[英]How to properly use Dagger2 with the new Android Architecture Components
我正在嘗試使用新的架構組件,但我還是匕首的新手,而且我缺少了東西。
使用下面的代碼,我得到一個NullPointerException,找不到位置。 如果還有其他我需要修復或改進的地方,請建議。
代碼: ViewModel
public class PostsVM extends ViewModel {
private LiveData<StoryPost> post;
private Repository repository;
@Inject
public PostsVM(Repository repository) {
this.repository = repository;
}
public void init() {
if (this.post != null) {
return;
}
post = repository.getPosts();
}
public LiveData<StoryPost> getPost() {
return post;
}
}
知識庫
@Singleton
public class Repository {
private final MutableLiveData<StoryPost> data = new MutableLiveData<>();
public LiveData<StoryPost> getPosts() {
//
new GetUser(post.getUid()) {
@Override
public void onSuccess(@NonNull User user) {
// this is where I setValue//
data.setValue(post);
}
@Override
public void onError() {
}
@Override
public void userNotFound() {
}
};
return data;
}
}
Singleton工廠
@Singleton
public class ViewModelFactory implements ViewModelProvider.Factory {
private final Map<Class<? extends ViewModel>, Provider<ViewModel>> creators;
@Inject
public ViewModelFactory(Map<Class<? extends ViewModel>, Provider<ViewModel>> creators) {
this.creators = creators;
}
@SuppressWarnings("unchecked")
@Override
public <T extends ViewModel> T create(Class<T> modelClass) {
Provider<? extends ViewModel> creator = creators.get(modelClass);
if (creator == null) {
for (Map.Entry<Class<? extends ViewModel>, Provider<ViewModel>> entry : creators.entrySet()) {
if (modelClass.isAssignableFrom(entry.getKey())) {
creator = entry.getValue();
break;
}
}
}
if (creator == null) {
throw new IllegalArgumentException("unknown model class " + modelClass);
}
try {
return (T) creator.get();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}
DAO
@Dao
public interface PostDao {
@Query("SELECT * FROM posts ORDER by time DESC")
LiveData<List<StoryPost>> getAll();
@Query("SELECT * FROM posts WHERE id = :id")
LiveData<List<StoryPost>> getPost(String id);
@Insert(onConflict = OnConflictStrategy.REPLACE)
@NonNull
void insert(StoryPost... posts);
@Delete
void delete(StoryPost post);
@Update
void update(StoryPost post);
}
然后在MainActivity中:
@Inject public ViewModelFactory factory;
//...
//*onCreate*
PostsVM model = ViewModelProviders.of(this, factory).get(PostsVM.class);
model.init();
final Observer<StoryPost> observer = post -> storyAdapter.insert(post);
model.getPost().observe(this, observer);
Logcat :
... java.lang.NullPointerException: Attempt to invoke interface method
'android.arch.lifecycle.ViewModel android.arch.lifecycle.ViewModelProvider
$Factory.create(java.lang.Class)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2479)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2539)
at android.app.ActivityThread.access$900(ActivityThread.java:168)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1378)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:150)
at android.app.ActivityThread.main(ActivityThread.java:5665)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:799)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:689)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'android.arch.lifecycle.ViewModel android.arch.lifecycle.ViewModelProvider$Factory.create(java.lang.Class)' on a null object reference
at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:128)
at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:96)
at com.aollay.smartpaper.MainActivity.bindDatabase(MainActivity.java:238)
at com.aollay.smartpaper.MainActivity.populateNews(MainActivity.java:233)
at com.aollay.smartpaper.MainActivity.config(MainActivity.java:159)
at com.aollay.smartpaper.MainActivity.onCreate(MainActivity.java:74)
at android.app.Activity.performCreate(Activity.java:6372)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2432)
該問題是由引起ViewModelFactory
實例為您的內部空MainActivity
,而NPE建議。 這本身很可能是因為沒有正確注入ViewModelFactory
,因此保持為null。 正如Orest在評論中建議的那樣,您需要確保從AppModule
正確注入MainActivity
:
主要活動:
public class MainActivity extends AppCompatActivity implements HasSupportFragmentInjector
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
AndroidInjection.inject(activity);
super.onCreate(savedInstanceState);
}
// if your Activity also has Fragments which need to be injected
@Inject
DispatchingAndroidInjector<Fragment> androidInjector;
@Override
public DispatchingAndroidInjector<Fragment> supportFragmentInjector()
{
return androidInjector;
}
}
您可以查看我之前在AndroidInjector上發布的相關問題中使用的大多數DI類。 如果沒有@ Provide-或@ Produces-annotated方法 , 則無法提供,並查看該設置是否有幫助你出去。
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