Android LiveData，无法创建 class viewModel 的实例

Question

按照android给出的示例，我扩展了 ViewModel class

public class NameViewModel extends ViewModel {
private MutableLiveData<String> currentName;

public MutableLiveData<String> getCurrentName() {
    if (currentName == null) {
        currentName = new MutableLiveData<String>();
    }
    return currentName;
}}

我观察了 LiveData 对象

public class NameActivity extends AppCompatActivity {

private NameViewModel model;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Other code to setup the activity...

    // Get the ViewModel.
    model = new ViewModelProvider(this).get(NameViewModel.class);

    // Create the observer which updates the UI.
    final Observer<String> nameObserver = new Observer<String>() {
        @Override
        public void onChanged(@Nullable final String newName) {
            // Update the UI, in this case, a TextView.
            nameTextView.setText(newName);
        }
    };

    // Observe the LiveData, passing in this activity as the LifecycleOwner and the observer.
    model.getCurrentName().observe(this, nameObserver);
}}

我在 build.gradle 中的依赖项是

dependencies {
    def lifecycle_version = "2.2.0"

    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'

    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    // Lifecycles only (without ViewModel or LiveData)
    implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"


    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
}

运行此代码给出无法创建 class NameViewModel 的实例，我在这里做错了什么？

Answer 1

您缺少 ViewModel class 中的构造函数，因此添加public NameViewModel() {}; 到您的 ViewModel。

Answer 2

不过，代码似乎没有任何问题。

你能检查一下你的 build.gradle 中是否有这个特定的依赖版本吗

 implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'

您能否添加您的 build.gradle。

Answer 3

您是否应该在ViewModel中添加一个空的构造函数并在那里调用super() ？

Answer 4

我在同一个 java 文件中有 NameViewModel 作为 NameActivity，它应该在单独的文件 NameViewModel.java 中，这修复了它。

Answer 5

而是： model = new ViewModelProvider(this).get(NameViewModel.class);

试试这个： model = new ViewModelProvider(this,new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(NameViewModel.class);