问题
Im pretty new in Android development and newer in DI. I am using Kotlin on a personal project where I am experimenting with Dagger 2. I managed to set it up for a util class, but where i need to have a context to use it to inject a class that requires a context (a sharedpref manager class), I failed. Here is my code, and here is the error ( NPE ) i am getting. Thank you in advance.
my module class
package com.android.pine
import android.content.Context
import com.android.pine.utils.SharedPreferencesManager
import dagger.Module
import dagger.Provides
import javax.inject.Singleton
@Module
class AppModule {
@Provides
@Singleton
fun context(pineApplication: PineApplication): Context = pineApplication.applicationContext
@Provides
@Singleton
fun provideSharedPrefManager(context: Context): SharedPreferencesManager = SharedPreferencesManager(context)
}
my component class:
package com.android.pine
import com.android.pine.home.HomePresenter
import com.android.pine.home.categories.CategoryAdapter
import dagger.Component
import javax.inject.Singleton
@Singleton
@Component(modules = arrayOf(AppModule::class))
interface AppComponent {
fun inject(categoryAdapter: CategoryAdapter)
fun inject(homePresenter: HomePresenter)
}
edit: added the info below, How I call the inject of sharedPreferencesManager:
class HomePresenter : BasePresenter<HomeView>() {
@Inject
lateinit var sharedPreferencesManager: SharedPreferencesManager
.
.
.
Also in my HomePresenter class, in onAttached method override:
DaggerAppComponent.create().inject(this)
My pineApplication class and SharedPrefManager class looks like this:
class PineApplication @Inject constructor(): Application()
SharedPref:
class SharedPreferencesManager @Inject constructor(context: Context) {
.
.
.
Crash, can not get the pineApplication.getContext() (edited, added the full stack trace)
06-02 11:57:01.028 14840-14840/com.android.pine.debug E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.android.pine.debug, PID: 14840
java.lang.RuntimeException: Unable to resume activity {com.android.pine.debug/com.android.pine.home.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3429)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3469)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2732)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
at com.android.pine.AppModule.context(AppModule.kt:12)
at com.android.pine.AppModule_ContextFactory.proxyContext(AppModule_ContextFactory.java:34)
at com.android.pine.DaggerAppComponent.getContext(DaggerAppComponent.java:29)
at com.android.pine.DaggerAppComponent.getSharedPreferencesManager(DaggerAppComponent.java:34)
at com.android.pine.DaggerAppComponent.injectHomePresenter(DaggerAppComponent.java:59)
at com.android.pine.DaggerAppComponent.inject(DaggerAppComponent.java:49)
at com.android.pine.home.HomePresenter.onAttached(HomePresenter.kt:31)
at com.android.pine.home.HomePresenter.onAttached(HomePresenter.kt:10)
at com.android.pine.core.BaseActivity.onResume(BaseActivity.kt:34)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1269)
at android.app.Activity.performResume(Activity.java:6783)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3406)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3469)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2732)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
回答1:
You can't use class PineApplication @Inject constructor(): Application() to create PineApplication. It's a framework class and has to be created by the Android Framework.
Doing so Dagger will create PineApplication, but applicationContext will return null as it has never been initialized (by the system).
Don't use constructor injection for framework classes and don't create the yourself. Use @Bindsintance to add the object to the component with its builder, or use a module to provide it.
回答2:
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)
回答3:
For provide application context, you can create class, like ComponentsManager
with:
public class ComponentsManager {
private Context context;
private AppComponent appComponent;
public ComponentsManager(Context context) {
this.context = context.getApplicationContext();
}
public AppComponent getAppComponent(){
if (appComponent == null){
appComponent = DaggerAppComponent.builder()
.appModule(new AppModule(context))
.build();
}
return appComponent;
}
}
And in your application class init this ComponentsManager like this:
public class YourApp extends Application {
private static ComponentsManager componentsManager;
@Override
public void onCreate() {
super.onCreate();
initComponentsManager();
initAppComponent();
}
public static ComponentsManager getComponentsManager(){
return componentsManager;
}
private void initComponentsManager(){
componentsManager = new ComponentsManager(this);
}
private void initAppComponent(){
componentsManager.getAppComponent();
}
}
回答4:
You can try modifying your App module like this.
@Module
class ApplicationModule(private val application: Application) {
@Provides
@Singleton
fun provideContext(): Context {
return application.applicationContext
}
@Provides
@Singleton
fun provideSharedPreferences(context: Context): SharedPreferences {
return context.getSharedPreferences(PREF_FILE_NAME, Context.MODE_PRIVATE)
}
}
Then you can build the dagger component like this from your Application class.
val appComponent = DaggerAppComponent.builder()
.applicationModule(ApplicationModule(this))
.build()
Inject values in presenter like this.
application.appComponent.inject(this)
来源：https://stackoverflow.com/questions/50655722/dagger-2-providing-application-context-in-module