小贝子编程

FragmentManager field null android



所以我试图切换到另一个片段通过点击其中一个标签在我的底部导航视图，但我一直得到NullPointer异常。

我尝试将我的xml文件中的fragment标记更改为androidx.fragment.app.FragmentContainerView，但我仍然得到相同的错误。

Logcat:

java.lang.NullPointerException: Attempt to read from field 'androidx.fragment.app.FragmentManager androidx.fragment.app.Fragment.mFragmentManager' on a null object reference
at androidx.fragment.app.BackStackRecord.hide(BackStackRecord.java:201)
at com.murkoff.MusicPlayer.MainActivity$1.onNavigationItemChanged(MainActivity.java:214)
at com.volcaniccoder.bottomify.BottomifyNavigationView.selectItem(BottomifyNavigationView.kt:240)
at com.volcaniccoder.bottomify.BottomifyNavigationView.access$selectItem(BottomifyNavigationView.kt:19)
at com.volcaniccoder.bottomify.BottomifyNavigationView$addListener$1.onTouch(BottomifyNavigationView.kt:207)
at android.view.View.dispatchTouchEvent(View.java:11830)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2994)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2659)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2673)
at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:592)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1977)
at android.app.Activity.dispatchTouchEvent(Activity.java:3405)
at androidx.appcompat.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:69)
at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:548)
at android.view.View.dispatchPointerEvent(View.java:12095)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4966)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4761)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4259)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4329)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4287)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4429)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4295)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4486)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4259)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4329)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4287)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4295)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4259)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6854)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6828)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6783)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6966)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:194)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.next(MessageQueue.java:329)
at android.os.Looper.loop(Looper.java:150)
2021-03-06 01:12:37.661 27094-27094/com.murkoff.MusicPlayer E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:6702)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:549)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:826)

活动的xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.motion.widget.MotionLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@android:color/black"
android:id="@+id/activity_main_motionlayout_test_container"
tools:context=".MainActivity" app:layoutDescription="@xml/activity_main_motionlayout_test_scene">

<androidx.constraintlayout.widget.ConstraintLayout android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintBottom_toTopOf="@+id/main_bottom_nav_bar"
android:id="@+id/main_activity_fragment_container"
app:layout_constraintTop_toTopOf="parent">
<androidx.fragment.app.FragmentContainerView
android:layout_width="0dp"
android:layout_height="0dp" android:name="com.murkoff.MusicPlayer.HomeFragment"
android:id="@+id/fragment_tag" tools:layout="@layout/fragment_home"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="1.0"
app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintTop_toTopOf="parent"/>
</androidx.constraintlayout.widget.ConstraintLayout>

<com.volcaniccoder.bottomify.BottomifyNavigationView android:layout_width="match_parent"
android:layout_height="60dp"
android:id="@+id/main_bottom_nav_bar"
android:background="#0E0E0E"
android:elevation="8dp"
app:menu="@menu/main_menu_bottom_bar"
app:item_text_size="13sp"
app:active_color="@color/bottomifyActiveColor"
app:passive_color="@color/bottomifyPassiveColor"
app:pressed_color="@color/bottomifyPressedColor"
android:orientation="horizontal"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
android:gravity="center_vertical"/>
</androidx.constraintlayout.motion.widget.MotionLayout>

活动:

我正在使用fragment1, fragment2, fragment3, fragment4变量来防止碎片的重新创建，如BottomNavigationView -如何避免碎片的重新创建并重用它们

public class MainActivity extends AppCompatActivity{
BottomifyNavigationView navigationView;
HomeFragment fragment1;
SearchFragment fragment2;
PlaylistsFragment fragment3;
SettingsFragment fragment4;
final FragmentManager fragmentManager = getSupportFragmentManager();
Fragment active = fragment1;
boolean added = false;
FragmentManager mFragmentManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main_motionlayout);
if(fragmentManager!=null) {
mFragmentManager = fragmentManager;
Log.d("main activity", "fragment manager not null");
}
navigationView = findViewById(R.id.main_bottom_nav_bar);
fragment1 = new HomeFragment();
fragment2 = new SearchFragment();
fragment3 = new PlaylistsFragment();
fragment4 = new SettingsFragment();

