java.lang.IllegalStateException ConstraintLayout 没有为片段设置 NavController

Question

I have an activity ( main ) with three fragments (first, second and third). I included the 3 fragments in my activity ( activity_main.xml ) by using <include layout="@layout/content_main"/> . The content_main.xml is using FragmentContainerView with id = nav_host_fragment . And this is my nav_graph.xml :

<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/FirstFragment">

    <fragment
        android:id="@+id/FirstFragment"
        android:name="com.example.makegroups.FirstFragment"
        android:label="@string/first_fragment_label"
        tools:layout="@layout/fragment_first">

        <action
            android:id="@+id/action_FirstFragment_to_SecondFragment"
            app:destination="@id/SecondFragment" />
    </fragment>

    <fragment
        android:id="@+id/SecondFragment"
        android:name="com.example.makegroups.SecondFragment"
        android:label="@string/second_fragment_label"
        tools:layout="@layout/fragment_second">

        <action
            android:id="@+id/action_SecondFragment_to_FirstFragment"
            app:destination="@id/FirstFragment" />
        <action
            android:id="@+id/action_SecondFragment_to_ThirdFragment"
            app:destination="@id/ThirdFragment" />
    </fragment>

    <fragment
        android:id="@+id/ThirdFragment"
        android:name="com.example.makegroups.ThirdFragment"
        android:label="@string/third_fragment_label"
        tools:layout="@layout/fragment_third">

        <action
            android:id="@+id/action_ThirdFragment_to_FirstFragment"
            app:destination="@id/FirstFragment" />
    </fragment>
</navigation>

I have a floatingactionbutton in my activity ( first fragment starts first) and when I click on it, I open the third fragment . On the third fragment I have a button (next) to navigate to the first fragment , and when I click on it, I am back to first fragment using:

Fragment frg = new FirstFragment();
FragmentManager fm = requireActivity().getSupportFragmentManager();

Now (while I am in the first fragment ), I click on the button next (another button to navigate to the second fragment ), then the app crashes. I found this error:

java.lang.IllegalStateException: View androidx.constraintlayout.widget.ConstraintLayout{c9572fa V.E...... ........ 0,0-1440,2112} does not have a NavController set

Why I am getting this error? -I tried these suggestions here , without success.

I am using Java.

EDIT: Read the last comment with @Zain to know why I got the error.

Answer 1

navController = Navigation.findNavController(activity, R.id.nav_host_fragment)

Answer 2

Bu using Navigation Architecture components, The NavController is the responsible for fragment transaction and managing the back stack instead of the Support FragmentManager .

So, instead of making tranditional framgnet transactions with FragmentManager

You can move from ThridFragment to the first one by:

 Navigation.findNavController(requireView()).navigate(R.id.action_ThirdFragment_to_FirstFragment);

Where action_ThirdFragment_to_FirstFragment is the id of the action you defined in the navigation graph to move from ThridFragment to FirstFragment

UPDATE:

As discussed from comments, besides replacing FragmentManager by NavController in all actions; there is another issue:

Missing of action_FirstFragment_to_ThirdFragment action from the navigation graph.

Answer 3

when using the Navigation Component you should not handle the transactions yourself, instead you define the actions between each fragment and then access those directly like this

override fun onCreate(){
  val navController = this.findNavController()
  button.setOnClickListener{
     navController.navigate(R.id.action_FirstFragment_to_SecondFragment, null)
  }
}

and your nav_graph should be like this

<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/FirstFragment">

<fragment
    android:id="@+id/FirstFragment"
    android:name="com.example.makegroups.FirstFragment"
    android:label="@string/first_fragment_label"
    tools:layout="@layout/fragment_first">

    <action
        android:id="@+id/action_FirstFragment_to_SecondFragment"
        app:destination="@id/SecondFragment" />
</fragment>

<fragment
    android:id="@+id/SecondFragment"
    android:name="com.example.makegroups.SecondFragment"
    android:label="@string/second_fragment_label"
    tools:layout="@layout/fragment_second">

    <action
        android:id="@+id/action_SecondFragment_to_FirstFragment"
        app:destination="@id/FirstFragment" />
    <action
        android:id="@+id/action_SecondFragment_to_ThirdFragment"
        app:destination="@id/ThirdFragment" />
</fragment>

<fragment
    android:id="@+id/ThirdFragment"
    android:name="com.example.makegroups.ThirdFragment"
    android:label="@string/third_fragment_label"
    tools:layout="@layout/fragment_third">

    <action
        android:id="@+id/action_ThirdFragment_to_FirstFragment"
        app:destination="@id/FirstFragment" />
</fragment>

Answer 4

   val navController  =Navigation.findNavController(requireActivity(),R.id.fragment_container)
   navigate.navigate(R.id.fragment)