Introduction

RoboGuice, also called Google Guice on Android, is an easy-to-use dependency injection framework, which can make Android development more intuitive and convenient. Using this framework, you can drastically reduce the amount of code you write for performing common tasks, such as initializing various resources, accessing Android system services, and handling events.

In this tutorial, I will be showing you how to make the most of RoboGuice 3 in your Android projects.

1. Understanding Dependency Injection

Traditionally, if an object depends on something, it is its own responsibility to satisfy that dependency. In simpler terms, if an instance of class A depends on an instance of class B, then the developer is usually expected to call the constructor of class B inside the code of class A. This obviously leads to tighter coupling between the two classes.

Dependency injection is a design pattern in which objects rely on external code, which is commonly referred to as a dependency injector, to satisfy their dependencies. This means that if an object depends on other objects, it doesn’t have to know how to create or initialize those objects. This reduces the coupling between the objects and leads to code that is more modular, easier to modify, and less complex to test.

Thus, by using dependency injection, you can largely do away with constructors and factory methods in your project’s business logic.

2. How RoboGuice Works

Google Guice is a framework that makes it easy for you to create, configure, and use a dependency injector in your Java projects. RoboGuice builds on Google Guice and comes with a pre-configured dependency injector for Android. Simply put, out of the box, RoboGuice knows how to initialize various Android objects, get references to various resources of an app and more.

RoboGuice uses Java annotations, which are nothing but metadata embedded inside Java code, to determine what has to be injected where. Earlier versions of RoboGuice used to process annotations using the Java Reflection API during runtime and were often criticized for being slow. RoboGuice 3, however, comes with RoboBlender, a compile-time annotation processor that drastically improves RoboGuice’s performance.

3. Setting Up RoboGuice

Before you use RoboGuice, you must add it as a compile dependency in your app module’s build.gradle file. As it is available on Android Studio’s default repository, jcenter, doing so requires just one line of code.

compile 'org.roboguice:roboguice:3.0.1'

To improve the performance of RoboGuice, it is recommended that you also add RoboBlender, an annotation processor, as a provided dependency.

provided 'org.roboguice:roboblender:3.0.1'

To be able to use RoboGuice’s annotations in your Android activities, their classes must extend RoboActivity instead of Activity. Similarly, if you want to use the annotations inside an Android service, its class must extend RoboService instead of Service.

4. Associating Layouts With Activities

Normally, you would use the setContentView method and pass a layout resource to it in order to set the layout of an Activity. RoboGuice offers an alternative means to do the same thing, the @ContentView annotation.

For example, here’s how you would apply the layout defined in an XML file called activity_main.xml to a RoboActivity called MainActivity:

@ContentView(R.layout.activity_main)
public class MainActivity extends RoboActivity {

}

5. Injecting Views

If you think using the findViewById method and type casting the View object it returns is a lot of work, you can use RoboGuice’s @InjectView annotation instead.

For example, consider the following layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView android:id="@+id/email"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />

    <Button android:id="@+id/okay"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>

To initialize the two UI widgets defined in the XML in a RoboActivity, you could write the following:

@InjectView(R.id.email)
private TextView email;

@InjectView(R.id.okay)
private Button okay;

6. Injecting Resources

Accessing the resources of your app using the Android API involves lots of different classes and functions. To fetch a Drawable, for example, you would have to use ContextCompat.getDrawable. To fetch an Animation, you would have to use AnimationUtils.loadAnimation.

RoboGuice’s @InjectResource annotation offers a more consistent way to fetch all types of resources. The following code snippet shows you how to inject a ColorStateList, a Drawable, a String, and an Animation resource:

@InjectResource(R.color.red)
private ColorStateList red;

@InjectResource(R.drawable.square)
private Drawable square;

@InjectResource(R.string.hello)
private String hello;

@InjectResource(R.anim.fade)
private Animation fade;

7. Injecting System Services

To get a reference to an Android system service, such as the PowerManager or the Vibrator, you can use the @Inject annotation instead of using the getSystemService method. For example, here’s how you would get a reference to the PowerManager:

@Inject
private PowerManager pm;

8. Injecting Extras

You can use the @InjectExtra annotation to inject the extras that were passed to a RoboActivity. The following code snippet injects an extra whose key is EMAIL_ADDRESS:

@InjectExtra("EMAIL_ADDRESS")
private String email;

Note that @InjectExtra will cause a runtime error if the extra is not present. If the extra is optional, you should include an optional flag whose value is set to true in order to avoid the error.

