Dependency Injection: Dagger

Ken Baldauf

Senior Software Engineer @ Acorns

Dagger 2

• Most popular Android DI framework
• Compile time DI framework for Java
• Doesn't use reflection
• Heavy use of annotation processing
• Lots of generated boilerplate
• Maintained by Google
  • Based of Square's Dagger 1
• Hilt: opinionated Dagger extension
  • Standardizes the way Dagger is incorporated into an Android project

Modules & Components

• Module
  • A provider of dependencies
  • Defines how dependencies are built
• Component
  • Resolver of dependencies
  • Container that bundles one or more modules together
  • Generates an object graph at compile time
    • Consists of objects it knows how to resolve
      • As well as their dependencies
  • User must manage a Component's lifecycle
  • SubComponents
    • Components that inherit from a Component 

Scopes

• By default Dagger creates a new instance of an object each time it is requested
• To avoid that, we can use scope annotations
• Allow us to persist a single instance of a dependency for a component's entire life
• Scoped & unscoped dependencies can live in same module
  • Scoped dependencies must live in a component with the same scope
• No 2 components can be assigned the same scope
• @Singleton is a predefined scope in Dagger
  • But we can define our own

Provides vs Binds

• @Provides & @Binds
  • ​2 ways to define dependencies in modules
  • Declare type being provided & dependencies needed
  • Can't be used in the same module
• @Provides
  • Declares exact construction of provided object
  • Useful when constructor injection can't be used
    • Ex: Retrofit's builder pattern
• @Binds
  • Construction of object is inferred by Dagger
  • Generates less code
  • Maps an interface or generic return type to an implementation

Questions?