如何在我的片段上保存项目的状态？

Question

I'm getting a ArrayList from an Url with AsyncTask , and inserting into an ListView on my Fragment but everytime I change fragments I have to get json data from the Url again so I tried to use onSaveInstanceState() to save my ArrayList as a String and transform to my ArrayList using JsonObject and Gson Library but I can't save the data.

OnSave method

      @Override
            public void onSaveInstanceState(@NonNull Bundle outState) {
            outState.putString("cursos", this.cursosString);

            super.onSaveInstanceState(outState);
      }

OnCreateView method

    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    View view = inflater.inflate(R.layout.fragment_cursos, container, false);
    final ListView lvCursos = view.findViewById(R.id.lvCursos);

    if(savedInstanceState != null){

        Type arrayListCurso = new TypeToken<ArrayList<Curso>>(){}.getType();
        ArrayList<Curso> cursos = new Gson().fromJson(savedInstanceState.getString("cursos"), arrayListCurso);
        ListaCursosAdapter adapter = new ListaCursosAdapter(cursos, getActivity());
        lvCursos.setAdapter(adapter);

    } else {
    ...

Change Fragments method

   public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {

    switch (menuItem.getItemId()){

        case R.id.navigation_profile:
            getSupportFragmentManager()
                    .beginTransaction()
                    .replace(R.id.nav_fragment, perfilFragment)
                    .commit();
            return true;

Answer 1

Here is the solution, you just left to decide how to implement getTagForFragmentId() and getFragmentForFragmentTag(fragmentTag) , which should be simple.

The idea of the code below is that you .hide() previous fragment presented and then you either .add(fragment) a new fragment, if it was not previously presented or .show() fragment if it is already inside the backstack.

String lastPresentedFragmentTag = "";

public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
    switch (menuItem.getItemId()){
        case R.id.navigation_profile:
            // getTagForFragmentId() can be just a switch case function
            // or you can hardcode value instead
            String fragmentTag = getTagForFragmentId(R.id.navigation_profile);
            presentFragment(fragmentTag);
            return true;
        //...


public void presentFragment(String fragmentTag) {
    if (lastPresentedFragmentTag == fragmentTag) {
        return;
    }

    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    // Hide previous fragment
    if (lastPresentedFragmentTag != null) {
        Fragment lastFragmenter code hereent = getSupportFragmentManager().findFragmentByTag(lastPresentedFragmentTag);
        ft.hide(lastFragment);
    }

    Fragment currentFragment = getSupportFragmentManager().findFragmentByTag(fragmentTag);
    if (currentFragment == null) {

            // Implement getFragmentForFragmentTag(fragmentTag) it will be another switch case function that will return you fragment
            val fragment = getFragmentForFragmentTag(fragmentTag);
            transaction.add(R.id.fragmentContainer, fragment, tag);
        } else {
            transaction.show(currentFragment);
        }
    }

    transaction.commitNow();
}