}
@Override
protected void onStart() {
super.onStart();
if(!added) {
fragmentManager.beginTransaction().add(R.id.main_activity_fragment_container_motionlayout, fragment4, "4").hide(fragment4).commit();
fragmentManager.beginTransaction().add(R.id.main_activity_fragment_container_motionlayout, fragment3, "3").hide(fragment3).commit();
fragmentManager.beginTransaction().add(R.id.main_activity_fragment_container_motionlayout, fragment2, "2").hide(fragment2).commit();
fragmentManager.beginTransaction().add(R.id.main_activity_fragment_container_motionlayout, fragment1, "1").commit();
added = true;
}
navigationView.setOnNavigationItemChangedListener(new OnNavigationItemChangeListener() {
@Override
public void onNavigationItemChanged(@NonNull BottomifyNavigationView.NavigationItem navigationItem) {
switch(navigationItem.getPosition()){
case 0:
mFragmentManager.beginTransaction().hide(active).show(fragment1).commit();
active = fragment1;
break;
case 1:
Log.d("fragment", "commiting");
mFragmentManager.beginTransaction().hide(active).show(fragment2).commit();
active = fragment2;
break;
case 2:
mFragmentManager.beginTransaction().hide(active).show(fragment3).commit();
active = fragment3;
break;
case 3:
mFragmentManager.beginTransaction().hide(active).show(fragment4).commit();
active = fragment4;
break;
}
}
});
}
}

如何解决这个问题?谢谢你的帮助!

你不能在类初始化期间调用final FragmentManager fragmentManager = getSupportFragmentManager();，因为超类中的片段管理器尚未初始化。删除所有的类成员，用getSupportFragmentManager()替换所有的mFragmentManager和fragmentManager

使用导航图定义片段

build.gragle

dependencies {
...
implementation 'androidx.appcompat:appcompat:1.0.2'
implementation 'androidx.core:core-ktx:1.0.2'
implementation 'com.google.android.material:material:1.1.0-alpha10'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
implementation 'androidx.navigation:navigation-fragment:2.0.0'
implementation 'androidx.navigation:navigation-ui:2.0.0'
implementation 'androidx.lifecycle:lifecycle-extensions:2.1.0'
implementation 'androidx.navigation:navigation-fragment-ktx:2.0.0'
implementation 'androidx.navigation:navigation-ui-ktx:2.0.0'
}

MainActivity.kt

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.navigation.findNavController
import androidx.navigation.ui.NavigationUI
import androidx.navigation.ui.setupWithNavController
import kotlinx.android.synthetic.main.activity_main1.*
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)

val navController = findNavController(R.id.nav_host_fragment)
bottomNavigationView_activityMain.setupWithNavController(navController)
bottomNavigationView_activityMain.setOnNavigationItemSelectedListener {
NavigationUI.onNavDestinationSelected(it, navController)
true
}
}
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingTop="?attr/actionBarSize">
<fragment
android:id="@+id/nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:defaultNavHost="true"
app:layout_constraintBottom_toTopOf="@id/bottomNavigationView_activityMain"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/nav_graph" />
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/bottomNavigationView_activityMain"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="0dp"
android:layout_marginEnd="0dp"
android:background="?android:attr/windowBackground"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:menu="@menu/bottom_nav_menu" />
</androidx.constraintlayout.widget.ConstraintLayout>

bottom_nav_menu.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item
android:id="@+id/navigation_home"
android:icon="@drawable/ic_home_black_24dp"
android:title="@string/home" />
<item
android:id="@+id/navigation_gallery"
android:icon="@drawable/ic_menu_gallery"
android:title="@string/gallery" />
<item
android:id="@+id/navigation_slideshow"
android:icon="@drawable/ic_menu_slideshow"
android:title="@string/slideshow" />

</menu>

nav_graph.xml

<?xml version="1.0" encoding="utf-8"?>
<navigation 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/mobile_navigation"
app:startDestination="@id/navigation_home">
<fragment
android:id="@+id/navigation_home"
android:name="com.pkg.android_navigation_sample.ui.home.HomeFragment"
android:label="@string/home"
tools:layout="@layout/fragment_home" />
<fragment
android:id="@+id/navigation_gallery"
android:name="com.pkg.android_navigation_sample.ui.gallery.GalleryFragment"
android:label="@string/gallery"
tools:layout="@layout/fragment_gallery" />
<fragment
android:id="@+id/navigation_slideshow"
android:name="com.pkg.android_navigation_sample.ui.slideshow.SlideshowFragment"
android:label="@string/slideshow"
tools:layout="@layout/fragment_slideshow" />
</navigation>

其他示例:https://github.com/android/architecture-components-samples/blob/main/NavigationAdvancedSample/app/src/main/java/com/example/android/navigationadvancedsample/MainActivity.kt

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