onAttach(Activity) deprecated: where I can check if the activity implements callback interface

Question

Before API 23 I used Fragment's onAttach methods to get my listener instance, then the reference is cleaned inside onDetach. ex:

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    mListener = null;
    try {
        mListener = (SellFragmentListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString()
                + " must implement SellFragmentListener");
    }
}

@Override
public void onDetach() {
    super.onDetach();
    mListener = null;
}

Is it safe to do the same check inside onAttach(Context context) or is there a better way to get the holder activity instance?

I usually do this kind of thing on `onResume()`/`onPause()`, especially if the listener will do anything that affects the UI. — Kevin Krumwiede, Aug 27 '15 at 20:04
@tibo I think that onActivityCreated it's not called when a Fragment is added to an existing activity. — Ariel Carbonaro, Aug 27 '15 at 20:29
Yes, this is good question. It's mean that documentation http://developer.android.com/training/basics/fragments/communicating.html is wrong or not actual — Kiryl Bielašeŭski, Oct 10 '15 at 21:38

Zsolt Mester · Accepted Answer · 2015-08-27T19:57:40.027

Check the source code:

/**
 * Called when a fragment is first attached to its context.
 * {@link #onCreate(Bundle)} will be called after this.
 */
public void onAttach(Context context) {
    mCalled = true;
    final Activity hostActivity = mHost == null ? null : mHost.getActivity();
    if (hostActivity != null) {
        mCalled = false;
        onAttach(hostActivity);
    }
}

/**
 * @deprecated Use {@link #onAttach(Context)} instead.
 */
@Deprecated
public void onAttach(Activity activity) {
    mCalled = true;
}

So the onAttach(Activity activity) is called by the onAttach(Context context) if there is a host activity. You can use the onAttach(Activity activity) safely.

mHost is not accessible from outside the package...So what then? — joninx, Nov 12 '15 at 12:37

score 5 · Answer 2 · edited May 23 '17 at 11:47

As shown in Zsolt Mester's answer, onAttach(Activity activity) is deprecated in favor of onAttach(Context context). Thus, all you need to do is check to make sure the context is an activity.

In Android Studio if you go to File > New > Fragment, you can get the auto generated code that will contain the proper syntax.

import android.support.v4.app.Fragment;
...

public class MyFragment extends Fragment {

    private OnFragmentInteractionListener mListener;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // inflate fragment layout
        return inflater.inflate(R.layout.fragment_myfragment, container, false);
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnFragmentInteractionListener) {
            mListener = (OnFragmentInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

Notes

Since the parent Activity must implement our OnFragmentInteractionListener (the arbitrarily named interface), checking (context instanceof OnFragmentInteractionListener) ensures that the context is actually the activity.
Note that we are using the support library. Otherwise onAttach(Context context) wouldn't be called by pre API 23 versions of Android.

See also

score 4 · Answer 3 · answered Aug 27 '15 at 19:45

4

Well, what is deprecated is onAttach(Activity activity); method, but the whole flow remains. so you may do nothing really as onAttach(Activity activity); is going to be still supported long enough.

answered Aug 27 '15 at 19:45

Marcin Orlowski

72,056
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The Original Android · Answer 4 · 2015-08-28T01:35:26.007

3

I never had to use onAttach(Context context) yet but I think your code is basically good. So here is my suggestion, using your code:

public void onAttach (Context context) {
   super.onAttach(context);
   try {
        Activity activity = (Activity) context;
        mListener = (SellFragmentListener) activity;
   } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString()
                + " must implement SellFragmentListener");
    }
}

The main difference is that I can typecast context to get the Activity. This is because Context can propagate to the subclass which is the activity.

Another issue, API 23 is still far away from now for us to worry. If you do worry, using build pragma (static Build) may be a good option.

edited Aug 28 '15 at 01:35

answered Aug 28 '15 at 00:49

The Original Android

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1

You should cast context directly. Or if you want the context to be an instance of an activity. Check that first by using if(context instanceof Activity) { ... } – Daverix Jan 11 '16 at 09:11
@Daverix, Interesting comment "You should cast context directly". Please be more specific. Are you saying object context can/should be cast to mListener directly? That will reduce code from 2 lines to only 1 line. If so, that is a coding style difference only. – The Original Android Jan 11 '16 at 19:28
Yes, that may be a coding style difference, but still an unecessary line imo. Otherwise it's better to use "instanceof" instead of catching ClassCastExceptions. But in the end maybe that only saves you a bit more lines and gain minimum performance. – Daverix Jun 29 '16 at 08:31

score 1 · Answer 5 · edited Sep 29 '15 at 08:03

1

I have faced the same situation. I have gone through various posts and finally implemented my code in onCreate() block of fragment.Till now i haven't faced any issue and i think it wouldn't be an issue as onCreate is called immediately after onAttach in lifecycle.

 @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        try {
            listnener = (TilesOnClickListnener)getActivity();
        } catch (ClassCastException e) {
            throw new ClassCastException(getActivity().toString() + " must implement OnArticleSelectedListener");
        }
    }

edited Sep 29 '15 at 08:03

Movsar Bekaev

888
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answered Sep 06 '15 at 13:23

SriMaharshi Manchem

524
5
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Also, i forgot to mention that casting context to Activity didn't work out well for me. – SriMaharshi Manchem Sep 06 '15 at 13:24
Why would Activity casting not work out well? Works fine for me, if you declare activity outside the try / catch block --- public void onAttach (Context context) { super.onAttach(context); Activity activity = (Activity) context; try { mListener = (SellFragmentListener) activity; } catch (ClassCastException e) { throw new ClassCastException(activity.toString() + " must implement SellFragmentListener"); } } – WallyHale Oct 16 '15 at 08:43
Great! if it is working. I haven't tried that.Thanks for the information. – SriMaharshi Manchem Oct 16 '15 at 08:51

score 1 · Answer 6 · answered Nov 11 '17 at 15:32

I use the native Fragment rather than the one from the Support Library. I put both onAttach() methods in my code and did some debugging on devices running different SDK versions. I found that:

SDK 22 and below - only onAttach(Activity) is called. Not surprising really because onAttach(Context) was only introduced in SDK 23.

SDK 23 and above - onAttach(Context) is called first and then onAttach(Activity). (This is consistent with what @Zsolt Mester said about the source code in another answer to this post.)

Since minSdkVersion for my app was below 23 I just decided to omit onAttach(Context) altogether and simply add the @SuppressWarnings("deprecation") annotation to my existing onAttach(Activity) method.

score 0 · Answer 7 · edited Jan 07 '17 at 09:19

 public class MainActivity extends AppCompatActivity implements  topsection.TopSectionListener {

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



 public void createMeme(String top, String bottom){
    bottomsection fragmentbottom = (bottomsection) getSupportFragmentManager().findFragmentById(R.id.fragment2);
    fragmentbottom.setMemeText(top, bottom);
  }
 }

onAttach(Activity) deprecated: where I can check if the activity implements callback interface

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