DevTut

# Library Dagger 2: Dependency Injection in Applications

Dagger 2, as explained on GitHub (opens new window), is a compile-time evolution approach to dependency injection. Taking the approach started in Dagger 1.x to its ultimate conclusion, Dagger 2.x eliminates all reflection, and improves code clarity by removing the traditional ObjectGraph/Injector in favor of user-specified @Component interfaces.

# Create @Module Class and @Singleton annotation for Object

import javax.inject.Singleton;
import dagger.Module;
import dagger.Provides;

@Module
public class VehicleModule {
 
    @Provides @Singleton
    Motor provideMotor(){
        return new Motor();
    }
 
    @Provides @Singleton
    Vehicle provideVehicle(){
        return new Vehicle(new Motor());
    }
}

Every provider (or method) must have the @Provides annotation and the class must have the @Module annotation. The @Singleton annotation indicates that there will be only one instance of the object.

# Request Dependencies in Dependent Objects

Now that you have the providers for your different models, you need to request them. Just as Vehicle needs Motor, you have to add the @Inject annotation in the Vehicle constructor as follows:

@Inject
public Vehicle(Motor motor){
    this.motor = motor;
}

You can use the @Inject annotation to request dependencies in the constructor, fields, or methods. In this example, I'm keeping the injection in the constructor.

# Connecting @Modules with @Inject

The connection between the provider of dependencies, @Module, and the classes requesting them through @Inject is made using @Component, which is an interface:

import javax.inject.Singleton;
import dagger.Component;

@Singleton
@Component(modules = {VehicleModule.class})
public interface VehicleComponent {
    Vehicle provideVehicle();
}

For the @Component annotation, you have to specify which modules are going to be used. In this example VehicleModule is used, which is defined in this example (opens new window). If you need to use more modules, then just add them using a comma as a separator.

# Using @Component Interface to Obtain Objects

Now that you have every connection ready, you have to obtain an instance of this interface and invoke its methods to obtain the object you need:

VehicleComponent component = Dagger_VehicleComponent.builder().vehicleModule(new VehicleModule()).build();
vehicle = component.provideVehicle();
Toast.makeText(this, String.valueOf(vehicle.getSpeed()), Toast.LENGTH_SHORT).show();

When you try to create a new object of the interface with the @Component annotation, you have to do it using the prefix Dagger_<NameOfTheComponentInterface>, in this case Dagger_VehicleComponent, and then use the builder method to call every module within.

# Remarks

  1. Library setup in application(for maven, gradle,java projects)
  2. Advantages of Dragger use
  3. Important Links (for Documentation and demos)
  4. How to integrate and use Dragger components

# Dagger 2 API:

Dagger 2 exposes a number of special annotations:

@Module for the classes whose methods provide dependencies

@Provides for the methods within @Module classes

@Inject to request a dependency (a constructor, a field, or a method)

@Component is a bridge interface between modules and injection

GitHub: https://github.com/google/dagger (opens new window)

UserGuide(Google): https://google.github.io/dagger/users-guide.html (opens new window)

Videos: https://google.github.io/dagger/resources.html (opens new window)

Vogella Tutorial: http://www.vogella.com/tutorials/Dagger/article.html (opens new window)

Codepath Tutorial: https://github.com/codepath/android_guides/wiki/Dependency-Injection-with-Dagger-2 (opens new window)

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