片段中的 Kotlin RecyclerView - 导航架构组件 - 空错误

Question

我正在尝试在片段中创建 RecyclerView，但在 RecyclerView 上出现空错误。我能够使用带有底部导航栏的新导航架构组件成功创建片段。我是运行 Android Studio canary 3.2 Beta 1.

请注意我哪里出错了，因为我在 Android Studio 中没有看到任何红线。

调试错误

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.projects.arise.mytestapp, PID: 28285 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.projects.arise.mytestapp/com.projects.arise.mytestapp.MainActivity}: java.lang.IllegalStateException: rview_keys must not be null at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) at android.os.Handler.dispatchMessage(Handler.java:105) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) Caused by: java.lang.IllegalStateException: rview_keys must not be null at com.projects.arise.mytestapp.MainActivity.onCreate(MainActivity.kt:55) at android.app.Activity.performCreate(Activity.java:6975) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) at android.os.Handler.dispatchMessage(Handler.java:105) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

MainActivity.kt

class MainActivity : AppCompatActivity() {

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

        val host: NavHostFragment = supportFragmentManager
                .findFragmentById(R.id.nav_host_fragment) as NavHostFragment? ?: return

        val navController = host.navController
        setupBottomNavMenu(navController)
    }

    override fun onSupportNavigateUp(): Boolean {
        return Navigation.findNavController(this, R.id.nav_host_fragment).navigateUp()
    }

    private fun setupBottomNavMenu(navController: NavController) {
        findViewById<BottomNavigationView>(R.id.navigation)?.let { bottomNavView ->
            NavigationUI.setupWithNavController(bottomNavView, navController)
        }
    }
}

FragmentKeys.kt

class FragmentKeys : Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        setHasOptionsMenu(true)
        return inflater.inflate(R.layout.fragment_keys_layout, container, false)
    }

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

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        //val rview_keys: RecyclerView = findViewById(R.id.rview_keys)
        rview_keys.layoutManager = LinearLayoutManager(context)
        rview_keys.adapter = AdapterKeys(KeysListObjects.keys)
    }
}

我注释掉了上面的一行，因为我在 "findViewById" 上收到 "unresolved reference" 错误。这一行似乎没有必要，因为它不在其他人的代码中。例如：

AdapterKeys.kt

class AdapterKeys(val keys: List<KeysList>) : RecyclerView.Adapter<AdapterKeys.KeysViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): KeysViewHolder {
        val view = LayoutInflater.from(parent?.context).inflate(R.layout.list_item_keys, parent, false)
        return KeysViewHolder(view)
    }

    override fun getItemCount() : Int {
        return keys.size
    } //override fun getItemCount() = keys.size

    override fun onBindViewHolder(holder: KeysViewHolder, position: Int) {
        val keysList = keys[position]
        holder.setData(keysList, position)
    }

    inner class KeysViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {

        fun setData(keysList: KeysList?, position: Int) {
            itemView.tview_Keys.text = keysList!!.keysText
        }
    }
}

ModelKeys.kt

object KeysListObjects {
    val keys = listOf<KeysList>(
            KeysList("Testing1"),
            KeysList("Testing2"),
            KeysList("Testing3")
    )
}

Answer 1

你收到错误是因为你评论了这个

//val rview_keys: RecyclerView = findViewById(R.id.rview_keys)

没有将 rview_keys 声明为变量。所以它正在考虑 rview_keys 作为一个视图，因为在你的 activity 中你有一个 ID 为 rview_keys.

的视图

以下错误行只需确认即可。

java.lang.IllegalStateException: rview_keys must not be null at com.projects.arise.mytestapp.MainActivity.onCreate(MainActivity.kt:55) at

检查 MainActivity 第 55 行的代码。

这些人没有那条线的原因是他们将它用作视图。而且您不能将其用作 onActivityCreated() 中的视图。

Answer 2

我必须做两件事才能让它工作：

将两行 rview_keys 代码从 onActivityCreated 移到 onViewCreated（我相信它在片段生命周期的早期）。感谢以上海报的反馈！所以现在我的片段 class 看起来像这样：

FragmentKeys.kt

class FragmentKeys : Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        setHasOptionsMenu(true)
        return inflater.inflate(R.layout.fragment_keys_layout, container, false)
    }

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

        rview_keys.layoutManager = LinearLayoutManager(context)
        rview_keys.adapter = AdapterKeys(KeysListObjects.keys)
    }
}

我仍然遇到构建错误，所以我尝试将 build.gradle 文件 (jdk7) 中的一个 Kotlin 依赖项更改回之前的状态 (jre7)，尽管我收到了弃用警告。

我被要求使用的新依赖项 - 它不起作用：

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

我切换回的旧依赖项 - 正在工作（目前）：

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"

片段中的 Kotlin RecyclerView - 导航架构组件 - 空错误

Kotlin RecyclerView in Fragment - Navigation architecture component - null error

navigation

android-fragments

kotlin

android-recyclerview

android-architecture-components