使用MVVM时无法创建ViewModel类的实例

问题描述 投票：0回答：1

我已经创建了ViewModel类，并尝试使用ViewModelProvider创建我的ViewModel类的实例。在运行时，我收到此错误“无法创建类className ViewModel的实例。这是必需的代码：

MainActivity.java

public class MainActivity extends AppCompatActivity {

MainActivityViewModel mainActivityViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

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

MainActivityViewModel.java

public class MainActivityViewModel extends AndroidViewModel {
private BookShopRepository bookShopRepository;

public MainActivityViewModel(@NonNull Application application) {
    super(application);
    bookShopRepository = new BookShopRepository(application);
   } 
}

BookShopRepository.java

public class BookShopRepository {

private CategoryDAO categoryDAO;
private BookDAO bookDAO;
private LiveData<List<Category>> categories;
private LiveData<List<Book>> books;

public BookShopRepository(Application application) {

    BooksDatabase booksDatabase = BooksDatabase.getInstance(application);
    categoryDAO = booksDatabase.categoryDAO();
    bookDAO = booksDatabase.bookDAO();
   } 
}

build.gradle依赖项

dependencies {
def room_version = "2.2.5"
def lifecycle_version = "2.2.0"

// ViewModel
implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
// LiveData
implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"

implementation "androidx.room:room-runtime:$room_version"
annotationProcessor "androidx.room:room-compiler:$room_version"

implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'androidx.test:runner:1.2.0'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

}

错误消息：

Process: com.example.ch5project, PID: 3754
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ch5project/com.example.ch5project.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.ch5project.viewmodel.MainActivityViewModel
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
    at android.app.ActivityThread.access$800(ActivityThread.java:151)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.ch5project.viewmodel.MainActivityViewModel
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
    at com.example.ch5project.MainActivity.onCreate(MainActivity.java:24)
    at android.app.Activity.performCreate(Activity.java:5990)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
    at android.app.ActivityThread.access$800(ActivityThread.java:151) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5254) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
 Caused by: java.lang.InstantiationException: class com.example.ch5project.viewmodel.MainActivityViewModel has no zero argument constructor
    at java.lang.Class.newInstance(Class.java:1597)
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
    at com.example.ch5project.MainActivity.onCreate(MainActivity.java:24) 
    at android.app.Activity.performCreate(Activity.java:5990) 
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
    at android.app.ActivityThread.access$800(ActivityThread.java:151) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5254) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
 Caused by: java.lang.NoSuchMethodException: <init> []
    at java.lang.Class.getConstructor(Class.java:531)
    at java.lang.Class.getDeclaredConstructor(Class.java:510)
    at java.lang.Class.newInstance(Class.java:1595)
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219) 
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
    at com.example.ch5project.MainActivity.onCreate(MainActivity.java:24) 
    at android.app.Activity.performCreate(Activity.java:5990) 
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
    at android.app.ActivityThread.access$800(ActivityThread.java:151) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5254) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
android mvvm viewmodel android-viewmodel viewmodelproviders
1个回答
1
投票

根据Lifecycle 2.2.0 release notes：

ViewModelProviders.of()弃用：ViewModelProviders.of()已弃用。您可以将Fragment或FragmentActivity传递给新的ViewModelProvider(ViewModelStoreOwner)构造函数以实现相同的功能使用Fragment 1.2.0时。

最后一部分很重要，否则new ViewModelProvider(this)将仅使用您在例外中看到的NewInstanceFactory，但不支持AndroidViewModel。 AppCompat 1.1.0仅取决于片段1.1.0，因此您没有获得对所使用的ViewModelProvider构造函数的支持。

因此，您应该升级到片段1.2.0或更高版本（当前是1.2.4）：

implementation 'androidx.fragment:fragment:1.2.4'
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