Новые грани возможного”
● Dagger2 official page
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Home readings for Kotlin
Mobile Architecture Club
● Juan Ignacio Saravia, Keddit
https://android.jlelse.eu/keddit-part-10-kotlin-dagger-
2-dependency-injection-a620052a7384
● Antonio Leiva
https://antonioleiva.com/dagger-android-kotlin/
● Koin https://github.com/Ekito/koin
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Thank you! :)
@RoyceMars
@DataArt