Dagger 2 Injecting Android Application Context

Dagger 2 injecting Android Application Context

Was not correctly building the Application component, needed to pass in the Application. This Dagger 2 example perfectly shows how to do this: https://github.com/google/dagger/tree/master/examples/android-simple/src/main/java/com/example/dagger/simple

Update:
Working link: https://github.com/yongjhih/dagger2-sample/tree/master/examples/android-simple/src/main/java/com/example/dagger/simple

Inject Application Context in Repository with Dagger 2

You can use a dependency which provides these assets to the repository. And this dependency can contain a reference to the context. So your repository can simply query this dependency to get the assets it requires.

Here's a gist of it:

AssetProvider:

class AssetProvider @Inject constructor(
    private val context: Context
) {
    
    fun getDescription() = context.assets.open("tab1/item1/description.txt")
}

Repository:

@Singleton
class Repository @Inject constructor(
    private val assetProvider: AssetProvider
) {

    fun loadList(pageIndex: Int): List<String> {
        val input = assetProvider.getDescription()
        ...
    }
}

I like having repositories that have minimal dependency on Android specific stuff. So the repository logic is agnostic to the platform it runs on. This also helps in unit tests where you don't have to inject context to test your repository.

How to provide context with Dagger 2?

You are all good at using @Component.Builder but it can be further optimized.

Following are the changes:

Step 1 : Use @Binds in Context Module to provide Context

package com.mamak.geobaza.di

import android.content.Context
import dagger.Module
import dagger.Provides

@Module
abstract class ContextModule {  // to allow abstract method make module abstract

  //@Binds works on an abstract method

    @Singleton
    @Binds   // @Binds, binds the Application instance to Context
    abstract fun context(appInstance:Application): Context //just return the super-type you need 

}

Step 2 : Remove redundant code from AppComponent

package com.mamak.geobaza.di

import android.app.Application
import dagger.BindsInstance
import dagger.Component
import dagger.android.support.AndroidSupportInjectionModule
import javax.inject.Singleton

@Singleton
@Component(modules = [
    ApiModule::class,
    ViewModelModule::class,
    AndroidSupportInjectionModule::class,
    InterfaceModule::class,
    ContextModule::class
])
interface AppComponent {
    @Component.Builder
    interface Builder {
        @BindsInstance
        fun application(application: Application): Builder

        @BindsInstance
        fun apiModule(apiModule: ApiModule): Builder

        @BindsInstance
        fun interfaceModule(interfaceModule: InterfaceModule): Builder

       // @BindsInstance   //this two  commented lines can be removed 
       // fun contextModule(contextModule: ContextModule): Builder

       // why? because dagger already knows how to provide Context Module 

        fun build(): AppComponent
    }

    fun inject(appController: AppController)
}

Step 3: Modify the Component Builder to take advantage of @Component.Builder and @Binds

package com.mamak.geobaza.di

import android.app.Application

import dagger.android.AndroidInjector
import dagger.android.DispatchingAndroidInjector
import dagger.android.HasAndroidInjector
import javax.inject.Inject

class AppController : Application(), HasAndroidInjector {
    @Inject
    lateinit var dispatchingAndroidInjector: DispatchingAndroidInjector<Any>

    override fun androidInjector(): AndroidInjector<Any> {
        return dispatchingAndroidInjector
    }

    override fun onCreate() {
        super.onCreate()
        DaggerAppComponent.Builder()
            .application(this)
            .apiModule(ApiModule())
            .interfaceModule(InterfaceModule())
           // .contextModule(ContextModule(this)) //this line can be removed 
            .build()
            .inject(this)
    }
}

Dagger 2, Providing Application Context in Module

This is how It is done. Using @BindsInstance in your component will inject application to all of your modules.In your case just to AppModule

@Singleton
@Component(modules = arrayOf(AppModule::class))
interface AppComponent {
    @Component.Builder
    interface Builder() {
        fun build(): AppComponent

        @BindsInstance
        fun application(application: Application): Builder
    }
}

** Delete code to "Provides application" function in your APP module and make sure you pass application context to create sharedPreferences.

