Dagger: inject into different instances of one class

Question

Can I use Dagger to inject different values into several instances of the same class that are deep inside the object graph? I want to avoid passing the values through the containing objects in the graph (so I can change the implementation of the contained objects without affecting their containers).

Here's a contrived example. The object graph is a Top, which contains a Left and Right, which each contain a Show. So there are two instances of Show.

class Top {
  Left left;
  Right right;
  void encodeTwice(String data) {
    left.encode(data);
    right.encode(data.getBytes());
  }
}
class Left {
  Leaf leaf;
  void encode(String data) {
    leaf.write(URLEncoder.encode(data));
  }
}
class Right {
  Leaf leaf;
  void encode(byte[] data) {
    leaf.write(DatatypeConverter.printBase64Binary(data));
  }
}
interface Leaf {
  void write(String data);
}
class Show implements Leaf {
  String label;
  @Override public void write(String data) {
    System.out.println(label + ": " + data);
  }
}
// There might be other classes that implement Leaf.

Can I use Dagger to inject different values into Top.left.leaf.label and Top.right.leaf.label?

Here's one attempt. This is unsatisfactory, because Left and Right depend on the implementation of Leaf. I want to inject Show.label without involving Left or Right.

ObjectGraph.create(new TopModule()).get(Top.class).encodeTwice("Hello!");

@Module(injects = Top.class)
public class TopModule {
  @Provides @Named("Left.leaf.label") String provideLeftLabel() {
    return "URL encoded";
  }
  @Provides @Named("Right.leaf.label") String provideRightLabel() {
    return "Base 64";
  }
}
class Top {
  @Inject Left left;
  @Inject Right right;
  void encodeTwice(String data) {
    left.encode(data);
    right.encode(data.getBytes());
  }
}
class Left {
  Leaf leaf;
  @Inject Left(@Named("Left.leaf.label") String label) {
    leaf = new Show(label);
  }
  void encode(String data) {
    leaf.write(URLEncoder.encode(data));
  }
}
class Right {
  Leaf leaf;
  @Inject Right(@Named("Right.leaf.label") String label) {
    leaf = new Show(label);
  }
  void encode(byte[] data) {
    leaf.write(DatatypeConverter.printBase64Binary(data));
  }
}
interface Leaf {
  void write(String data);
}
class Show implements Leaf {
  String label;
  Show(String label) {
    this.label = label;
  }
  @Override public void write(String data) {
    System.out.println(label + ": " + data);
  }
}

Answer 1

So this is hard to reason about with such abstract types because based on what Left, Right, and Top actually are I might change approaches.

There are three approaches to this problem that I can think of: a Leaf factory, differentiation of Leafs by qualifier, differentiation of Leafs by child graphs.

Factory

Hide the Leaf implementation behind a factory that takes its required dependencies.

interface LeafFactory {
  Leaf create(String name);
}

class TopModule {
  // ...

  @Provide @Singleton LeafFactory provideLeafFactory() {
    return new LeafFactory() {
      @Override public Leaf create(String name) {
        return new Show(name);
      }
    };
  }
}

class Left {
  private final Leaf leaf;

  @Inject Left(@Named("..") String name, LeafFactory leafFactory) {
    leaf = leafFactory.create(name);
  }

  // ...
}

Qualifier

Right now you are injecting the label via a qualifier annotation. There's no reason you couldn't do the same thing but with Leaf instances.

class TopModule {
  @Provides @Named("..") Leaf provideLeftLeaf() {
    return new Show("URL encoded");
  }

  @Provides @Named("..") Leaf provideRightLeaf() {
    return new Show("Base64 encoded");
  }
}

class Left {
  @Inject Left(@Named("..") Leaf leftLeaf) { .. }
}

Note: You can still inject the label names as arguments to the provideFooLeaf methods if you need that abstraction as well.

Child Graphs

By extending the graph with a "leaf scope" you can create separate parts of your application (e.g., "right", "left") which only have a single Leaf instance.

@Module(addsTo = TopModule.class)
class LeafModule {
  private final String label;

  LeafModule(String label) {
    this.label = label;
  }

  @Provide @Singleton Leaf provideLeaf() {
    return new Show(label);
  }
}

Now we can .plus() this onto the root object graph to get a scope.

ObjectGraph ogRoot = ObjectGraph.create(new TopModule());
// ...
ObjectGraph ogLeft = ogRoot.plus(new LeafModule("URL encoded"));
Leaf left = ogLeft.get(Leaf.class);
ObjectGraph ogRight = ogRoot.plus(new LeafModule("Base64 encoded"));
Leaf right = ogRight.get(Leaf.class);

Like I said, this is hard to generalize to fit the abstract sample app you've outlined. But I hope you can see how child graphs allow you create a specialized version of the graph which is a superset of the "root" graph. By doing this multiple times you can alter the dependencies which are used to inject your types.