ViewModel 没有零参数构造函数错误 - 即使它具有零参数构造函数

Question

我是 Android 和 Java 的新手，正在尝试制作基于位置的应用程序。

编辑

我做了一个非常非常简单的测试代码，但得到了同样的错误。这是 java:

package com.example.viewmodeltest;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;

import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    public class MyViewModel extends ViewModel {
        public int scoreTeamA = 0;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        MyViewModel locationViewModel = new ViewModelProvider(this).get(MyViewModel.class);
    }
}

我得到了同样的错误。这是我的应用程序级别 build.gradle:

中的依赖项

dependencies {
    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.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

    //dependencies for ViewModel, LiveData, etc.
    def lifecycle_version = "2.2.0"
    def arch_version = "2.1.0"

    // 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:$lifecycle_version"
    // Saved state module for ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"
    // Annotation processor
    annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
}

原版POST

我正在尝试使用 ViewModel 和 LiveData 来更新用户位置，据我所知，这是了解生命周期的最佳方式。我有一个默认地图 activity...

public class MapsActivity extends AppCompatActivity implements OnMapReadyCallback {...}

扩展 LiveData 以存储用户位置的 class...

public class LocationLiveData extends LiveData<Location> {
        private final Context context;
        private FusedLocationProviderClient fusedLocationClient;
        private LocationRequest locationRequest;

        public LocationLiveData(Context context) {
            this.context = context;
            this.fusedLocationClient = LocationServices.getFusedLocationProviderClient(context);
        }

        private void setLocationData(Location location) {
            Location value = new Location("SetInternal");
            value.setLatitude(location.getLatitude());
            value.setLongitude(location.getLongitude());
            setValue(value);
        }

        protected void createLocationRequest() {
            LocationRequest locationRequest = LocationRequest.create();
            locationRequest.setInterval(1000);
            locationRequest.setFastestInterval(500);
            locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
        }

        private LocationCallback locationCallback = new LocationCallback() {
            @Override
            public void onLocationResult(LocationResult locationResult) {
                if (locationResult == null) {
                    return;
                }
                for (Location location : locationResult.getLocations()) {
                    setLocationData(location);
                }
            }
        };

        private void startLocationUpdates() {
            createLocationRequest();
            fusedLocationClient.requestLocationUpdates(locationRequest,
                    locationCallback,
                    Looper.getMainLooper());
        }

        @Override
        protected void onInactive() {
            super.onInactive();
            fusedLocationClient.removeLocationUpdates(locationCallback);
        }

        @Override
        protected void onActive() {
            super.onActive();
            fusedLocationClient.getLastLocation()
                    .addOnSuccessListener(new OnSuccessListener<Location>() {
                        @Override
                        public void onSuccess(Location location) {
                            if (location != null)
                                setValue(location);
                        }
                    });
            startLocationUpdates();
        }
    }

还有一个 class 扩展 ViewModel 以允许主要 activity 到达 LocationLiveData。

 public class LocationViewModel extends ViewModel {
        private LocationLiveData locationLiveData;

        public LocationViewModel () {
            locationLiveData = new LocationLiveData(getApplicationContext());
        }

        public LocationLiveData getLocationLiveData() {
            return locationLiveData;
        }
    }

然后当我尝试在 onMapReady 方法中创建 locationViewModel 的实例时：

LocationViewModel locationViewModel = new ViewModelProvider(this).get(LocationViewModel.class);

我在该行收到错误：

Cannot create an instance of class com.example.MapsActivity$LocationViewModel

Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor

即使我完全在 locationViewModel 中取出构造函数，并且如果我尝试扩展 AndroidViewModel，我也会收到此错误。

有什么想法吗？我见过其他类似的查询，但答案始终是从构造函数中取出参数——我已经这样做了！

非常感谢您的帮助

Answer 1

或者：

将 MyViewModel 移动到单独的 Java 文件，或
使 MyViewModel 成为 static class

现在，您已将 MyViewModel 定义为 MainActivity 的内部 class。这是行不通的，因为只有 MainActivity 的实例才能创建 MyViewModel 的实例。特别是，ViewModelProvider 无法创建 MyViewModel.

的实例

Answer 2

写的时候

public class MainActivity extends AppCompatActivity {

    public class MyViewModel extends ViewModel {
       public int scoreTeamA = 0;
    }
}

这使得 MyViewModel 成为 MainActivity 的内部 class，这意味着它具有对外部 class 的隐式引用（根据 Java documentation）并且不能与 MainActivity 分开构造。这实际上意味着在字节码中，它的构造函数总是采用 MainActivity 的实例——即使你没有写出那个构造函数。

您可以通过添加 static 关键字使内部 class 成为静态嵌套 class。这删除了对外部 class 的隐式引用，并使其类似于任何其他顶级 class:

public class MainActivity extends AppCompatActivity {

    public static class MyViewModel extends ViewModel {
       public int scoreTeamA = 0;
    }
}

Answer 3

while using hilt I am facing the same issue when I forgot to add

@AndroidEntryPoint

在 activity 声明上方

Answer 4

在我的例子中忘记添加 HasSupportFragmentInjector 我的 Activity !!!

public class MainActivity extends AppComaptActivity
        implements HasSupportFragmentInjector{


@Override
public void onCreate(Bundle savedInstanceState) {
    AndroidInjection.inject(this);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    @Inject
    DispatchingAndroidInjector<Fragment> fragmentDispatchingAndroidInjector;


    @Override
    public AndroidInjector<Fragment> supportFragmentInjector() {
        return fragmentDispatchingAndroidInjector;
    }
}

ViewModel 没有零参数构造函数错误 - 即使它具有零参数构造函数

ViewModel has no zero argument constructor error - even when it has a zero argument constructor

java

android

android-fusedlocation

android-livedata

android-viewmodel