@Module
class AppModule {

@Provides
@Singleton
fun provideSharedPrefManager(context: Application): SharedPreferencesManager = 
    SharedPreferencesManager(context)
}

and now in your onCreate of applicationClass

DaggerAppComponent.builder().application(this).build()

Optional: if you want to inject something into your applicationClass then do the following

 DaggerAppComponent.builder().application(this).build().inject(this)

Injecting application context in library module with Dagger 2

Dagger 2 for Android comes to the rescue. It provides the concept of AndroidInjector, which is a Component that can be used to inject an instance in a static way, without having to know the dependency provider. Moreover, using the Dagger- prefixed classes provided out of the box, the injected dependencies look like coming from nowhere. Awesome.

All you have to do is declare in the library a top-level Module which is installed in the Application Component. This Module will provide all the dependencies and the SubComponents needed by the library, which will automatically inherit the @AppContext Context that you seeded in the dependency graph, ready to be injected anywhere in you library, as well as every dependency you provide through the main Application Component.

Here's a short example (written in Kotlin):

@Component(modules = [
    AndroidSupportInjectionModule::class,
    AppModule::class,
    LibraryModule::class //plug-in the library to the dependency graph
])
@Singleton
interface AppComponent : AndroidInjector<App> {

    @Component.Builder
    abstract class Builder : AndroidInjector.Builder<App>() {

        @BindsInstance
        abstract fun appContext(@AppContext context: Context)

        override fun seedInstance(instance: App) {
            appContext(instance)
        }
    }
}

Edit: extended examples

An example of the Application subclass:

// DaggerApplication provides out-of-the-box support to all the AndroidInjectors.
// See the class' code to understand the magic.
public class App extends DaggerApplication {

@Override
protected AndroidInjector<? extends DaggerApplication> applicationInjector() {
    // We only provide its own Injector, the Application Injector,
    // that is the previous AppComponent
    return DaggerAppComponent.builder().create(this);
}

And in your Android library:

@Module
public abstract class LibraryModule {

    @ContributesAndroidInjector
    public abstract LibraryActivity contributeLibraryActivityInjector();

}

---

public class LibraryActivity extends DaggerAppCompatActivity {

    @Inject
    @AppContext
    Context appContext;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceSate);
        // here you automagically have your injected application context!
        ExternalSingleton.getInstance(appContext)
    }
}

How to inject Context into a Presenter using Dagger 2

You're using CustomApplication with Dagger in your ApplicationComponent, so that's what it knows about. It doesn't try to resolve types on its own, so Application is some class Dagger never heard about.

You can either add another @Provides / @Binds to bind CustomApplication > Application > Context or just go the direct way and change your code to require a CustomApplication instead of Application:

@Provides
@Singleton
fun provideContext(application: CustomApplication): Context {
    return application
}

  // ... or alternatively ...

@Provides
@Singleton
fun provideApplication(application: CustomApplication): Application {
    return application
}

@Provides
@Singleton
fun provideContext(application: Application): Context {
    return application
}

Either way your application can then be used as a Context.

Dagger 2 Inject Context in Kotlin Object

Is this because I am using an object (Singleton)?

Yes, objects' properties are Java's static fields under the hood. Your Manager class would decompile to something similar to:

public final class Manager {
    @Inject
    @NotNull
    public static Context context;

    public static final Manager INSTANCE;

    static {
        INSTANCE = new Manager();
    }

    private Manager() {
    }

    @NotNull
    public final Context getContext() {
        return context;
    }

    public final setContext(Context var1) {
        context = var1;
    }
}

and Dagger 2 simply does not support injection into static fields. However, even if Dagger let you do it, the dependency wouldn't be satisfied. That's because Dagger won't be able to inject into an object unless is explicitly told to do so (like you do when injecting for example into activities) or creates the object by itself. Obviously the latter doesn't apply when it comes to Kotlin's objects. In order to ask Dagger to inject the dependency you would have to somehow provide a MainApp instance:

init {
    mainApp.appComponent.inject(this)
}

which doesn't make much sense since you'd like to inject it (as a Context) in the first place.

Therefore you have to either satisfy the dependency manually and don't bother with Dagger in this one or leave the object idea behind, use just a standard class and let Dagger handle its scope:

@Singleton
class Manager @Inject constructor(private val context: Context) {

}

The only drawback is that you would have to inject a Manager instance (created by Dagger) into every class that needs to use it instead of calling its methods statically.

Dagger failing to inject Application context

Injecting the repository into the viewmodel fixed this issue instead of trying to pass application context through the chain.

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