How to properly get DatePicker value in parent fragment instead of parent activity

Question

I have complex application with tons of fragments and sub-fragments, and I need listeners to listen in a fragment, not in activity. This usually works, but not with date or time pickers.

Here is the sample application with activity with one fragment inflated - TestFragmentMain. That inflated fragment have two more fragments - one with single EditText (TestFragment_InputBox) and another with single TextView (TestFragment_DateBox) and it is used to listen events (TextChangeListener and DateChangeListener).

On text change in first fragment, all works like a charm, main fragment is receiving result.

However, on date change, I receive and error:

java.lang.NullPointerException: Attempt to invoke interface method 'void com.gmail.xxx.xxx.test.DateChangeListener.dateChanged(int, int, int)' on a null object reference
    at com.gmail.xxx.xxx.test.TestFragment_DatePicker.onDateSet(TestFragment_DatePicker.java:32)

I really do not understand why. Any help is appreciated.

Main activity:

public class TestClass extends AppCompatActivity {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.test_activity);
        TestFragmentMain testFragmentMain = new TestFragmentMain();
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        ft.replace(R.id.test_fragment_container, testFragmentMain);
        ft.commitAllowingStateLoss();
    }
}

Main fragment, with listeners

public class TestFragmentMain extends Fragment implements TextChangeListener, DateChangeListener  {

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

        TestFragment_InputBox  testFragmentInputBox = new TestFragment_InputBox();
        FragmentTransaction ft = getChildFragmentManager().beginTransaction();
        ft.replace(R.id.test_fragment_inputbox_container, testFragmentInputBox);
        ft.commitAllowingStateLoss();

        TestFragment_DateBox  testFragmentDateBox = new TestFragment_DateBox();
        ft = getChildFragmentManager().beginTransaction();
        ft.replace(R.id.test_fragment_date_container, testFragmentDateBox);
        ft.commitAllowingStateLoss();

        return view;
    }

    @Override
    public void onAttach(@NotNull Context context) {
        super.onAttach(context);
    }

    @Override
    public void textChanged() {
        Log.d("LISTEN","Text has been changed...");
    }

    @Override
    public void dateChanged(int year, int month, int day) {
        Log.d("LISTEN","Date has been changed to ...");
    }
}

Fragment with input box:

public class TestFragment_InputBox extends Fragment {

    private TextChangeListener textChangeListener;

    @Override
    public View onCreateView(@NotNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.test_input_box, container, false);
        EditText editText = view.findViewById(R.id.input_view);

        editText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                textChangeListener.textChanged();
            }
        });
        return view;
    }

    @Override
    public void onAttach(@NotNull Context context) {
        super.onAttach(context);
        try {
            textChangeListener = (TextChangeListener) getParentFragment();
        } catch (ClassCastException e) {
            e.printStackTrace();
        }
    }
}

Fragment with date box:

public class TestFragment_DateBox extends Fragment implements DateChangeListener {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.test_date_box, container, false);
        TextView textView = view.findViewById(R.id.date_view);

        textView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DialogFragment datePicker = new TestFragment_DatePicker();
                datePicker.show(getActivity().getSupportFragmentManager(), "datePicker");
            }
        });
        return view;
    }


    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
    }

    @Override
    public void dateChanged(int year, int month, int day) {
        Log.d("LISTEN","Date has been changed in box fragment with box ...");

    }
}

Date Picker fragment

public class TestFragment_DatePicker extends DialogFragment implements DatePickerDialog.OnDateSetListener {

    public DateChangeListener dateChangeListener;

    @NotNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        final Calendar c = Calendar.getInstance();
        int year = c.get(Calendar.YEAR);
        int month = c.get(Calendar.MONTH);
        int day = c.get(Calendar.DAY_OF_MONTH);
        return new DatePickerDialog(Objects.requireNonNull(getContext()), this, year, month, day);
    }

    @Override
    public void onDateSet(DatePicker view, int year, int month, int day) {
        dateChangeListener.dateChanged(year,month,day);
    }

    @Override
    public void onAttach(@NotNull Context context) {
        super.onAttach(context);
        try {
            dateChangeListener = (DateChangeListener) getParentFragment();
        } catch (ClassCastException e) {
            e.printStackTrace();
        }
    }
}

And listeners:

public interface TextChangeListener {
    void textChanged();
}

public interface DateChangeListener {
    void dateChanged(int year,int month,int day);
}

Answer 1

The problem is that you not inflate your TestFragment_DatePicker which is a DialogFragment which is a Fragment but return a DatePickerDialog which is an AlertDialog which is a Dialog which don't have a mParentFragment . That's why getParentFragment() return null.

This looks like a bad architecture design that we inherited from old versions of android.

Answer 2

I have found a solution.

In TestFragmentMain, TestFragment_DateBox need to be transacted with same FragmentManager as DatePicker is. And this is the code in TestFragmentMain:

    TestFragment_DateBox testFragmentDateBox = new TestFragment_DateBox();
    FragmentTransaction  ft2 = this.getChildFragmentManager().beginTransaction();
    ft2.replace(R.id.test_fragment_date_container, testFragmentDateBox);
    ft2.commitAllowingStateLoss();

Change is also in TestFragment_DateBox when opening DatePicker:

    DialogFragment datePicker = new TestFragment_DatePicker();
    datePicker.setTargetFragment(this, 0);
    datePicker.show(this.getFragmentManager(), "datePicker");

We need to run setTargetFragment in order to work.

Now this test works.

Answer 3

Your fragment TestFragment_DateBox calls the date picker and inherits DatePickerDialog.OnDateSetListener

public class TestFragment_DateBox extends Fragment implements DatePickerDialog.OnDateSetListener

and show the date picker fragment like so

TestFragment_DatePicker().show(this.childFragmentManager, "datepicker")

and override onDateSet

@Override
public void onDateSet(DatePicker view, int year, int month, int day) {

    // As you are in the calling fragment you can use the values as you see fit
    TextView textView = view.findViewById(R.id.date_view);
    textView.text = year.toString()

}

In TestFragment_DatePicker have a listener

private lateinit var listener: DatePickerDialog.OnDateSetListener

then override onAttach

    override fun onAttach(context: Context) {
    super.onAttach(context)

    // Verify that the dialog parent implements the callback interface
    try {
        // Instantiate the OnDateSetListener so we can send events to the host
        listener = parentFragment as DatePickerDialog.OnDateSetListener
    } catch (e: ClassCastException) {
        // The parent doesn't implement the interface, throw exception
        throw ClassCastException(("$context must implement OnDateSetListener"))
    }
}

Apologies for the mix of java and kotlin but you get the idea!