Lateinit 属性 viewModel 尚未初始化

Question

我viewModel用这一行进行了初始化，viewModel = (activity as NewsActivity).viewModel但是BreakingNewsFragment收到了以下错误。我该如何解决这个问题？感谢您的帮助

突发新闻片段

import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.View
import com.example.simplenewsapp.R
import com.example.simplenewsapp.main.MainViewModel
import com.example.simplenewsapp.NewsActivity
import com.example.simplenewsapp.adapter.NewsRecyclerViewAdapter
import kotlinx.android.synthetic.main.fragment_breaking_news.*


class BreakingNewsFragment : Fragment(R.layout.fragment_breaking_news) {

    lateinit var viewModel: MainViewModel


    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        viewModel = (activity as NewsActivity).viewModel


         ....

新闻活动

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.lifecycle.ViewModelProvider
import androidx.navigation.fragment.findNavController
import androidx.navigation.ui.setupWithNavController
import com.example.simplenewsapp.data.local.ArticleDatabase
import com.example.simplenewsapp.main.MainRepository
import com.example.simplenewsapp.main.MainViewModel
import com.example.simplenewsapp.main.MainViewModelProviderFactory
import kotlinx.android.synthetic.main.activity_news.*

class NewsActivity : AppCompatActivity() {

    lateinit var viewModel: MainViewModel

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

        val newsRepository = MainRepository(ArticleDatabase(this))
        val mainViewModelProviderFactory = MainViewModelProviderFactory(newsRepository)
        viewModel =
            ViewModelProvider(this, mainViewModelProviderFactory).get(MainViewModel::class.java)
        
        bottomNavigationView.setupWithNavController(newsNavHostFragment.findNavController())
    }
}

MainViewModelProviderFactory

import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

class MainViewModelProviderFactory(val mainRepository:MainRepository): ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        return MainViewModel(mainRepository) as T
    }
}

主视图模型

import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.example.simplenewsapp.data.local.models.NewsResponse
import com.example.simplenewsapp.util.Resource
import kotlinx.coroutines.launch
import retrofit2.Response

class MainViewModel(
    val newsRepository: MainRepository
) : ViewModel() {

    val breakingNews: MutableLiveData<Resource<NewsResponse>> = MutableLiveData()
    var breakingNewsPage = 1

    init {
        getBreakingNews("us")
    }

    fun getBreakingNews(countyCode: String) = viewModelScope.launch {
        breakingNews.postValue(Resource.Loading())

        val response = newsRepository.getBreakingNews(countyCode, breakingNewsPage)

        breakingNews.postValue(handleBreakingNewsResponse(response))
    }

    private fun handleBreakingNewsResponse(response: Response<NewsResponse>): Resource<NewsResponse> {
        if (response.isSuccessful) {
            response.body()?.let { resultResponse ->
                return Resource.Success(resultResponse)
            }
        }
        return Resource.Error(response.message())
    }


}

错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.simplenewsapp, PID: 4455
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.simplenewsapp/com.example.simplenewsapp.NewsActivity}: android.view.InflateException: Binary XML file line #17: Binary XML file line #17: Error inflating class fragment
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: android.view.InflateException: Binary XML file line #17: Binary XML file line #17: Error inflating class fragment
     Caused by: android.view.InflateException: Binary XML file line #17: Error inflating class fragment
     Caused by: kotlin.UninitializedPropertyAccessException: lateinit property viewModel has not been initialized
        at com.example.simplenewsapp.NewsActivity.getViewModel(NewsActivity.kt:16)
        at com.example.simplenewsapp.ui.fragments.BreakingNewsFragment.onViewCreated(BreakingNewsFragment.kt:29)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2974)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:543)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:282)
        at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:112)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1636)
        at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3112)
        at androidx.fragment.app.FragmentManager.dispatchViewCreated(FragmentManager.java:3049)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2975)
        at androidx.fragment.app.FragmentStateManager.ensureInflatedView(FragmentStateManager.java:389)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:281)
        at androidx.fragment.app.FragmentLayoutInflaterFactory.onCreateView(FragmentLayoutInflaterFactory.java:141)
        at androidx.fragment.app.FragmentController.onCreateView(FragmentController.java:135)
        at androidx.fragment.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:313)
        at androidx.fragment.app.FragmentActivity.onCreateView(FragmentActivity.java:292)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:780)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:730)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:863)
        at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:824)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:866)
        at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:824)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:515)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:423)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:374)
        at androidx.appcompat.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:696)
        at androidx.appcompat.app.AppCompatActivity.setContentView(AppCompatActivity.java:170)
        at com.example.simplenewsapp.NewsActivity.onCreate(NewsActivity.kt:20)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
I/Process: Sending signal. PID: 4455 SIG: 9

Answer 1

1. View Model首先，为什么您对片段和活动使用相同的内容？
   这种方法是不正确的，可能会导致数据不一致和内存泄漏。
   理想情况下fragment和activity应该有自己独立的View Model。
   例如：
   a) NewsActivity 和 NewsViewModel
   b) BreakingNewsFragment 和 BreakingNewsViewModel

2. 这通过遵循原则来分离代码Separation of concerns，随着应用程序逻辑的增长，您将从中受益。
   如果您正在寻找共享行为/数据，请使用Shared View Model。

3. 其次，您应该像 在 中那样在 中完全初始化它，而不是使用View Modelfrom 。NewsActivityBreakingNewsFragmentNewsActivity

// Add below code either in your fragment or activity.

 val newsRepository = MainRepository(ArticleDatabase(this))
 val mainViewModelProviderFactory = MainViewModelProviderFactory(newsRepository)
 viewModel = ViewModelProvider(this, mainViewModelProviderFactory).get(MainViewModel::class.java)