@InjectExtra(value = "EMAIL_ADDRESS", optional = true)
String email;

9. Injecting Your Own Classes

So far we have been injecting items that were specific to the Android SDK. To inject your own classes, you should use the @Inject annotation. @Inject behaves much like Java’s new keyword and you don’t have to make any changes to a class to make it injectable, provided it has a default constructor. For example, consider the following class:

public class Employee {
	public Employee() {
		Log.d("RG", "Hello");
	}
}

To inject an instance of the Employee class, you would use the following code:

@Inject
Employee p;	// equivalent to new Employee()

10. Using Custom Providers

If you want finer control over what’s injected when @Inject is used, you need to create your own custom providers.

Let’s create a simple provider that returns a random number every time @Inject is used to initialize an Integer.

Step 1: Create a Provider

A provider is just a class that implements the Provider interface. Therefore, create a new class called MyRandomNumberProvider that implements Provider and override its get method.

public class MyRandomNumberProvider implements Provider<Integer> {

    @Override
    public Integer get() {
        
    }
}

As you might have guessed, the return value of the get method is what will be injected when @Inject is used. To return a random integer, add the following code to the get method:

// Returns a random integer between 0 and 999
return (int)(Math.random()*1000);

Step 2: Create a Module

To be able to use your custom provider, you need to create a module for it. A module is a class that extends the AbstractModule class. Inside the module, you override the configure method and specify which class the provider should bind to using the bind and toProvider methods.

To create a module for MyRandomNumberProvider, create a new Java class called MyRandomNumberModule and add the following code to it:

public class MyRandomNumberModule extends AbstractModule {

    @Override
    public void configure() {
        // Bind Integer to MyRandomNumberProvider
        bind(Integer.class)
                .toProvider(MyRandomNumberProvider.class);
    }
}

Step 3: Register the Module

To let RoboGuice know about your module, you must register it in your app’s AndroidManifest.xml using a meta-data tag. The name attribute of the tag should be set to roboguice.modules and its value attribute should contain the class name of the module.

<meta-data
  android:name = "roboguice.modules"
  android:value="com.github.hathibelagal.roboguicetutorial.MyRandomNumberModule"
/>

The provider is now ready. At this point, if you annotate an Integer with @Inject, it will be initialized to a random number.

@Inject
Integer random1; // Initialized to a random number

@Inject
Integer random2; // Initialized to another random number

11. Working With Singletons

If you are a developer who prefers using a singleton to share data between multiple activities and services, you can use the @Singleton and @Inject annotations to simplify your code.

By adding the @Singleton annotation to a class, you can let RoboGuice know that it shouldn’t create more than one instance of the class. The following code creates a singleton called MySingleton:

@Singleton
class MySingleton {
}

You can now use the @Inject annotation to inject the singleton into your classes. For example, here’s how you inject MySingleton:

@Inject
MySingleton singleton;

12. Observing Events

By using the @Observes annotation, you can observe various events associated with an Activity. This means, you don’t have to override the onCreate, onResume, and other life cycle methods of the Activity class.

The following code shows you how the @Observes annotation can be used as an alternative to overriding the onCreate and onDestroy methods:

public class MainActivity extends RoboActivity {

	// Called when the Activity is created
	public void initialize(@Observes OnCreateEvent e) {
		// Initialization code goes here
	}

	// Called when the Activity is destroyed
	public void cleanup(@Observes OnDestroyEvent e) {
		// Clean up code goes here
	}
}

Conclusion

In this tutorial, you learned how to use RoboGuice to make your code more concise and readable. While doing so, you also learned how to use Java annotations and the dependency injection pattern.

To learn more about RoboGuice, I recommend browsing its wiki on GitHub.