DAGGER 2 Tutorial Part 2: Component, Module and Essential Applications

It will be beneficial to view whiteboard animation of this tutorial in the first place.

Component: @Component is the means to declaratively specify an object graph that Dagger 2 will incarnate during the precompilation process. It is essentially @Component annotated Java interface. Dagger 2 will implement this interface and give us a concrete class implementation. @Component is the bedrock element of Dagger 2. For a complete Dagger 2 application we may do without @Module but @Component is indispensable.

We observe here simplest Dagger 2 Application. Moreover it is complete. @Inject decorated constructor results in immediate object graph residency that our component entertains. CommandRouter has that @Inject decorated contructor therefore this type is object graph resident. Also it is the only type CommandRouterFactory component needs in graph conclusion.

Module: @Module is there for modularization of @Component construction. Remember we are bosses of our enterprise architecture, we like to give commands by emitting Dagger 2 configuration interfaces but we are frugal souls we don’t like to repeat ourselves. @Module is a reusable encapsulation of a number of object providing functions. We declare what we want to @Provide inside @Module boundaries. Sets of @Modules are then declared on top of @Components. @Modules are all about encapsulating object provision declaration and modular graph construction. @Modules are just classes or interfaces with @Module annotation.

We will now observe slightly more complicated Dagger 2 Application. Drip coffee maker will aid us conceiving the subject matter.

Here comes our first class. It is ElectricHeater implementing Heater interface. Coffee is hot therefore we need a heat source. ElectricHeater class has no dependency. This fact deems its residency at the root of our object graph. But its constructor is not @Inject decorated. Therefore Heater type is not in our object graph yet. In order to be so Heater type must be the return type of a @Provides annotated method inside @Module class. Always remember, there are two primary ways of a type’s object graph inclusion. Declaring @Inject decorated class or returning the type from @Provides method of a @Module.

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