如何在Java中使用androidx.lifecycle.ViewModelProvider

huangapple
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英文:

How to use the androidx.lifecycle.ViewModelProvider in Java

问题

我最近在学习关于架构组件的内容，正在按照旧教程使用旧方法进行操作：

  1. mainActivityViewModel =
  2.     new ViewModelProvider(this).get(MainActivityViewModel.class);

但在ViewModelProvider的文档中，唯一可用的构造函数是：

  1. ViewModelProvider(ViewModelStoreOwner, Factory) 和 ViewModelProvider(ViewModelStore, Factory)。

所以我尝试了以下方法，但不确定在重写的方法中应该返回什么，它目前返回null，导致程序崩溃。

  1. public class MainActivity extends AppCompatActivity {
  2.     private NoteViewModel noteViewModel;
  4.     @Override
  5.     protected void onCreate(Bundle savedInstanceState) {
  6.         super.onCreate(savedInstanceState);
  7.         setContentView(R.layout.activity_main);
  9.         noteViewModel = new ViewModelProvider(this, new ViewModelProvider.Factory() {
  10.             @NonNull
  11.             @Override
  12.             public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
  13.                 return null;
  14.             }
  15.         }).get(NoteViewModel.class);
  17.         noteViewModel.getAllNotes().observe(this, new Observer<List<NoteEntity>>() {
  18.             @Override
  19.             public void onChanged(List<NoteEntity> noteEntities) {
  20.                 Toast.makeText(MainActivity.this,"Changed",Toast.LENGTH_LONG).show();
  21.             }
  22.         });
  23.     }
  24. }

我的方法是否正确？我现在完全迷失了。我应该从重写的方法中返回什么？

英文:

I'm recently learning about the architectural components and was following the old tutorial where they used the old method:
<!-- language-all: java -->

  1. mainActivityViewModel =
  2.     new ViewModelProvider(this).get(MainActivityViewModel.class);

But in the documentation for the ViewModelProvider, the only constructors available are
ViewModelProvider(ViewModelStoreOwner, Factory) &
ViewModelProvider(ViewModelStore, Factory).

So I did something like this but I'm not sure what to do in the overridden method and it currently returns null that crashes the program.

  1. public class MainActivity extends AppCompatActivity {
  2.     private NoteViewModel noteViewModel;
  4.     @Override
  5.     protected void onCreate(Bundle savedInstanceState) {
  6.         super.onCreate(savedInstanceState);
  7.         setContentView(R.layout.activity_main);
  9.         noteViewModel = new ViewModelProvider(this, new ViewModelProvider.Factory() {
  10.             @NonNull
  11.             @Override
  12.             public &lt;T extends ViewModel&gt; T create(@NonNull Class&lt;T&gt; modelClass) {
  13.                 return null;
  14.             }
  15.         }).get(NoteViewModel.class);
  17.         noteViewModel.getAllNotes().observe(this, new Observer&lt;List&lt;NoteEntity&gt;&gt;() {
  18.             @Override
  19.             public void onChanged(List&lt;NoteEntity&gt; noteEntities) {
  20.                 Toast.makeText(MainActivity.this,&quot;Changed&quot;,Toast.LENGTH_LONG).show();
  21.             }
  22.         });
  23.     }
  24. }

Is my approach correct? I'm absolutely lost right now. What am I supposed to return from the overridden method?

答案1

得分: 2

noteViewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(NoteViewModel.class);

当我们在 ViewModel 的构造函数中传递参数时（除了 Application 参数），我们使用自定义的工厂。

在 Gradle 依赖中添加以下代码：

  1. def lifecycle_version = "2.2.0"
  3. // LiveData
  4. implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
  5. //
  6. implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
  7. // ViewModel
  8. implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
英文:

Use this

  1. noteViewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(NoteViewModel.class;

We use custom factory, when we pass param to the constructor of ViewModel (apart from Application param).

and gradle dependency in case,

  1. def lifecycle_version = &quot;2.2.0&quot;
  3.     // LiveData
  4.     implementation &quot;androidx.lifecycle:lifecycle-livedata:$lifecycle_version&quot;
  5.     //
  6.     implementation &quot;androidx.lifecycle:lifecycle-common-java8:$lifecycle_version&quot;
  7.     // ViewModel
  8.     implementation &quot;androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version&quot;

