Android 侦听器始终为 null 附加在 DialogFragment 中时

Question

high school teachers here trying to teach how to implement a DialogFragment with listener attach with an interface to the fragment that show it. for some reason the listener is always null and while the dialog show, when I press the ok or Cancel button of the Dialog, the transfert to the implement method dont work, it stop a the line where I call the listener class (always null) in the alertDialog.Builder Here my code, thank

here my Fragment that call the dialogfragment

    package net.ccl.monapp.ui;

import android.os.Bundle;
import androidx.fragment.app.DialogFragment;
import androidx.fragment.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import net.ccl.monapp.R;


public class AnimationFragment extends Fragment implements MonDialogFragment.MonDialogListener {


    public AnimationFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container, Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_animation,container,false);
        Button myButton = root.findViewById(R.id.bt_dialog);
        final TextView tvTitre = root.findViewById(R.id.tv_titre);

        myButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                DialogFragment myDialog = new MonDialogFragment();

                // Show Alert DialogFragment
                myDialog.show(getFragmentManager(), "Dialog");
            }
        });

        return root;
    }





    @Override
    public void onDialogPositiveClick(DialogFragment dialog) {
        TextView myTitre = getView().findViewById(R.id.tv_titre);
        myTitre.setText("Mon nouveau titre");
        dialog.dismiss();
    }

    @Override
    public void onDialogNegativeClick(DialogFragment dialog) {
        Toast.makeText(getActivity(),"Vous avez canceler l'action du dialog",Toast.LENGTH_SHORT);
    }
}

here my DialogFragment

import android.app.Dialog;
import android.content.Context;
import android.content.DialogInterface;
import android.os.Bundle;

import androidx.appcompat.app.AlertDialog;
import androidx.fragment.app.DialogFragment;




public class MonDialogFragment extends DialogFragment {
    public MonDialogFragment() {

    }

    public interface MonDialogListener {
        void onDialogPositiveClick(DialogFragment dialog);
        void onDialogNegativeClick(DialogFragment dialog);
    }

    // Use this instance of the interface to deliver action events
    public MonDialogListener listener;



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

        try {
            // Instantiate the NoticeDialogListener so we can send events to the host
            listener = (MonDialogListener) context;
        } catch (ClassCastException e) {

        }
    }

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {


        AlertDialog.Builder builder =  new AlertDialog.Builder(getActivity());

        builder.setTitle("Mon interface dialog");

        builder.setMessage("Ceci est un message qui explique que tu peux changer le titre du fragment animation en cliquant sur ok");

        builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        listener.onDialogPositiveClick(MonDialogFragment.this);

                    }
                });

        builder .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        listener.onDialogNegativeClick(MonDialogFragment.this);
                    }
                });

        return builder.create();
    }


}

and here my logcat

 Process: net.ccl.monapp, PID: 13691
java.lang.NullPointerException: Attempt to invoke interface method 'void net.ccl.monapp.ui.MonDialogFragment$MonDialogListener.onDialogPositiveClick(androidx.fragment.app.DialogFragment)' on a null object reference
    at net.ccl.monapp.ui.MonDialogFragment$1.onClick(MonDialogFragment.java:56)
    at androidx.appcompat.app.AlertController$ButtonHandler.handleMessage(AlertController.java:167)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7356)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

Answer 1

Your onAttach() isn't actually doing anything since the Context you're attempting to cast is your Activity , not your AnimationFragment .

Instead of reaching up to get your listener, your AnimationFragment should set the listener on the Dialog by overriding onAttachFragment() .

First, you need to make sure that your MonDialogFragment is a child fragment of your AnimationFragment by changing your show() to use getChildFragmentManager() :

myDialog.show(getChildFragmentManager(), "Dialog");

Then, override onAttachFragment() in your AnimationFragment :

@Override
public void onAttachFragment(Fragment fragment) {
    super.onAttachFragment(fragment);

    if (fragment instanceof MonDialogFragment) {
        ((MonDialogFragment) fragment).listener = this;
    }
}

You can then remove onAttach() from MonDialogFragment entirely.