Dagger- Should we create each component and module for each Activity/ Fragment
I've been working with dagger2 for a while. And I got confused wether to create an own component/module for each Activity/ Fragment. Please help me clarify this:
For example, We have an app, and the app has about 50 screens. We will implement the code following the MVP pattern and Dagger2 for DI. Suppose that we have 50 activities and 50 presenters.
In my opinion, usually we should organize the code like this :
Create an AppComponent and AppModule which will provide all objects that will be used while the app is open.
@Module public class AppModule { private final MyApplicationClass application; public AppModule(MyApplicationClass application) { this.application = application; } @Provides @Singleton Context provideApplicationContext() { return this.application; } //... and many other providers } @Singleton @Component( modules = { AppModule.class } ) public interface AppComponent { Context getAppContext(); Activity1Component plus(Activity1Module module); Activity2Component plus(Activity2Module module); //... plus 48 methods for 48 other activities. Suppose that we don't have any other Scope (like UserScope after user login, ....) }Create ActivityScope :
@Scope @Documented @Retention(value=RUNTIME) public @interface ActivityScope { }Create Component and Module for each Activity. Usually I will put them as static classes within the Activity class:
@Module public class Activity1Module { public LoginModule() { } @Provides @ActivityScope Activity1Presenter provideActivity1Presenter(Context context, /*...some other params*/){ return new Activity1PresenterImpl(context, /*...some other params*/); } } @ActivityScope @Subcomponent( modules = { Activity1Module.class } ) public interface Activity1Component { void inject(Activity1 activity); // inject Presenter to the Activity } // .... Same with 49 remaining modules and components.
Those are just very simple examples to show how I would implement this.
But a friend of mine just gave me another implementation:
Create PresenterModule which will provide all presenters:
@Module public class AppPresenterModule { @Provides Activity1Presenter provideActivity1Presentor(Context context, /*...some other params*/){ return new Activity1PresenterImpl(context, /*...some other params*/); } @Provides Activity2Presenter provideActivity2Presentor(Context context, /*...some other params*/){ return new Activity2PresenterImpl(context, /*...some other params*/); } //... same with 48 other presenters. }Create AppModule and AppComponent:
@Module public class AppModule { private final MyApplicationClass application; public AppModule(MyApplicationClass application) { this.application = application; } @Provides @Singleton Context provideApplicationContext() { return this.application; } //... and many other provides } @Singleton @Component( modules = { AppModule.class, AppPresenterModule.class } ) public interface AppComponent { Context getAppContext(); public void inject(Activity1 activity); public void inject(Activity2 activity); //... and 48 other methods for 48 other activities. Suppose that we don't have any other Scope (like UserScope after user login, ....) }
His explaination is: He doesn't have to create components and modules for each activity. I think my friends idea is absolutely not good at all, but please correct me if I am wrong. Here are the reasons:
A lot of memory leaks :
- The app will create 50 presenters even if the user has only 2 Activities open.
- After the user closes an Activity, its presenter will still remain
What happens if I want to create two instances of one Activity ? (how can he create two presenters )
It will take a lot of time for the app to initialize (because it has to create many presenters, objects, ...)
Sorry for a long post, but please help me clarify this for me and my friend, I can't convince him. Your comments will be very appreciated.
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Edit after doing a demo.
First, thanks for @pandawarrior answer. I should have created a Demo before I asked this question. I hope my conclusion here could help someone else.
- What my friend has done does not cause memory leaks unless he puts any Scope to the Provides-methods. (For example @Singleton, or @UserScope, ...)
- We can create many presenters, if the Provides-method doesn't have any Scope. (So, my second point is wrong, too)
- Dagger will create the presenters only when they are needed. (So, the app will not take a long time to initialize, I was confused by Lazy Injection)
So, all the reasons I have said above are mostly wrong. But it does not mean that we should follow my friend idea, for two reasons:
It's not good for the source's architecture, when he inits all presenters in module / component. (It violates Interface segregation principle, maybe Single Responsibility priciple, too).
When we create a Scope Component, we will know when it's created and when it's destroyed which is a huge benefit for avoiding memory leaks. So, for each Activity we should create a Component with an @ActivityScope. Let's imagine, with my friends implementation, that we forgot to put some Scope in the Provider-method => memory leaks will occur.
In my opinion, with a small app (just a few screens without many dependencies or with similar dependencies), we could apply my friends idea, but of course it's not recommended.
Prefer to read more on: What determines the lifecycle of a component (object graph) in Dagger 2? Dagger2 activity scope, how many modules/components do i need?
