android - 无法创建类 ViewModel 的实例

Question

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 Cannot create instance of class com.nailuj29gaming.CountdownViewModel
堆栈跟踪是

    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 very similar question, this one ，但这是针对 Java 的，我的代码是用 Kotlin 编写的。我的 ViewModel类的文本是

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)
    }
}

我创建了 ViewModel在我的 MainActivity.onCreate() 的第一行方法。我用来创建它的代码是 viewModel = ViewModelProvider(this).get(CountdownViewModel::class.java)
我的 build.gradle 文件(至少在这个问题中很重要)是

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，但不推荐使用 ViewModelProviders 类


当我使用委托(delegate)时，IDE没有给我错误，但是编译器说

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

我的代码被编译成使用 JVM 目标 1.8 构建的字节码，所以这个错误不应该出现，对吧？

Answer 1

您需要升级到 Fragment 1.2.0 或更高版本，这增加了对 ViewModelProvider 的支持。根据 Lifecycle 2.2.0 release notes 的构造函数:

You can pass a Fragment or FragmentActivity to the new ViewModelProvider(ViewModelStoreOwner) constructor to achieve the same functionality when using Fragment 1.2.0.

当使用旧版本的 Fragments(即 AppCompat 1.1.0 引入的 Fragment 1.1.0)时，您只会得到 ViewModelProvider$NewInstanceFactory您在异常跟踪中看到，而不是支持 AndroidViewModel 的 ViewModel.Factory .
因此，您应该添加对 Fragment 1.3.5(当前最新版本)的显式依赖:

implementation 'androidx.fragment:fragment:1.3.5'