为什么我有这个错误：view.findViewById<MapView>(R.id.map)不能为空

Question

我试图在我的片段中获取mainActivity上下文和mapView，我试图写一些代码，但当我写这个时：

contextMain = activity?.baseContext!!
mapView = view.findViewById<MapView>(R.id.map)

我得到这个错误：

java.lang.NullPointerException: view.findViewById<MapView>(R.id.map) must not be null

这是我的碎片代码

import android.content.Context
import android.os.Bundle
import android.util.Log
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.Button
import android.widget.EditText
import android.widget.Toast
import androidx.fragment.app.Fragment
import org.osmdroid.views.MapView
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private const val ARG_PARAM1 = "param1"
private const val ARG_PARAM2 = "param2"
/**
* A simple [Fragment] subclass.
* Use the [CreatePathFragment.newInstance] factory method to
* create an instance of this fragment.
*/
class CreatePathFragment() : Fragment() {
// TODO: Rename and change types of parameters
private var param1: String? = null
private var param2: String? = null
private lateinit var editTextLatitudineP1: EditText
private lateinit var editTextLongitudineP1: EditText
private lateinit var editTextLatitudineP2: EditText
private lateinit var editTextLongitudineP2: EditText
private lateinit var  contextMain: Context
private lateinit var mapView: MapView
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
arguments?.let {
param1 = it.getString(ARG_PARAM1)
param2 = it.getString(ARG_PARAM2)
}
}
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View {
val view: View = inflater.inflate(R.layout.fragment_create_path, container, false)
contextMain = activity?.baseContext!!
mapView = view.findViewById<MapView>(R.id.map)
// traccia percorso tra i punti degli editText
editTextLatitudineP1 = view.findViewById<EditText>(R.id.editTextLatitudineP1)
editTextLongitudineP1 = view.findViewById<EditText>(R.id.editTextLongitudineP1)
editTextLatitudineP2 = view.findViewById<EditText>(R.id.editTextLatitudineP2)
editTextLongitudineP2 = view.findViewById<EditText>(R.id.editTextLongitudineP2)
val b = view.findViewById<Button>(R.id.location)
b.setOnClickListener{
try {
Log.d("Percorso", "Premuto")
val p1lati: Double = editTextLatitudineP1.text.toString().toDouble()
val p1long: Double = editTextLongitudineP1.text.toString().toDouble()
val p2lati: Double = editTextLatitudineP2.text.toString().toDouble()
val p2long: Double = editTextLongitudineP2.text.toString().toDouble()
println("latitudine p1: $p1lati, longitudine p1: $p1long")
println("latitudine p2: $p2lati, longitudine p2: $p2long")
val navFun: NavFun = NavFun(contextMain, mapView)
navFun.routePath(p1lati, p1long, p2lati, p2long)
mapView.invalidate()
} catch (e: Exception) {
Toast.makeText(activity, "Errore nel inserimento delle coordinate", Toast.LENGTH_SHORT).show()
}
}
// Inflate the layout for this fragment
return view
}

companion object {
/**
* Use this factory method to create a new instance of
* this fragment using the provided parameters.
*
* @param param1 Parameter 1.
* @param param2 Parameter 2.
* @return A new instance of fragment CreatePathFragment.
*/
// TODO: Rename and change types and number of parameters
@JvmStatic
fun newInstance(param1: String, param2: String) =
CreatePathFragment().apply {
arguments = Bundle().apply {
putString(ARG_PARAM1, param1)
putString(ARG_PARAM2, param2)
}
}
}
}

这是我的片段xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/white"
tools:context=".CreatePathFragment"
android:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/crea_percorso"
android:textColor="@color/black"
android:textSize="40sp"
android:textStyle="bold" />
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/inserisci_le_coordinate_dei_punti_tra_cui_vuoi_creare_un_percorso" />
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/latitudine_punto_di_partenza" />
<EditText
android:id="@+id/editTextLatitudineP1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:ems="10"
android:hint="@string/latitudine_p1"
android:inputType="numberSigned|numberDecimal" />
<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/longitudine_punto_di_partenza" />
<EditText
android:id="@+id/editTextLongitudineP1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:ems="10"
android:hint="@string/longitudine_p1"
android:inputType="numberSigned|numberDecimal" />
<TextView
android:id="@+id/textView4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/latitudine_punto_di_arrivo" />
<EditText
android:id="@+id/editTextLatitudineP2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:ems="10"
android:hint="@string/latitudine_p2"
android:inputType="numberSigned|numberDecimal" />
<TextView
android:id="@+id/textView5"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/longitudine_punto_di_arrivo" />
<EditText
android:id="@+id/editTextLongitudineP2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:ems="10"
android:hint="@string/longitudine_p2"
android:inputType="numberSigned|numberDecimal" />
<Button
android:id="@+id/location"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:text="@string/crea_percorso"
/>
</LinearLayout>

这是我的错误

E/AndroidRuntime: FATAL EXCEPTION: main
Process: uk.co.lorenzopulcinelli.navigationdrawer, PID: 21670
java.lang.NullPointerException: view.findViewById<MapView>(R.id.map) must not be null
at uk.co.lorenzopulcinelli.navigationdrawer.CreatePathFragment.onCreateView(CreatePathFragment.kt:54)
at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2963)
at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:518)
at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:282)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2189)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2100)
at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:2002)
at androidx.fragment.app.FragmentManager$5.run(FragmentManager.java:524)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:224)
at android.app.ActivityThread.main(ActivityThread.java:7590)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
I/Process: Sending signal. PID: 21670 SIG: 9

我已经以这种方式在mainActivityxml文件中声明并显示了映射：

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/drawerLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<org.osmdroid.views.MapView
android:id="@+id/map"
tilesource="Mapnik"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:layout_editor_absoluteX="-181dp"
tools:layout_editor_absoluteY="57dp">
</org.osmdroid.views.MapView>
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/frameLayout" />
</androidx.constraintlayout.widget.ConstraintLayout>
<com.google.android.material.navigation.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
app:headerLayout="@layout/nav_header"
app:menu="@menu/nav_menu"
android:layout_gravity="start"
android:fitsSystemWindows="true"/>
</androidx.drawerlayout.widget.DrawerLayout>

Answer 1

您在活动中定义了R.id.map，但您正在片段的视图上执行findViewById。为了让片段找到定义为活动一部分的地图，您可以尝试以下操作：

val map = activity?.findViewById<MapView>(R.id.map)

这将返回在两个条件下为null的可选项：活动为null，或者活动上不存在映射。

此时，为了安全访问地图的属性，您需要使用安全呼叫运算符"？"，例如：

map?.invalidate()

或

map?.let{
it.invalidate()
}

在第二种情况下，只有当map不为空时，才执行块的内容。此外，在块中，您可以通过名为it的默认字段访问对map的不可为null的引用。