DialogFragment throws ClassCastException if called from Fragment

Question

My DialogFragment throws ClassCastException if called from Fragment, while it is working normally if called from an Activity. I have already looked at few other questions with similar problem and basically those are related to imports, but I haven't been able to solve it in my implementation. Here is my implementation for DialogFragment.

import android.app.AlertDialog;
import android.app.Dialog;
import android.app.DialogFragment;

public class HotspotScanDialog extends DialogFragment {

    SetupHotspotDialogListener mListener;

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        ...

        .setAdapter(hotspotAdapter, new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                mListener.onHotspotSelectedListener(hotspotAdapter.getItem(
                        which).toString());
            }
        })...
    }

    public interface SetupHotspotDialogListener {
        public void onHotspotSelectedListener(String selection);

    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        try {
            mListener = (SetupHotspotDialogListener) activity;
        } catch (ClassCastException ignore) {
            // Just to make sure if anyone will be pointing at my throwing
            // ClassCastException myself I have tried without this code as well.
            throw new ClassCastException(activity.toString()
                    + " must implement NoticeDialogListener");
        }
    }
}

Here is my Fragment that is using the above DialogFragment:

import android.app.AlertDialog;
import android.app.DialogFragment;
import android.support.v4.app.Fragment;
import com.xxx.yyy.ui.compontent.dialog.HotspotScanDialog;
import com.xxx.yyy.ui.compontent.dialog.HotspotScanDialog.SetupHotspotDialogListener;

public class SmartMode extends Fragment implements SetupHotspotDialogListener {

    private void showWifiSelectionDialog() {
        DialogFragment setupWifiSelectionDialog = new HotspotScanDialog();

        /*
         * using getFragmentManager() only says "The method
         * show(FragmentManager, String) in the type DialogFragment is not
         * applicable for the arguments (FragmentManager, String)"
         */

        setupWifiSelectionDialog.show(getActivity().getFragmentManager(),
                Keys.TAG.toString());
    }

    @Override
    public void onHotspotSelectedListener(String selection) {
        // Log.d(TAG,selection);
    }
}

This is the error log:

02-01 13:11:32.750: E/AndroidRuntime(15061): FATAL EXCEPTION: main 02-01 13:11:32.750: E/AndroidRuntime(15061): java.lang.ClassCastException: com.milanix.tuki.UiMainActivity@41d75350 must implement NoticeDialogListener 02-01 13:11:32.750: E/AndroidRuntime(15061): at com.xxx.yyy.ui.compontent.dialog.HotspotScanDialog.onAttach(HotspotScanDialog.java:122) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:787) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1035) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.app.BackStackRecord.run(BackStackRecord.java:635) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1397) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.app.FragmentManagerImpl$1.run(FragmentManager.java:426) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.os.Handler.handleCallback(Handler.java:615) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.os.Handler.dispatchMessage(Handler.java:92) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.os.Looper.loop(Looper.java:137) 02-01 13:11:32.750: E/AndroidRuntime(15061): at android.app.ActivityThread.main(ActivityThread.java:4898) 02-01 13:11:32.750: E/AndroidRuntime(15061): at java.lang.reflect.Method.invokeNative(Native Method) 02-01 13:11:32.750: E/AndroidRuntime(15061): at java.lang.reflect.Method.invoke(Method.java:511) 02-01 13:11:32.750: E/AndroidRuntime(15061): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1006) 02-01 13:11:32.750: E/AndroidRuntime(15061): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773) 02-01 13:11:32.750: E/AndroidRuntime(15061): at dalvik.system.NativeStart.main(Native Method)

I am wondering if anyone can give a hint about this issue.

Answer 1

From docs:

 onAttach(Activity) called once the fragment is associated with its activity.

In your code

 mListener = (SetupHotspotDialogListener) activity;

line throws ClassCastException because your activity don't implement SetupHotspotDialogListener interface. (Fragment is directly tied to the activity that contains it, as well as DialogFragment because DialogFragment extends Fragment).

Again from docs

In some cases, you might need a fragment to share events with the activity. A good way to do that is to define a callback interface inside the fragment and require that the host activity implement it. When the activity receives a callback through the interface, it can share the information with other fragments in the layout as necessary.

So if you want to create FragmentDialog from Fragment I suggest to organize it via callbacks to activity:

1. create callback interface into your SmartMode Fragment class (like you do into dialogFragment) with one method like createDialogRequest().
2. let your activity implement that interface
3. then, when you need to create dialog, send callback from Fragment to Activity
4. place "show dialog logics" into Activity

It's look like fragment ask activity to create dialog, activity shows dialog.

EDITED: I think i had found better implementation of what you need. I've write a simple example of creating fragment dialog from fragment with receiving fragment dialog callback events into fragment.

Activity:

public class MyFragmentActivity extends FragmentActivity {

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

        // first start of activity
        if (savedInstanceState == null) {
            // put fragment to activity layout 
            MyFragment fragment = new MyFragment();
            FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
            ft.replace(R.id.fragmentContainer, fragment, "fragment");
            ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
            ft.commit();
        }
    }

}

Fragment:

public class MyFragment extends Fragment implements MyDialogInterface {

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

        View fragmentView = inflater.inflate(R.layout.fragment, null);

        // button which shows dialog
        Button showDialogButton = (Button) fragmentView.findViewById(R.id.showDialog);
        showDialogButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                // create fragment dialog.
                FragmentDialog dialog = FragmentDialog.getInstance(MyFragment.this);
                dialog.show(getActivity().getSupportFragmentManager(), "dialog");
            }
        });

        return fragmentView;
    }

    @Override
    public void onClickEvent() {
        // receive click events from dialog fragment
        Log.e(getClass().getSimpleName(), "onClickEvent");
    }

}

FragmentDialog:

public class FragmentDialog extends DialogFragment {

    public interface MyDialogInterface extends Serializable {
        public void onClickEvent();
    }

    private MyDialogInterface callbackListener;

    /**
     * dialogInterface - instance of MyDialogInterface which will handle
     * callback events
     */
    public static FragmentDialog getInstance(MyDialogInterface dialogInterface) {
        FragmentDialog fragmentDialog = new FragmentDialog();

        // set fragment arguments
        Bundle args = new Bundle();
        args.putSerializable("dialogInterface", dialogInterface);
        fragmentDialog.setArguments(args);

        return fragmentDialog;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setStyle(STYLE_NO_TITLE, 0);
    }

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

        View pushDialogView = getActivity().getLayoutInflater().inflate(R.layout.fragment_dialog, null);

        // get reference to MyDialogInterface instance from arguments
        callbackListener = (MyDialogInterface) getArguments().getSerializable("dialogInterface");

        Button cancelButton = (Button) pushDialogView.findViewById(R.id.button);
        cancelButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                // send click event
                callbackListener.onClickEvent();
            }
        });

        return pushDialogView;
    }

}

I used support 4 library fragments

android.support.v4.app.Fragment
android.support.v4.app.DialogFragment
android.support.v4.app.FragmentActivity

And layouts:

activity_my_fragment.xml :

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:id="@+id/fragmentContainer"
   android:layout_width="match_parent"
   android:layout_height="match_parent" />

fragment.xml :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:background="#a00"
   android:orientation="vertical" >

   <Button
     android:id="@+id/showDialog"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="show doalog" />
</LinearLayout>

fragment_dialog.xml :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:background="#fe3"
   android:orientation="vertical" >

   <Button
      android:id="@+id/button"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="click me" />
 </LinearLayout>

The idea is to send reference to interface which will catch callback events.