How do I inject fields at runtime using Dagger 2?

Question

I need to inject fields of an instance of one of my classes on-demand i.e., at runtime because I'm instantiating them on-the-fly.

I used to use Guice for this where I would call MembersInjector#injectMembers or Injector#injectMembers. How can I have something like this in Dagger 2?

Answer 1

Dagger 2 Components are the counterpart to Guice Injectors so the way to do this in Dagger 2 would be to specify the object whose field you want to inject at runtime as an injection site and request injection from the component.

Let's say you have a CoffeeShop with fields you want to inject:

class CoffeeShop {

    @Inject CoffeeMaker coffeeMaker;

    CoffeeShop() { 
        //we're not using constructor injection here 
        //although we probably should be :/
    }
}

You can specify CoffeeShop as an injection site inside a component and request injection from it:

@Component(modules = { CoffeeModule.class })
interface CoffeeComponent {
    void inject(CoffeeShop coffeeShop);
}

So inside another class you can do something like this:

private CoffeeComponent coffeeComponent;

void initComponent() {
    coffeeComponent = DaggerCoffeeComponent
                         .builder()
                         .coffeeModule(new CoffeeModule())
                         .build();
}

void makeCoffee() {  
    CoffeeShop coffeeShop = new CoffeeShop();
    coffeeComponent.inject(coffeeShop); //inject members of coffeeShop
    coffeeShop.makeCoffee();
}

Alternatively, you can define provision methods inside your Dagger 2 Components which will allow you to resolve instances of a class ad hoc.

If you look at Jeff Bowman's example in the linked question, you can see there is a Component like this:

@Component(modules = {/* ... */})
public interface CoffeeShopComponent {
  CoffeeShop getCoffeeShop();

  void inject(CoffeeService serviceToInject); // to be discussed below
}

Say you then have a CoffeeService. You can now call getCoffeeShop() to obtain arbitrary instances of CoffeeShop:

class CoffeeService extends SomeFrameworkService {

    private CoffeeComponent coffeeComponent;

    void initComponent() {
        coffeeComponent = DaggerCoffeeComponent
                              .builder()
                              .coffeeModule(new CoffeeModule());
                              .build();
    }

    public CoffeeShop createCoffeeShop() {
        return coffeeComponent.getCoffeeShop(); //equivalent to Injector.getInstance();
    }
}