Cannot create an instance of class AndroidViewModel (androidx ViewModelProvider AndroidViewModel)

使用 androidx viewmodel 2.2.0 后，对 ViewModel 的创建有一些变化

原：ViewModelProviders.of(activity).get(ViewModel.class)

被替换为：ViewModelProvider(activity).get(ViewModel.class)

而使用 ViewModelProvider 创建继承自 AndroidViewModel 的类会报错 Cannot create an instance of class AndroidViewModel

原因是无法创建带参数的构造方法，原因是 ViewModelProvider 默认创建无参的构造方法，如果有参数的构造方法将无法正常创建。而 AndroidViewModel  需要使用带 application 参数的构造方法创建，解决办法是使用 AndroidViewModelFactory 创建 AndroidViewModel。如下

ViewModelProvider(
            activity,
            ViewModelProvider.AndroidViewModelFactory.getInstance(application)
        ).get(ViewModel.class)

如果你需要创建更多参数的构造方法，需要自定义 AndroidViewModelFactory，如下是多了一个 String 型的AndroidViewModelFactory

class AndroidViewModelFactory private constructor(
    private val mApplication: Application,
    private val mName: String
) : ViewModelProvider.NewInstanceFactory() {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        return if (AndroidViewModel::class.java.isAssignableFrom(modelClass)) {
            try {
                modelClass.getConstructor(
                    Application::class.java, String::class.java
                ).newInstance(mApplication, mName)
            } catch (e: Exception) {
                throw RuntimeException("Cannot create an instance of $modelClass", e)
            }
        } else super.create(modelClass)
    }

    companion object {
        private var sInstance: AndroidViewModelFactory? = null
        fun getInstance(
            application: Application,
            name: String
        ): AndroidViewModelFactory {
            if (sInstance == null) {
                sInstance = AndroidViewModelFactory(application, name)
            }
            return sInstance as AndroidViewModelFactory
        }
    }
}

使用方式如下

ViewModelProvider(
            activity,
            AndroidViewModelFactory.getInstance(application, name)
        ).get(ViewModel.class)

另附上我创建 ViewModel 的方式

1.AndroidViewModelFactory

class AndroidViewModelFactory private constructor(
    private val mApplication: Application,
    private val mName: String
) : ViewModelProvider.NewInstanceFactory() {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        return if (AndroidViewModel::class.java.isAssignableFrom(modelClass)) {
            try {
                modelClass.getConstructor(
                    Application::class.java, String::class.java
                ).newInstance(mApplication, mName)
            } catch (e: Exception) {
                throw RuntimeException("Cannot create an instance of $modelClass", e)
            }
        } else super.create(modelClass)
    }

    companion object {
        private var sInstance: AndroidViewModelFactory? = null
        fun getInstance(
            application: Application,
            name: String
        ): AndroidViewModelFactory {
            if (sInstance == null) {
                sInstance = AndroidViewModelFactory(application, name)
            }
            return sInstance as AndroidViewModelFactory
        }
    }
}

2.ViewModelFactory

/**
 * @author: Eli Shaw
 * @Date: 2020-04-27 17:20:45
 * @Description：创建相应的 ViewModel
 */
object ViewModelFactory {

    /**
     * @Description: 创建 FragmentActivity 页面的 ViewModel
     * @Author: Eli
     * @Date: 2020-05-09 14:41:04
     */
    fun <T : ViewModel?> createViewModel(
        activity: FragmentActivity,
        cls: Class<T>?
    ): T {
        return ViewModelProvider(activity).get(cls!!)
    }

    /**
     * @Description: 创建 Fragment 页面的 ViewModel
     * @Author: Eli
     * @Date: 2020-05-09 14:40:36
     */
    fun <T : ViewModel?> createViewModel(
        fragment: Fragment,
        cls: Class<T>?
    ): T {
        return ViewModelProvider(fragment).get(cls!!)
    }

    /**
     * @Description:创建 FragmentActivity 页面的 AndroidViewModel
     * @Author: Eli
     * @Date: 2020-05-09 14:36:39
     */
    fun <T : ViewModel?> createViewModel(
        activity: FragmentActivity,
        application: Application,
        cls: Class<T>?
    ): T {
        return ViewModelProvider(
            activity,
            ViewModelProvider.AndroidViewModelFactory.getInstance(application)
        ).get(cls!!)
    }

    /**
     * @Description:创建 Fragment 页面的 AndroidViewModel
     * @Author: Eli
     * @Date: 2020-05-09 14:36:39
     */
    fun <T : ViewModel?> createViewModel(
        fragment: Fragment,
        application: Application,
        cls: Class<T>?
    ): T {
        return ViewModelProvider(
            fragment,
            ViewModelProvider.AndroidViewModelFactory.getInstance(application)
        ).get(cls!!)
    }

    /**
     * @Description: 创建带参数的 AndroidViewModel
     * @Author: Eli
     * @Date: 2020-05-09 14:36:24
     */
    fun <T : ViewModel?> createViewModel(
        activity: FragmentActivity,
        application: Application,
        name: String,
        cls: Class<T>?
    ): T {
        return ViewModelProvider(
            activity,
            AndroidViewModelFactory.getInstance(application, name)
        ).get(cls!!)
    }
}

3.NameViewModel

class NameViewModel(application: Application, name: String) : AndroidViewModel(application) {

    val name = ObservableField<String>()

    init {
        this.name.set(name)
    }
}

4.NameActivity

class NameActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding =
            DataBindingUtil.setContentView(this, R.layout.activity_name) as ActivityNameBinding
        val viewModel = ViewModelFactory.createViewModel(
            this,
            application,
            "AndroidViewModel",
            NameViewModel::class.java
        )
        binding.viewModel = viewModel
    }
}

5.activity_name

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <data>
        <variable
            name="viewModel"
            type="cn.eli.demo.viewmodel.NameViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".viewmodel.NameActivity">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            android:text="@{viewModel.name}"/>
    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>