android - Fragment inner class should be static

Question

I have a FragmentActivity class with inner class that should display Dialog. But I am required to make it static. Eclipse offers me to suppress error with @SuppressLint("ValidFragment"). Is it bad style if I do it and what are the possible consequences?

public class CarActivity extends FragmentActivity {
//Code
  @SuppressLint("ValidFragment")
  public class NetworkConnectionError extends DialogFragment {
    private String message;
    private AsyncTask task;
    private String taskMessage;
    @Override
    public void setArguments(Bundle args) {
      super.setArguments(args);
      message = args.getString("message");
    }
    public void setTask(CarActivity.CarInfo task, String msg) {
      this.task = task;
      this.taskMessage = msg;
    }
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
      // Use the Builder class for convenient dialog construction
      AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
      builder.setMessage(message).setPositiveButton("Go back", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int id) {
          Intent i = new Intent(getActivity().getBaseContext(), MainScreen.class);
          startActivity(i);
        }
      });
      builder.setNegativeButton("Retry", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int id) {
          startDownload();
        }
      });
      // Create the AlertDialog object and return it
      return builder.create();
    }
  }

startDownload() starts Asynctask.

Answer 1

Non static inner classes do hold a reference to their parent classes. The problem with making a Fragment inner class non-static is that you always hold a reference to the Activity. The GarbageCollector cannot collect your Activity. So you can 'leak' the Activity if for example the orientation changes. Because the Fragment might still live and gets inserted in a new Activity.

EDIT:

Since some people asked me for some example I started writing one, while doing this I found some more problems when using non static Fragments:

• They cannot be used in a xml file since they do not have a empty constructor (They can have an empty constructor, but you usually instantiate nonstatic nested classes by doing myActivityInstance.new Fragment() and this is different to only calling an empty constructor)
• They cannot be reused at all - since the FragmentManager sometimes calls this empty constructor too. If you added the Fragment in some Transaction.

So in order to make my example work I had to add the

wrongFragment.setRetainInstance(true);

Line to not make the app crash on orientation change.

If you execute this code you will have an activity with some textviews and 2 buttons - the buttons increase some counter. And the Fragments show the orientation which they think their activity has. At the start everything works correctly. But after changing the screen orientation only the first Fragment works correcly - the second one is still calling stuff at its old activity.

My Activity class:

package com.example.fragmenttest;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.res.Configuration;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;

public class WrongFragmentUsageActivity extends Activity
{
private String mActivityOrientation="";
private int mButtonClicks=0;
private TextView mClickTextView;


private static final String WRONG_FRAGMENT_TAG = "WrongFragment" ;

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    int orientation = getResources().getConfiguration().orientation;
    if (orientation == Configuration.ORIENTATION_LANDSCAPE)
    {
        mActivityOrientation = "Landscape";
    }
    else if (orientation == Configuration.ORIENTATION_PORTRAIT)
    {
        mActivityOrientation = "Portrait";
    }

    setContentView(R.layout.activity_wrong_fragement_usage);
    mClickTextView = (TextView) findViewById(R.id.clicksText);
    updateClickTextView();
    TextView orientationtextView = (TextView) findViewById(R.id.orientationText);
    orientationtextView.setText("Activity orientation is: " + mActivityOrientation);

    Fragment wrongFragment = (WrongFragment) getFragmentManager().findFragmentByTag(WRONG_FRAGMENT_TAG);
    if (wrongFragment == null)
    {
        wrongFragment = new WrongFragment();
        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.add(R.id.mainView, wrongFragment, WRONG_FRAGMENT_TAG);
        ft.commit();
        wrongFragment.setRetainInstance(true); // <-- this is important - otherwise the fragment manager will crash when readding the fragment
    }
}

private void updateClickTextView()
{
    mClickTextView.setText("The buttons have been pressed " + mButtonClicks + " times");
}

private String getActivityOrientationString()
{
    return mActivityOrientation;
}


@SuppressLint("ValidFragment")
public class WrongFragment extends Fragment
{


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        LinearLayout result = new LinearLayout(WrongFragmentUsageActivity.this);
        result.setOrientation(LinearLayout.VERTICAL);
        Button b = new Button(WrongFragmentUsageActivity.this);
        b.setText("WrongFragmentButton");
        result.addView(b);
        b.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                buttonPressed();
            }
        });
        TextView orientationText = new TextView(WrongFragmentUsageActivity.this);
        orientationText.setText("WrongFragment Activities Orientation: " + getActivityOrientationString());
        result.addView(orientationText);
        return result;
    }
}

public static class CorrectFragment extends Fragment
{
    private WrongFragmentUsageActivity mActivity;


    @Override
    public void onAttach(Activity activity)
    {
        if (activity instanceof WrongFragmentUsageActivity)
        {
            mActivity = (WrongFragmentUsageActivity) activity;
        }
        super.onAttach(activity);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        LinearLayout result = new LinearLayout(mActivity);
        result.setOrientation(LinearLayout.VERTICAL);
        Button b = new Button(mActivity);
        b.setText("CorrectFragmentButton");
        result.addView(b);
        b.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                mActivity.buttonPressed();
            }
        });
        TextView orientationText = new TextView(mActivity);
        orientationText.setText("CorrectFragment Activities Orientation: " + mActivity.getActivityOrientationString());
        result.addView(orientationText);
        return result;
    }
}

public void buttonPressed()
{
    mButtonClicks++;
    updateClickTextView();
}

}

Note that you should probably not cast the activity in onAttach if you want to use your Fragment in different Activities - but for here its working for the example.

The activity_wrong_fragement_usage.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".WrongFragmentUsageActivity" 
android:id="@+id/mainView">

<TextView
    android:id="@+id/orientationText"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="" />

<TextView
    android:id="@+id/clicksText"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="" />



<fragment class="com.example.fragmenttest.WrongFragmentUsageActivity$CorrectFragment"
          android:id="@+id/correctfragment"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content" />


</LinearLayout>