答案2

得分: 1

首先，您需要在活动类中初始化 ViewModel，如下所示：

  1. noteViewModel = ViewModelProvider(this).get(NoteViewModel.class);

在 NoteViewModel.java 中：

您需要为存储模型提供的更新数据定义 LiveData 变量，并将其更新传递到 ViewModel。

  1. public class NoteViewModel extends AndroidViewModel {
  2.     AppRepository appRepository;
  3.     MediatorLiveData<List<NoteEntity>> mediatorData;
  4.    
  5.     public NoteViewModel (@NonNull Application application) {
  6.         super(application);
  7.         mediatorData = new MediatorLiveData<>();
  8.         appRepository = new AppRepository(MyApplication.apiService, application.retrofit);
  9.     }
  11.     public MediatorLiveData<List<NoteEntity>> getMediatorLiveData() {
  12.         return mediatorData;
  13.     }
  15.     public void getNoteEntry() {
  16.         try {
  17.             mediatorData.addSource(appRepository.getNoteEntry(), new Observer<List<NoteEntity>>() {
  18.                 @Override
  19.                 public void onChanged(@Nullable List<NoteEntity> data) {
  20.                     mediatorData.postValue(data);
  21.                 }
  22.             });
  23.         } catch (Exception e) {
  25.         }
  26.     }
  27. }

在 MainActivity 的 onCreate() 方法中，注册观察者并从 ViewModel 调用 API：

  1. noteViewModel.getMediatorLiveData().observe(this, new Observer<List<NoteEntity>>() {
  2.     @Override
  3.     public void onChanged(List<NoteEntity> noteEntities) {
  4.         Toast.makeText(MainActivity.this, "Changed", Toast.LENGTH_LONG).show();
  5.     }
  6. });
  8. noteViewModel.getNoteEntry();

AppRepository.java 文件如下：

  1. class AppRepository() {
  2.     ApiService apiService;
  3.     Retrofit retrofit;
  5.     public AppRepository(ApiService apiService, Retrofit retrofit) {
  6.         this.apiService = apiService;
  7.         this.retrofit = retrofit;
  8.     }
  10.     public MediatorLiveData<List<NoteEntity>> getNotes() {
  11.         MediatorLiveData<List<NoteEntity>> data = new MediatorLiveData<List<NoteEntity>>();
  13.         apiService.getNotes().enqueue(new Callback<List<NoteEntity>>() {
  14.             @Override
  15.             void onResponse(Call<List<NoteEntity>> call, Response<List<NoteEntity>> response) {
  16.                 if (response.isSuccessful()) {
  17.                     if (response.body() != null) {
  18.                         data.postValue(response.body()); //成功数据
  19.                     } else {
  20.                         data.postValue(null); //错误
  21.                     }
  22.                 } else {
  23.                     data.postValue(null); //错误
  24.                 }
  25.             }
  27.             @Override
  28.             void onFailure(Call<List<NoteEntity>> call, Throwable t) {
  29.                 data.postValue(null); //错误
  30.             }
  31.         });
  33.         return data;
  34.     }
  35. }
英文:

First you need to initialize viewmodel in activity class like :

  1. noteViewModel = ViewModelProvider(this).get(NoteViewModel.class);

In NoteViewModel.java

You need to define livedata variable for storing updated data provided by model and update post to view model

NoteViewModel.java file look like :

  1. public class NoteViewModel extends AndroidViewModel {
  2.     AppRepository appRepository;
  3.     MediatorLiveData&lt;List&lt;NoteEntity&gt;&gt; mediatorData;
  4.    
  5.     public NoteViewModel (@NonNull Application application) {
  6.         super(application);
  7.         mediatorData=new MediatorLiveData&lt;&gt;();
  8.         appRepository=new AppRepository((MyApplication).apiService, application.retrofit);
  10.     }
  12.     public MediatorLiveData&lt;List&lt;NoteEntity&gt;&gt; getMediatorLiveData() {
  13.         return mediatorVideoData;
  14.     }
  16.    
  18.     public void getNoteEntry()
  19.     {
  20.         try {
  22.             mediatorData.addSource(appRepository.getNoteEntry(), new Observer&lt;List&lt;NoteEntity&gt;&gt;() {
  23.                 @Override
  24.                 public void onChanged(@Nullable List&lt;NoteEntity&gt; data) {
  25.                     mediatorData.postValue(data);
  26.                 }
  27.             });
  28.         }catch (Exception e)
  29.         {
  31.         }
  32.     }
  34.    
  35. }

