默认Fragment调用requireActivity().findViewById()返回null崩溃求助

项目中使用BottomNavigationView，默认/首个Fragment调用requireActivity().findViewById()获取父Activity的视图时返回null并崩溃，但该操作在其他Fragment中可正常执行，仅初始Fragment出现此问题。

相关代码 #

首个Fragment代码 #

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View {
    _binding = FragmentCallregBinding.inflate(inflater, container, false)
    return binding.root
}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
   
    progress = requireActivity().findViewById(R.id.progress_circle)
}

主Activity代码 #

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityHomeBinding.inflate(layoutInflater)

    setContentView(binding.root)
    setSupportActionBar(binding.toolbar)

    navView = binding.navView
    progress = binding.progressCircle

    val navController = findNavController(R.id.nav_host_fragment_activity_home)
    navView.setupWithNavController(navController)
}

activity_main.xml代码 #

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:app="http://schemas.android.com/apk/res-auto">
<androidx.coordinatorlayout.widget.CoordinatorLayout
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_above="@id/nav_view"
    android:layout_alignParentEnd="true"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true">

    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/appbarlayout"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:fitsSystemWindows="true">
        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:background="@color/daynight"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_scrollFlags="scroll|enterAlways|snap">
            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="match_parent">
                <ImageView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:adjustViewBounds="true"
                    android:scaleType="centerCrop"
                    android:visibility="visible"
                    android:src="@drawable/logo"/>
            </LinearLayout>
        </androidx.appcompat.widget.Toolbar>
    </com.google.android.material.appbar.AppBarLayout>

    <androidx.core.widget.NestedScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fillViewport="true"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        tools:showIn="@layout/activity_home">
        <fragment
            android:id="@+id/nav_host_fragment_activity_home"
            android:name="androidx.navigation.fragment.NavHostFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:defaultNavHost="true"
            app:layout_constraintBottom_toTopOf="@id/nav_view"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:navGraph="@navigation/mobile_navigation" />
    </androidx.core.widget.NestedScrollView>

    <ProgressBar
        android:id="@+id/progress_circle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:indeterminate="true"
        android:indeterminateTint="@color/french_blue"
        android:indeterminateTintMode="src_in"/>

</androidx.coordinatorlayout.widget.CoordinatorLayout>

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/nav_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:menu="@menu/bottom_nav_menu"
    app:labelVisibilityMode="labeled"
    android:elevation="@dimen/_10sdp"
    android:layout_gravity="bottom"
    android:layout_alignParentBottom="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentEnd="true" />
</RelativeLayout>

fragment布局说明 #

当前fragment布局为空

mobile_navigation.xml代码 #

<?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_callreg">

    <fragment 
        android:id="@+id/navigation_callreg" 
        android:name="neo.starcool.app.ui.fragments.CallRegFragment" 
        android:label="@string/title_home" 
        tools:layout="@layout/fragment_callreg" /> 

    <fragment 
        android:id="@+id/navigation_open" 
        android:name="neo.starcool.app.ui.fragments.OpenCallFragment" 
        android:label="@string/title_home" 
        tools:layout="@layout/fragment_closed" /> 

    <fragment 
        android:id="@+id/navigation_pending" 
        android:name="neo.starcool.app.ui.fragments.PendingFragment" 
        android:label="@string/title_dashboard" 
        tools:layout="@layout/fragment_opencall" /> 

    <fragment 
        android:id="@+id/navigation_closed" 
        android:name="neo.starcool.app.ui.fragments.ClosedFragment" 
        android:label="@string/title_notifications" 
        tools:layout="@layout/fragment_pending" /> 
 </navigation>

原因 #

初始Fragment是在Activity初始化NavController时立即加载的，其onViewCreated生命周期方法可能在Activity的视图层级完全构建完成之前执行，导致findViewById无法找到目标视图。而其他Fragment是在用户点击BottomNavigation时才加载，此时Activity视图已经完全初始化，因此可以正常获取。

解决方案 #

方案1：延迟到Activity视图完全初始化后获取 #

将视图获取逻辑移到Fragment的onActivityCreated方法中，该方法会在Activity的onCreate执行完成后调用：

override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)
    progress = requireActivity().findViewById(R.id.progress_circle)
}

方案2：通过View.post延迟执行 #

利用View的post方法，将获取操作放到主线程消息队列末尾，确保Activity视图已构建完成：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
   
    view.post {
        progress = requireActivity().findViewById(R.id.progress_circle)
    }
}

方案3：通过Activity提供统一访问方法（更安全） #

在主Activity中封装获取ProgressBar的方法，Fragment调用该方法避免直接操作Activity视图：

• 主Activity添加代码：

fun getProgressCircle(): ProgressBar {
    return binding.progressCircle
}

• Fragment中调用：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
   
    view.post {
        progress = (requireActivity() as HomeActivity).getProgressCircle()
    }
}

方案4：使用共享ViewModel（推荐解耦方式） #

通过ViewModel在Activity和Fragment之间共享视图引用或状态，避免Fragment直接依赖Activity视图：

• 创建共享ViewModel：

class MainViewModel : ViewModel() {
    lateinit var progressBar: ProgressBar
}

• 主Activity中初始化：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityHomeBinding.inflate(layoutInflater)
    setContentView(binding.root)
    // ...其他代码
    val viewModel = ViewModelProvider(this)[MainViewModel::class.java]
    viewModel.progressBar = binding.progressCircle
}

• Fragment中获取：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    val viewModel = ViewModelProvider(requireActivity())[MainViewModel::class.java]
    progress = viewModel.progressBar
}

内容的提问来源于stack exchange，提问作者Abhinand A K