And one more note: If you want to see when the object are destroyed, you can call those of method together and the GC will run immediately:
System.runFinalization();
System.gc();
If you use only one of these methods, GC will run later, and you may get wrong results.
Answer
Declaring a separate module for each Activity is not a good idea at all. Declaring separate component for each Activity is even worse. The reasoning behind this is very simple - you don't really need all these module/components (as you have already seen by yourself).
However, having just one component that is tied to Application's life-cycle and using it for injection into all Activities is also not the optimal solution (this is your friend's approach). It is not optimal because:
- It restricts you to just one scope (
@Singletonor a custom one) - The only scope you're restricted to makes the injected objects "application singletons", therefore mistakes in scoping or incorrect usage of scoped objects can easily cause global memory leaks
- You'll want to use Dagger2 in order to inject into
Servicestoo, butServicescan require different objects thanActivities(e.g.Servicesdon't need presenters, don't haveFragmentManager, etc.). By using a single component you loose the flexibility of defining different object graphs for different components.
So, a component per Activity is an overkill, but single component for the entire application is not flexible enough. The optimal solution is in between these extremes (as it usually is).
I use the following approach:
- Single "application" component that provides "global" objects (e.g. objects that hold global state which is shared between all components in the application). Instantiated in
Application. - "Controller" subcomponent of "application" component that provides objects which are required by all user-facing "controllers" (in my architecture these are
ActivitiesandFragments). Instantiated in eachActivityandFragment. - "Service" subcomponent of "application" component that provides objects which are required by all
Services. Instantiated in eachService.
Following is an example of how you could implement the same approach.
Edit July 2017
I published a video tutorial that shows how to structure Dagger dependency injection code in Android application: Android Dagger for Professionals Tutorial.
Edit February 2018
I published a complete course about dependency injection in Android.
In this course I explain the theory of dependency injection and show how it emerges naturally in Android application. Then I demonstrate how Dagger constructs fit into the general dependency injection scheme.
If you take this course you will understand why the idea of having a separate definition of module/component for each Activity/Fragment is basically flawed in the most fundamental way.
Such an approach causes the structure of presentation layer from "Functional" set of classes to be mirrored into the structure of "Construction" set of classes, thus coupling them together. This goes against the main objective of dependency injection which is to keep the "Construction" and "Functional" sets of classes disjoint.
Application scope:
@ApplicationScope
@Component(modules = ApplicationModule.class)
public interface ApplicationComponent {
// Each subcomponent can depend on more than one module
ControllerComponent newControllerComponent(ControllerModule module);
ServiceComponent newServiceComponent(ServiceModule module);
}
@Module
public class ApplicationModule {
private final Application mApplication;
public ApplicationModule(Application application) {
mApplication = application;
}
@Provides
@ApplicationScope
Application applicationContext() {
return mApplication;
}
@Provides
@ApplicationScope
SharedPreferences sharedPreferences() {
return mApplication.getSharedPreferences(Constants.PREFERENCES_FILE, Context.MODE_PRIVATE);
}
@Provides
@ApplicationScope
SettingsManager settingsManager(SharedPreferences sharedPreferences) {
return new SettingsManager(sharedPreferences);
}
}
Controller scope:
@ControllerScope
@Subcomponent(modules = {ControllerModule.class})
public interface ControllerComponent {
void inject(CustomActivity customActivity); // add more activities if needed
void inject(CustomFragment customFragment); // add more fragments if needed
void inject(CustomDialogFragment customDialogFragment); // add more dialogs if needed
}
@Module
public class ControllerModule {
private Activity mActivity;
private FragmentManager mFragmentManager;
public ControllerModule(Activity activity, FragmentManager fragmentManager) {
mActivity = activity;
mFragmentManager = fragmentManager;
}
@Provides
@ControllerScope
Context context() {
return mActivity;
}
@Provides
@ControllerScope
Activity activity() {
return mActivity;
}
@Provides
@ControllerScope
DialogsManager dialogsManager(FragmentManager fragmentManager) {
return new DialogsManager(fragmentManager);
}
// @Provides for presenters can be declared here, or in a standalone PresentersModule (which is better)
}
And then in Activity:
public class CustomActivity extends AppCompatActivity {
@Inject DialogsManager mDialogsManager;
private ControllerComponent mControllerComponent;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getControllerComponent().inject(this);
}
private ControllerComponent getControllerComponent() {
if (mControllerComponent == null) {
mControllerComponent = ((MyApplication)getApplication()).getApplicationComponent()
.newControllerComponent(new ControllerModule(this, getSupportFragmentManager()));
}
return mControllerComponent;
}
}
Additional information on dependency injection: