Android Dependency Injection
I recently came across a very good collection of open source projects maintained by the guys from Square, a commercial payment service.
One of their projects came in use in one of my current Android projects: Dagger, a light-weight dependency injection library that can be used in Java and Android projects. Dagger kind of introduced me to the world of dependency injection libraries for Android.
DI and Android
If you are like me with a history (and present) in writing Java backend code, you will consider dependency injection as an almost given pattern when you start writing applications. For Android applications, dependency injection not only gets interesting for injecting things such as services or repositories, but also for injecting views.
Let’s have a look at the more familiar pattern of injecting Java objects/beans like services into activities.
DI with Dagger
Dagger is a light-weight dependency injection library that comes with support for Java’s JSR-330 annotations like
@Singleton . The creation of object instances that need to be injected at runtime are created in a so-called module definition:
In the case above, the module definition provides a REST interface called
AppConnector. Therefore, it configures an
OkHttpClient instance and the REST endpoint. At runtime, the
ApplicationModule needs to be instantiated and the dependency object graph defined by the module needs to be bootstrapped. This can be done in a custom
In order to actually trigger the dependency injection for an activity,
ObjectGraph#inject() needs to be called. This is typically done inside a common activity base class:
Once this initial setup is done, activities can be injected with the objects created in the module:
Of course, Dagger also comes with more advanced conecepts and features. But with the features shown above, you can already start to use dependency injection for services, repositories and other patterns.
VDI with Butterknife
When developing on Android, glue code to set instance variables which are all
View descendants is frequently needed:
View dependecy injection is a way to get rid of glue code that is needed to link instance variables with resources from the layout, and to speed up development. One library that is determined to be used exactly for this case is Butterknife. It comes with annotations that can be used to inject views, but also to annotated methods which at compile-time are transformed to listener instances.
But let’s first of all have a look at view injection. Actually, it is pretty simple, the annotation to use is
@InjectView. It needs a single annotation parameter which is the resource ID of the targeted view:
No more code needed for manually retrieving and casting the view instances in the
onCreate Activity callback. One little detail that is hidden here is the call to
ButterKnife.inject(this) which actually causes Butterknife to inject all the instance fields with the target views at runtime. I do the calls to
inject usually in my
BaseActivity which is shared across all activities in my project:
Among other nice Butterknife features, I really like the annotations for various listeners, take
@OnClick for example. This is a method level annotation that annotates methods which are at compile-time transformed/wrapped with an actual listener implementation:
Or suppose the layout has a Spinner and we want to react when an item is selected:
The method targeted by
OnItemSelected may come with any parameters also found in the actual listener interface.
Butterknife in one of my latest Android projects and I won’t go without them anymore. Especially
Butterknife comes in very handy as it allows to get rid of view glue code that is needed to get references to UI view elements.
It should be noted that there is also the AndroidAnnotations project that comes with an even richer set of annotations to do basically everything with annotations. In my current projects, our needs were totally satisfied with the
Butterknife feature set, so I can’t say anything about
AndroidAnnotations in depth.