android fragment state restored only on back button , not when i select a fragment randomly from listview

问题

Hi i have a listview sidebar and i am displaying fragments based on user selection in listview.

This is how i am replacing fragments

public void switchFragment(Fragment fragment, boolean addBackStack) {
        try {
            FragmentManager manager = getSupportFragmentManager();
            FragmentTransaction ft = manager.beginTransaction();

            ft.replace(R.id.content, fragment);
            currentFragment = fragment;
           //if (addBackStack)
                ft.addToBackStack(null);
            ft.commit();
        } catch (Exception e) {

        }
    }

This is my sample fragment code.Now when i replace fragments i am saving instance state in onpause and restoring it in onresume but it only works when i press back button. When i manually navigate back to fragment from listview ,fragment state is not restored.Why?

public class Fragment1 extends BaseFragment {   

    int currentFragmentInd = 1;  

    private Button startButton;
    private  Button endButton;
    private long savedStartTime;
    private TextView setStartText;
    private TextView setEndText;
    private String starttime;
    private String endtime;

    public int getIndex() {
        MyApplication.getApplication().setCurrentChild(0);
        MyApplication.getApplication().setCurrentGroup(0);
        return currentFragmentInd;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        if (savedInstanceState !=null)
        {      



        }
    }

    @Override
    public void onResume() {
        super.onResume();
        setStartText= (TextView)getActivity().findViewById(R.id.MAtextView2);
        setEndText= (TextView)getActivity().findViewById(R.id.MAtextView3);
        setEndText.setText(endtime);
        setStartText.setText(starttime);
    }

    @Override
    public void onPause() {
        super.onPause();
        setStartText= (TextView)getActivity().findViewById(R.id.MAtextView2);
        setEndText= (TextView)getActivity().findViewById(R.id.MAtextView3);
        starttime=setStartText.getText().toString();
        endtime=setEndText.getText().toString();

    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);


    }

    FrameLayout frameLayout;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View contentView = inflater.inflate(R.layout.layout1, null, false);

        ((MainActivity) getActivity()).openList(0, 0);




        if (savedInstanceState == null) {

        }





        startButton= (Button) contentView.findViewById(R.id.button);
        endButton= (Button) contentView.findViewById(R.id.button2);
        endButton.setEnabled(false);

        setStartText= (TextView)contentView.findViewById(R.id.MAtextView2);
        setEndText= (TextView)contentView.findViewById(R.id.MAtextView3);





        startButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {


                    Time now = new Time();
                    now.setToNow();




            }
        });



        endButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {

                    Time now = new Time();
                    now.setToNow();

                    setEndText.setText(now.hour+" : "+now.minute);


            }
        });


        return contentView;
    }




}

回答1:

Late replay but might help somebody else. This happens because when you click a listview item you create a new inctance of that fragment. "I assume the fragment you send to switchFragment(Fragment fragment), is created using a 'new' keyword." Therefore this new instance of a fragment doesnt hold your old data.

This is how I solved this. There are probably better ways, but since nobody replied, I will give my solution.

1. When you replace the fragment (ft.replace, fragment), give a string reference to that transaction: -ft.replace(R.id.content, fragment, "FRAGMENT_NAME");
2. When you add the fragment to the backstack with addToBackStack(null); put the name of your fragment where you have null.: -ft.addToBackStack("FRAGMENT_NAME");

3. Create a method which tells you if that fragment has already been created, and therefore exists in the back stack.:

   public boolean isTagInBackStack(String tag){

       Log.i(TAG, "isTagInBackStack() Start");
       int x;
       boolean toReturn = false;
       int backStackCount = getSupportFragmentManager().getBackStackEntryCount();
       Log.i(TAG, "backStackCount = " + backStackCount);
   
       for (x = 0; x < backStackCount; x++){
           Log.i(TAG, "Iter = " + x +" "+ getSupportFragmentManager().getBackStackEntryAt(x).getName());
           if (tag == getSupportFragmentManager().getBackStackEntryAt(x).getName()){
               toReturn = true;
   
           }
   
       }
   
       Log.i(TAG, "isTagInBackStack() End, toReturn = " + toReturn);
       return toReturn;
   }
   

4. Now before you create a new instance of that fragment check in the backstack if a backstack item named "FRAGMENT_NAME" exists. if it exists, use that item (fragment) instead of creating a new one.

   if (isTagInBackStack("FRAGMENT_NAME")){
       Log.i(TAG, "Tag is in BackStack!!!! frag is = " + getSupportFragmentManager().findFragmentByTag("FRAGMENT_NAME"));
   
       FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
       transaction.replace(R.id.main_activity_container, getSupportFragmentManager().findFragmentByTag("FRAGMENT_NAME"));
       transaction.addToBackStack("FRAGMENT_NAME");
       transaction.commit();
   }else{
       Create the fragment (this happens the first time. 
   }
   

来源：https://stackoverflow.com/questions/22468977/android-fragment-state-restored-only-on-back-button-not-when-i-select-a-fragme