使用MVVM时无法创建类ViewModel的实例

问题描述

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，出现此错误无法创建类 className ViewModel的实例.这是必需的代码:

I have created ViewModel class, and trying to create an instance of my ViewModel class using ViewModelProvider. At runtime, I'm getting this error "Cannot create an instance of class className ViewModel. Here are the required codes:

MainActivity.java

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

MainActivityViewModel.java

public class MainActivityViewModel extends AndroidViewModel {
private BookShopRepository bookShopRepository;

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

BookShopRepository.java

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

build.gradle dependencies

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) 

推荐答案

按照 Lifecycle 2.2.0发行说明:

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，该异常不支持AndroidViewModel. AppCompat 1.1.0仅取决于Fragment 1.1.0，因此您没有获得对所使用的ViewModelProvider构造函数的支持.

That last part is important, as otherwise new ViewModelProvider(this) will only use the NewInstanceFactory you see in the exception, which doesn't support AndroidViewModel. AppCompat 1.1.0 only depends on Fragment 1.1.0, so you're not getting the support for the ViewModelProvider constructor you're using.

因此，您应该升级到Fragment 1.2.0或更高版本(当前为1.2.4):

Therefore, you should upgrade to Fragment 1.2.0 or higher (currently 1.2.4):

implementation 'androidx.fragment:fragment:1.2.4'

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