无法创建类ViewModel的实例 [英] Cannot Create Instance Of Class ViewModel

问题描述

我目前正在开发一个倒计时应用程序，每当我打开它时，它都会失败并显示一条消息，提示Cannot create instance of class com.nailuj29gaming.CountdownViewModel

I'm currently working on a countdown app, and whenever I open it, it fails with a message saying Cannot create instance of class com.nailuj29gaming.CountdownViewModel

堆栈跟踪为

    Process: com.nailuj29gaming.countdown.debug, PID: 17800
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.nailuj29gaming.countdown.debug/com.nailuj29gaming.countdown.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.nailuj29gaming.countdown.CountdownViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2750)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6316)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.nailuj29gaming.countdown.CountdownViewModel
        at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
        at com.nailuj29gaming.countdown.MainActivity.onCreate(MainActivity.kt:23)
        at android.app.Activity.performCreate(Activity.java:6757)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6316) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762) 
     Caused by: java.lang.InstantiationException: java.lang.Class<com.nailuj29gaming.countdown.CountdownViewModel> has no zero argument constructor
        at java.lang.Class.newInstance(Native Method)
        at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
        at com.nailuj29gaming.countdown.MainActivity.onCreate(MainActivity.kt:23) 
        at android.app.Activity.performCreate(Activity.java:6757) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6316) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762) 

我知道有一个非常类似的问题，这个问题 ，但这是针对Java的，而我的代码是用Kotlin编写的.我的ViewModel课的文字是

I know there is a very similar question, this one, but that is target at Java, and my code is written in Kotlin. My ViewModel class's text is

package com.nailuj29gaming.countdown

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

class CountdownViewModel(application: Application) : AndroidViewModel(application) {
    private val repo: CountdownRepo
    val countdowns: LiveData<List<Countdown>>

    init {
        val countdownDao = AppDatabase.getDatabase(application).countdownDao()
        repo = CountdownRepo(countdownDao)
        countdowns = repo.countdowns

    }

    fun insert(countdown: Countdown) = viewModelScope.launch {
        repo.insert(countdown)
    }

    fun delete(countdown: Countdown) = viewModelScope.launch {
        repo.delete(countdown)
    }
}

我在MainActivity.onCreate()方法的第一行中创建ViewModel.我用来创建它的代码是viewModel = ViewModelProvider(this).get(CountdownViewModel::class.java)

I create the ViewModel in the first line of my MainActivity.onCreate() method. The code I used to create it is viewModel = ViewModelProvider(this).get(CountdownViewModel::class.java)

我的build.gradle文件(至少在此问题中很重要)是

My build.gradle file (at least what matters in this question) is

apply plugin: 'kotlin-kapt'

dependencies {
    def room_version = "2.2.3"
    def lifecycle_version = "2.2.0"
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.core:core-ktx:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.13'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation 'com.google.code.gson:gson:2.8.6'
    implementation 'androidx.preference:preference:1.1.0'
    implementation 'net.steamcrafted:materialiconlib:1.1.5'
    implementation 'com.google.android.material:material:1.0.0'
    implementation "androidx.room:room-runtime:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    implementation "androidx.room:room-ktx:$room_version"
}

我尝试使用工厂，但是不赞成使用ViewModelProviders类

I tried using a factory, but the ViewModelProviders class is deprecated

当我使用委托时，IDE并没有给我一个错误，但是编译器说

When I used a delegate, the IDE did not give me an error, but the compiler said

Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6. Please specify proper '-jvm-target' option

我的代码被编译为使用JVM target 1.8构建的字节码，因此不应显示此错误，对吧?

My code is compiled into bytecode built with JVM target 1.8, so this error shouldn't show, right?

推荐答案

您需要升级到Fragment 1.2.0或更高版本，这根据

You need to upgrade to Fragment 1.2.0 or higher, which is what adds a support for the ViewModelProvider constructor as per the Lifecycle 2.2.0 release notes:

使用使用较旧版本的Fragments(即AppCompat 1.1.0插入的Fragment 1.1.0)时，您只会得到在异常跟踪中看到的ViewModelProvider$NewInstanceFactory，而不是得到的ViewModel.Factory支持AndroidViewModel.

When using an older version of Fragments (i.e., the Fragment 1.1.0 that AppCompat 1.1.0 pulls in), you'll only get the ViewModelProvider$NewInstanceFactory you see in your exception trace, rather than a ViewModel.Factory that supports AndroidViewModel.

因此，您应该添加对片段1.2.1(当前最新版本)的显式依赖项:

Therefore you should add an explicit dependency on Fragment 1.2.1 (the current latest):

implementation 'androidx.fragment:fragment:1.2.1'

这篇关于无法创建类ViewModel的实例的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！