Question

按照android给出的示例，我扩展了ViewModel类

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'
}

运行此代码将导致无法创建类NameViewModel的实例，我在这里做什么错了？

Answer 1

您在ViewModel类中缺少构造函数，因此将public NameViewModel() {};添加到ViewModel中。

Answer 2

代码似乎没什么问题。

您能否检查build.gradle中是否具有此特定的依赖版本

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

，您能添加您的build.gradle吗？

Answer 3

也许您应该在ViewModel调用super()中添加一个空的构造函数吗？

Answer 4

我将NameViewModel与NameActivity放在同一个Java文件中，该文件应放在单独的文件NameViewModel.java中，对此进行了修复。

Answer 5

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

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

Android LiveData，无法创建类viewModel的实例

5 个答案: