Android: Can not get view from activity in fragment when starting up application

Question

I am using NavHostFragment to display fragments. When I am trying to obtain activity's FloatingActionButton in fragment application crashes.

MainActivity.kt

class MainActivity : AppCompatActivity(), ImageDrawerListDialogFragment.OnImageClickListener {
    ...

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

}

SearchFragment.kt

class SearchFragment : Fragment(), ImageDrawerListDialogFragment.OnImageClickListener {
   
private lateinit var fab: FloatingActionButton
   ...
   override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                          savedInstanceState: Bundle?): View {
      _binding = FragmentSearchBinding.inflate(inflater, container, false)
      return binding.root
   }

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

     fab = (requireActivity() as MainActivity).findViewById(R.id.floatingActionButton) // <-- 
     fab.setOnSafeClickListener { requestPermission() }
     ...
   }

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/appBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/Theme.AppName.AppBarOverlay">

        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|enterAlways"
            app:popupTheme="@style/Theme.AppName.PopupOverlay">

            <androidx.appcompat.widget.SearchView
                android:id="@+id/searchView"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:focusable="true"
                android:imeOptions="actionDone"
                android:inputType="text"
                app:iconifiedByDefault="false" />

        </androidx.appcompat.widget.Toolbar>

    </com.google.android.material.appbar.AppBarLayout>

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/floatingActionButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:contentDescription="create new search request"
        app:layout_anchorGravity="bottom|end"
        app:srcCompat="@android:drawable/ic_input_add" />
    
    <include layout="@layout/content_main" />


</androidx.coordinatorlayout.widget.CoordinatorLayout>

content_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:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />
</androidx.constraintlayout.widget.ConstraintLayout>

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/nav_graph"
    app:startDestination="@id/SearchFragment">

    ...
</navigation>

error

Caused by: android.view.InflateException: Binary XML file line #36 in com.dmytroa.appName:layout/activity_main: Binary XML file line #18 in com.dmytroa.appName:layout/content_main: Error inflating class fragment Caused by: android.view.InflateException: Binary XML file line #18 in com.dmytroa.appName:layout/content_main: Error inflating class fragment Caused by: java.lang.NullPointerException: requireActivity() as MainActivity).findViewById(R.id.floatingActionButton) must not be null

Answer 1

fab = (requireActivity() as MainActivity).findViewById(R.id.floatingActionButton) // <-- fab.setOnSafeClickListener { requestPermission() }

I would suggest you not "reach up" from the Fragment into the Activity to operate on a view that exists in its layout. This creates an explicit dependency on that Activity from your Fragment which will break if you every try to use that Fragment anywhere else.

Execute logic that operates on the Activity's views within the Activity itself.

MainActivity:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main) 
    
    fab = findViewById(R.id.floatingActionButton)
    fab.setOnSafeClickListener { requestPermission() }
    ...
}