In onCreate() of Mainactivity register observer like and call the API from view model like

  1.  noteViewModel.getMediatorLiveData().observe(this, new Observer&lt;List&lt;NoteEntity&gt;&gt;() {
  2.             @Override
  3.             public void onChanged(List&lt;NoteEntity&gt; noteEntities) {
  4.                 Toast.makeText(MainActivity.this,&quot;Changed&quot;,Toast.LENGTH_LONG).show();
  5.             }
  6.         });
  8.   noteViewModel.getNoteEntry();

AppRepository.java file look like

  1. class AppRepository() {
  2. ApiService apiService;
  3. Retrofit retrofit;
  4. public AppRepository(ApiService apiService ,Retrofit retrofit){
  5.  this.apiService = apiService;
  6.  this.retrofit = retrofit;
  7. }
  9.     public MediatorLiveData&lt;List&lt;NoteEntity&gt;&gt;  getNotes() {
  11.         MediatorLiveData&lt;List&lt;NoteEntity&gt;&gt; data = new MediatorLiveData&lt;List&lt;NoteEntity&gt;&gt;()
  12.        
  13.         apiService.getNotes()
  14.             .enqueue(new Callback&lt;List&lt;NoteEntity&gt;&gt; {
  16.                 @Override 
  17.                 void onResponse(
  18.                     Call&lt;List&lt;NoteEntity&gt;&gt; call,
  19.                     Response&lt;List&lt;NoteEntity&gt;&gt; response
  20.                 ) {
  21.                     if (response.isSuccessful()) {
  22.                         if(response.body()!=null){
  23.                         data.postValue(response.body()); //successfull data
  24.                          }else{
  25.                          data.postValue(null); //error
  26.                          }
  27.                         
  28.                     } else {
  29.                         data.postValue(null); //error
  30.                     }
  31.                 }
  32.                  @Override
  33.                 fun onFailure(
  34.                     Call&lt;List&lt;NoteEntity&gt;&gt; call,
  35.                     Throwable t
  36.                 ) {
  37.                      data.postValue(null); //error 
  38.                 }
  39.             })
  41.         return data;
  42.     }
  43. }

答案3

得分: 1

  1. Kotlin 代码。
  3. 在 build.gradle 中添加
  5.     implementation "androidx.activity:activity-ktx:1.1.0"
  7. 然后
  9.     val noteViewModel by viewModels<NoteViewModel>()
英文:

Kotlin Code.

In build.gradle add

  1. implementation &quot;androidx.activity:activity-ktx:1.1.0&quot;

then

  1. val noteViewModel by viewModels&lt;NoteViewModel&gt;()

答案4

得分: 0

找到了解决方法，经过4个小时的研究。

  1. noteViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(this.getApplication())).get(NoteViewModel.class);

另外，这些依赖项应该添加到 gradle 文件中。

  1. def room_version = "2.2.5";
  2. def lifecycle_version = "2.2.0";
  4. implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
  5. // LiveData
  6. implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
  7. annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
  9. implementation "androidx.room:room-runtime:$room_version"
  10. annotationProcessor "androidx.room:room-compiler:$room_version"

不确定错误是否是因为我漏掉了注解依赖，但现在一切都正常工作了。

英文:

Figured it out after 4 hours.

  1. noteViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(this.getApplication())).get(NoteViewModel.class);

Also these dependencies should be added to the gradle file.

  1.     def room_version = &quot;2.2.5&quot;
  2.     def lifecycle_version = &quot;2.2.0&quot;
  4.     implementation &quot;androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version&quot;
  5. // LiveData
  6.     implementation &quot;androidx.lifecycle:lifecycle-livedata:$lifecycle_version&quot;
  7.     annotationProcessor &quot;androidx.lifecycle:lifecycle-compiler:$lifecycle_version&quot;
  9.     implementation &quot;androidx.room:room-runtime:$room_version&quot;
  10.     annotationProcessor &quot;androidx.room:room-compiler:$room_version&quot;

Not sure if the error was caused because I missed the annotation dependency. But everything works now.

huangapple
  • 本文由 发表于 2020年8月24日 14:24:47
  • 转载请务必保留本文链接：https://go.coder-hub.com/63555733.html
  • android
  • android-room
  • java
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