由以下原因引起：java.lang.InstantiationException：类WordViewModel没有零参数构造函数

如何解决由以下原因引起：java.lang.InstantiationException：类WordViewModel没有零参数构造函数

无法实例化Wordviewmodel 原因：java.lang.InstantiationException：类com.example.roomwordsample.Wordviewmodel没有零参数构造函数

我试图用回收者视图显示房间数据，但无法通过viewmodelProvider get方法实例化类

Wordviewmodel.kt

package com.example.roomwordsample

import android.app.Application
import androidx.lifecycle.Androidviewmodel
import androidx.lifecycle.LiveData
import androidx.lifecycle.viewmodelScope
import kotlinx.coroutines.dispatchers
import kotlinx.coroutines.launch

class Wordviewmodel(application: Application) : Androidviewmodel(application) {

    private val repository: WordRepository
    // Using LiveData and caching what getAlphabetizedWords returns has several benefits:
    // - We can put an observer on the data (instead of polling for changes) and only update the
    //   the UI when the data actually changes.
    // - Repository is completely separated from the UI through the viewmodel.
    val allWords: LiveData<List<Word>>


    init {
        val wordsDao = WordRoomDatabase.getDatabase(application,viewmodelScope).wordDao()
        repository = WordRepository(wordsDao)
        allWords = repository.allWords
    }

    /**
     * Launching a new coroutine to insert the data in a non-blocking way
     */
    fun insert(word: Word) = viewmodelScope.launch(dispatchers.IO) {
        repository.insert(word)
    }
}

MainActivity.kt

package com.example.roomwordsample

import android.app.Activity
import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import androidx.lifecycle.Observer
import androidx.lifecycle.viewmodelProvider
import androidx.recyclerview.widget.linearlayoutmanager
import androidx.recyclerview.widget.RecyclerView
import com.google.android.material.floatingactionbutton.FloatingActionButton

class MainActivity : AppCompatActivity() {
    private val newWordActivityRequestCode = 1
    private lateinit var wordviewmodel: Wordviewmodel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)
        val adapter = Wordlistadapter(this)
        recyclerView.adapter = adapter
        recyclerView.layoutManager = linearlayoutmanager(this)

        wordviewmodel = viewmodelProvider(this).get(Wordviewmodel::class.java)
        wordviewmodel.allWords.observe(this,Observer { words ->
            // Update the cached copy of the words in the adapter.
            words?.let { adapter.setWords(it) }
        })

        val fab = findViewById<FloatingActionButton>(R.id.fab)
        fab.setonClickListener {
            val intent = Intent(this@MainActivity,NewWordActivity::class.java)
            startActivityForResult(intent,newWordActivityRequestCode)
        }
    }

    override fun onActivityResult(requestCode: Int,resultCode: Int,data: Intent?) {
        super.onActivityResult(requestCode,resultCode,data)

        if (requestCode == newWordActivityRequestCode && resultCode == Activity.RESULT_OK) {
            data?.getStringExtra(NewWordActivity.EXTRA_REPLY)?.let {
                val word = Word(it)
                wordviewmodel.insert(word)
            }
        } else {
            Toast.makeText(
                applicationContext,R.string.empty_not_saved,Toast.LENGTH_LONG).show()
        }
    }


}

解决方法

尝试将视图模型工厂添加到您的视图模型类中。

package com.example.roomwordsample

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch

class WordViewModel(application: Application) : AndroidViewModel(application) {

    private val repository: WordRepository
    // Using LiveData and caching what getAlphabetizedWords returns has several benefits:
    // - We can put an observer on the data (instead of polling for changes) and only update the
    //   the UI when the data actually changes.
    // - Repository is completely separated from the UI through the ViewModel.
    val allWords: LiveData<List<Word>>


    init {
        val wordsDao = WordRoomDatabase.getDatabase(application,viewModelScope).wordDao()
        repository = WordRepository(wordsDao)
        allWords = repository.allWords
    }

    /**
     * Launching a new coroutine to insert the data in a non-blocking way
     */
    fun insert(word: Word) = viewModelScope.launch(Dispatchers.IO) {
        repository.insert(word)
    }

class Factory(
        private val app: Application
    ) : ViewModelProvider.Factory {
        @Suppress("UNCHECKED_CAST")
        override fun <T : ViewModel?> create(modelClass: Class<T>): T {
            if (modelClass.isAssignableFrom(WordViewModel::class.java)) {
                return WordViewModel(app) as T
            }
            throw IllegalArgumentException("Unknown ViewModel class")
        }
    }
}

然后在MainActivity中像这样初始化视图模型：

package com.example.roomwordsample

import android.app.Activity
import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import com.google.android.material.floatingactionbutton.FloatingActionButton

class MainActivity : AppCompatActivity() {
    private val newWordActivityRequestCode = 1
    private val wordViewModel by viewModels<WordViewModel> {
        WordViewModel.Factory(application)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)
        val adapter = WordListAdapter(this)
        recyclerView.adapter = adapter
        recyclerView.layoutManager = LinearLayoutManager(this)

        //wordViewModel = ViewModelProvider(this).get(WordViewModel::class.java)
        wordViewModel.allWords.observe(this,Observer { words ->
            // Update the cached copy of the words in the adapter.
            words?.let { adapter.setWords(it) }
        })

        val fab = findViewById<FloatingActionButton>(R.id.fab)
        fab.setOnClickListener {
            val intent = Intent(this@MainActivity,NewWordActivity::class.java)
            startActivityForResult(intent,newWordActivityRequestCode)
        }
    }

    override fun onActivityResult(requestCode: Int,resultCode: Int,data: Intent?) {
        super.onActivityResult(requestCode,resultCode,data)

        if (requestCode == newWordActivityRequestCode && resultCode == Activity.RESULT_OK) {
            data?.getStringExtra(NewWordActivity.EXTRA_REPLY)?.let {
                val word = Word(it)
                wordViewModel.insert(word)
            }
        } else {
            Toast.makeText(
                applicationContext,R.string.empty_not_saved,Toast.LENGTH_LONG).show()
        }
    }


}

,

我通过以下方式解决了这个问题

wordViewModel = ViewModelProvider(this).get(WordViewModel::class.java)

以此更改上面的代码：

wordViewModel = ViewModelProvider.AndroidViewModelFactory.getInstance(application).create(WordViewModel::class.java)