[Android / Java] Have the host Fragment (caller) call back the event from DialogFragment

Introduction

I'm developing an app in Android Studio (java) There was a scene where "I want to call back the button click event of the dialog and process it with the calling fragment". To be more specific, when I implemented API communication + screen transition processing in the dialog, I implemented a callback because the screen transition could not be done probably because the thread was different. I got an error like this `ʻAndroid IllegalStateException: fragment not attached to activity``

So I was able to realize the processing using the callback, so I will write what I learned at that time.

Overview of the created sample application

Display a fragment with a button in the activity, and press the button to display a dialog. Press the dialog OK button to change the button text. Just a simple app

1. View fragment
2. Click the button

Implementation code

layout file

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"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/layout_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <FrameLayout
        android:id="@+id/fragment_sample"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintTop_toTopOf="parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>

fragment_sample.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:id="@+id/layout_sample"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SampleFragment">

    <Button
        android:id="@+id/bt_show_dialog"
        android:layout_width="142dp"
        android:layout_height="96dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        android:text="Dialog display"/>

</androidx.constraintlayout.widget.ConstraintLayout>

java file

MainActivity.java

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Set fragment in Activity
        getSupportFragmentManager()
                .beginTransaction()
                .add(R.id.fragment_sample, new SampleFragment())
                .commit();
    }
}

** * The parts that are called after pressing the button are listed in the order in which they are called **

SampleFragment.java

            //Implements to implement the interface
public class SampleFragment extends Fragment implements SampleDialogFragment.SampleDialogListener {

    //Define the button element of the dialog display as a global variable
    private Button button;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_sample, container, false);

        //Set the title on the action bar
        MainActivity activity = (MainActivity) getActivity();
        activity.setTitle("Dialog callback sample");

        //Get button elements from layout
        button = view.findViewById(R.id.bt_show_dialog);

        //Processing when the button is clicked ・ ・ ・ ①
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //Get Fragment Manager
                FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
                //Create an instance object of SampleDialogFragment
                SampleDialogFragment dialogFragment = SampleDialogFragment.newInstance();
                //Set the calling SampleFragment object to the SampleDialogFragment object
                dialogFragment.setTargetFragment(SampleFragment.this, 0);
                //Show dialog
                dialogFragment.show(fragmentManager, "");
            }
        });
        return view;
    }

    //Process that is called back and executed ... ⑤
    @Override
    public void onDialogPositiveClick(DialogFragment dialog) {
        //Change the characters displayed on the button
        button.setText("Callback success");
    }
}

SampleDialogFragment.java

public class SampleDialogFragment extends DialogFragment {

    //Method to create an instance
    public static SampleDialogFragment newInstance() {
        return new SampleDialogFragment();
    }

    //Implemented interface to receive event callback
    public interface SampleDialogListener {
        void onDialogPositiveClick(DialogFragment dialog);
    }

    //Define an interface instance to use to signal click event firing
    private SampleDialogListener listener;

    // onAttach()Verify that the calling parent fragment implements the interface
    // onAttach():The first method called in the fragment life cycle,
    //Called only once when a fragment is associated with an activity. The context contains the parent activity ... ②
    @Override
    public void onAttach(Context context) {
        super.onAttach(context);

        try {
            //Get the calling SampleFragment object so that you can send the event to the parent fragment
            //Create an instance of listener
            listener = (SampleDialogListener) getTargetFragment();
        } catch (ClassCastException e) {
            //Throw an exception if the parent fragment does not implement the interface
            throw new ClassCastException(getTargetFragment().toString() + "Does not implement the interface");
        }
    }

    //Generate dialog onCreateDialog
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {

        //Generate a dialog ... ③
        return new AlertDialog.Builder(getActivity())
                .setTitle("Verification")
                .setMessage("Do you want to start a callback?")
                .setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        //Call back the process to the parent fragment ... ④
                        listener.onDialogPositiveClick(SampleDialogFragment.this);
                    }
                })
                .setNegativeButton("Cancel", null) // Cancelボタンでは何もしないためnull
                .create();
    }
}

Finally

I haven't understood much yet, but I learned a lot. I would appreciate it if you could comment on any mistakes.

Reference material

It was very helpful! Thank you very much!

Android developers Dialog Fragment and Fragment Callback Relationship