Explore other Box tech blog posts here . The most effective way to improve the testability of your Android app is through a technique such as dependency injection. Dependency injection on Android is a notoriously difficult problem to tackle. The problem stems from the fact that Android manages the creation and destruction of all activity instances, thereby making it difficult for us to provide dependencies. The diagram below, taken from the Android developer docs, illustrates the activity lifecycle:

So why is this an issue? When building testable apps, we want to be able to inject dependencies into our activities. This allows us to swap out our "real" objects with testable mocks. However, we can't do this since we don't have control over the creation of our activities. Unfortunately, this tends to promote a design involving heavy use of static singletons. Let's take a look at an example of a typical Android activity class.

Note: I'm not going to go into detail over what dependency injection and inversion of control (IoC) is. It has become a fairly common design pattern in the past few years, and there are plenty of other resources that can explain it far better than I can.

Basic Activity Example

onCreate()

Activity

Using The Service Locator Pattern

The basic idea behind a service locator is to have an object that knows how to get hold of all of the services that an application might need. - http://martinfowler.com/articles/injection.html

Android Service Locator

ServiceLocator

WebServiceClient

Testing With Service Locator

ServiceLocator

Using Dagger

ObjectGraph

Dagger Example

MainActivity

ObjectGraph

Advantages

All of our dependencies are grouped together. Instead of having calls to ServiceLocator sprinkled throughout our code, our injected field variables are conveniently listed in a single place.

Instead of having calls to sprinkled throughout our code, our injected field variables are conveniently listed in a single place. Reduced boiler plate. Manually injecting dependencies can become cumbersome as our app becomes more complex. This leads to the next point...

Manually injecting dependencies can become cumbersome as our app becomes more complex. This leads to the next point... Developers are more likely to use it. When your design is easier to implement, other team members are more likely to stick to it. It also means that less time is spent passing around dependencies and figuring out the order in which to instantiate them.

When your design is easier to implement, other team members are more likely to stick to it. It also means that less time is spent passing around dependencies and figuring out the order in which to instantiate them. Using Dagger also allows us to use regular dependency injection. Activities need to use a service locator because of their lifecycle, but other objects do not have this limitation. With Dagger we can satisfy as many dependencies as possible at the start of our app, but fall back to using a service locator pattern with our activities.

Summary

[java]/** * A simple activity that makes a web service call when a button is clicked. */ public class MainActivity extends Activity { private WebServiceClient client; @Override public void onCreate(final Bundle savedInstanceState) { client = WebServiceClient.getInstance(); Button btn = (Button) findViewById(R.id.btn); btn.setOnClickListener(new OnClickListener() { @Override public void onClick(final View v) { client.doSomething(); } }); } }[/java] This makes sense at a first glance, but now say we want to test our activity. How do we do this without making real web service calls? We can create an integration test that makes calls to our server, but those are slow, less reliable, and require a separate testing environment. Also notice that there is no straightforward way of providing objects to themethod. We cannot add any additional parameters because it is an overridden method defined by the baseclass. We also cannot create a constructor because the Android OS only knows how to call the default empty constructor. So how do we fix this?The first option is to use the service locator pattern. If you aren't familiar with this pattern, it's basically a way for our activity to grab the services (or dependencies) it needs.The following is an example of how this might work in the context of an Android app.[java]public class MainActivity extends Activity { private WebServiceClient client; @Override public void onCreate(final Bundle savedInstanceState) { client = ServiceLocator.getServiceLocator().getWebServiceClient(); Button btn = (Button) findViewById(R.id.btn); btn.setOnClickListener(new OnClickListener() { @Override public void onClick(final View v) { client.doSomething(); } }); } } public class ServiceLocator { private static ServiceLocator instance; private IWebServiceClient webServiceClient; public static void setServiceLocator(ServiceLocator serviceLocator) { this.instance = serviceLocator; } public static ServiceLocator getServiceLocator() { return this.serviceLocator; } public ServiceLocator(IWebServiceClient webServiceClient) { this.webServiceClient = webServiceClient; } public IWebServiceClient getWebServiceClient() { return this.webServiceClient; } }[/java] Now we can provide ourwith a mockwhen writing tests.[java]public class MainActivityTests { @Test testClickingButtonDoesSomethingWithWebService() { IWebServiceClient mockWebService = createMock(WebServiceClient.class); ws.doSomething(); expectLastCall(); replay(mockWebService); ServiceLocator sl = new ServiceLocator(mockWebService); verify(mockWebService); } }[/java] Our code is now much better. We can easily test logic that makes web service calls, our dependencies are clearly defined through, and we've reduced the coupling between classes.The nice part about using the service locator pattern is that it can work nicely with regular dependency injection. Dagger is a lightweight IoC framework that was designed with Android in mind. Dagger can automatically handle our dependencies, alleviating a lot of the effort. The standard way of using an IoC framework, like Dagger, is to resolve all of your dependencies at startup. However, on Android this isn't possible because we cannot instantiate activities ourself. Instead, we can use Dagger'sas a type of service locator.[java]public class MainActivity extends Activity { @Inject private IWebServiceClient client; @Override public void onCreate(final Bundle savedInstanceState) { MyApp app = (MyApp) getApplication(); app.getObjectGraph().inject(this); Button btn = (Button) findViewById(R.id.btn); btn.setOnClickListener(new OnClickListener() { @Override public void onClick(final View v) { client.doSomething(); } }); } } @Module(entryPoints = MainActivity.class) public class DefaultModule { @Provides IWebServiceClient provideWebServiceClient() { return new WebServiceClient(); } } public class MyApp extends Application { private ObjectGraph objectGraph; @Override public void onCreate() { super.onCreate(); this.objectGraph = ObjectGraph.create(new DefaultModule()); } public ObjectGraph getObjectGraph() { return this.objectGraph; } }[/java] So there we have it. I won't go into the details of how to use Dagger, but if you've used an IoC framework before, the syntax is fairly self-explanatory. Here, we get an instance of Dagger's object graph and use it to inject dependencies into ourobject. Our Daggerinstance is essentially acting as a service locator.Using IoC provides a few advantages over our manual service locator:As mobile devices become more powerful, the apps we create become more complex. It's important to take the time to ensure your Android apps are designed in a modular, testable way. It is generally agreed that a lack of automated testing is no longer acceptable for desktop and web apps, and mobile shouldn't be any exception. Greg works at Box as a Code Reliability Engineer. Interested especially in mobile software design and best practices, he works to improve the quality of apps on Android, iOS, and Windows 8. He can also be found at gregcurt.is