使用 MVVM 时无法创建类 ViewModel 的实例答案

【问题标题】：Cannot create instance of class ViewModel while using MVVM使用 MVVM 时无法创建类 ViewModel 的实例
【发布时间】：2020-07-17 01:15:03
【问题描述】：

我创建了ViewModel 类，并尝试创建我的实例 ViewModel 类使用 ViewModelProvider。在运行时，我收到此错误 “无法创建类 className ViewModel 的实例。这里是所需代码：

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 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'
}

错误信息：

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) 
```text

【问题讨论】：

标签： android mvvm viewmodel android-viewmodel viewmodelproviders

【解决方案1】：

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，以实现使用Fragment 1.2.0时相同的功能。

最后一部分很重要，否则new ViewModelProvider(this) 将只使用您在异常中看到的NewInstanceFactory，它不支持AndroidViewModel。 AppCompat 1.1.0 仅依赖于 Fragment 1.1.0，因此您无法获得对您正在使用的 ViewModelProvider 构造函数的支持。

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

implementation 'androidx.fragment:fragment:1.2.4'

【讨论】：