add fragment to pager adapter on button click

Question

I have a tabbed activity with 1 fragment, this fragment has a button, i want when i click the button it create a new fragment and i can swipe between the two fragments

this is the main activity

public class ActivityBeamRec extends AppCompatActivity {

    private SectionsPagerAdapter mSectionsPagerAdapter;

    public static CustomViewPager mViewPager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_activity_beam_rec);

        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
        // Set up the ViewPager with the sections adapter.
        mViewPager = (CustomViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);
        mViewPager.setPagingEnabled(false);

            }

    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
                       switch (position){
                case 0 : return PlaceholderFragment.newInstance(position + 1);
         //       case 1 : return the new fragment ;
            }
            return null;
        }

        @Override
        public int getCount() {

            return 1 ; 
        }

            }
}

this is the fragment i have.

public  class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    public PlaceholderFragment() {
    }

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        final View rootView = inflater.inflate(R.layout.fragment_activity_beam_rec, container, false);

        final EditText etxb;
     etxb = (EditText)rootView.findViewById(R.id.editText);
     final Button buDesign = (Button)rootView.findViewById(R.id.buDesign);

        buDesign.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
    double b;          
    b = Double.valueOf(etxb.getText().toString());

\\here i want the button to create the second fragment and pass the variable d to it
 ActivityBeamRec.mViewPager.setPagingEnabled(true); // this is to enable the siwpe between the fragments
ActivityBeamRec.mViewPager.setCurrentItem(2); // ths is to set the new fragment as the current view

            }
        });
        return rootView;
    }



}

the second fragment should go through on create view stage after the button is pressed, and please tell where i put each code if there is a way to do this.

Answer 1

ActivityBeamRec:

public class ActivityBeamRec extends AppCompatActivity {

    private SectionsPagerAdapter mSectionsPagerAdapter;

    private ViewPager mViewPager;

    private TabLayout tabLayout;

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {
        // This list holds the fragments for the pages displayed by view pager.
        private List < Fragment > fragments = new ArrayList < Fragment > ();

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
            // Add the first fragment:
            fragments.add(PlaceholderFragment.newInstance(1));
        }

        private void addFragment(Fragment fragment) {
            fragments.add(fragment);

            // Notify view pager that the contents of the adapter has changed and that it needs to update
            // its pages:
            notifyDataSetChanged();

            // Since content of view pager has changed, re-wire tab layout:
            tabLayout.setupWithViewPager(mViewPager);

            // Set the current page in the view pager to the last added fragment:
            mViewPager.setCurrentItem(fragments.size() - 1);
        }

        @Override
        public Fragment getItem(int position) {
            return fragments.get(position);
        }

        @Override
        public int getCount() {
            return fragments.size();
        }

        @Override
        public CharSequence getPageTitle(int position) {
            // Set the tab title as the number of the current section:
            return "SECTION " + (position + 1);
        }
    }

    /**
     * Adds a new fragment (page) to the view pager.
     * This method can either be public or package-private (without any modifier) depending on the package
     * of 'PlaceholderFragment'. Since you're new to Java please refer the link to access modifiers below.
     *
     * @param fragment the fragment to be added to the view pager
     **/
    public void addFragment(Fragment fragment) {
        mSectionsPagerAdapter.addFragment(fragment);
    }

    /**
     * Returns the number of the next section (page) to be added.
     *
     * @return the next section number
     */
    public int getNextSectionNumber() {
        return mSectionsPagerAdapter.getCount() + 1;
    }

}

PlaceholderFragment:

public class PlaceholderFragment extends Fragment {

    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    public PlaceholderFragment() {}

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        final View rootView = inflater.inflate(R.layout.fragment_activity_beam_rec, container, false);

        final EditText etxb;
        etxb = (EditText) rootView.findViewById(R.id.editText);
        final Button buDesign = (Button) rootView.findViewById(R.id.buDesign);

        buDesign.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                double b;
                b = Double.valueOf(etxb.getText().toString());

                //here i want the button to create the second fragment and pass the variable d to it
                int nextSectionNumber = ((ActivityBeamRec) getActivity()).getNextSectionNumber();
                ((ActivityBeamRec) getActivity()).addFragment(PlaceholderFragment.newInstance(nextSectionNumber));
            }
        });
        return rootView;
    }
}

When the button in the fragment is clicked the following things occur in sequence:

1. addFragment() method of parent activity is called by passing the fragment instance to be added. This public method is used to access the private member mSectionsPagerAdapter of parent activity. We can do away with this method if mSectionsPagerAdapter is made public or package-private.
2. SectionsPagerAdapter adds the passed-in fragment to its list of fragments and then notifies the view pager that its data set has changed.
3. TabLayout is refreshed to accommodate the new fragment (page).
4. Finally the view pager is made to scroll to the added fragment using the setCurrentItem() method.

Reference:

• Java Access Control Modifiers
• List Data Structure in Java

Answer 2

In your SectionsPagerAdapter

private ArrayList<Fragment> fragments = new ArrayList<>();

@Override
public int getCount() {
   return fragments.size();
}

@Override
public Fragment getItem(int position) {
  if(position<fragments.size())
      return fragments.get(position);
  throw new NullPointerException("No Fragment with this position found.");
}
public void addFragment(Fragment newFragment){
  fragments.add(newFragment);
}

In the MainActivity before setting the adapter with mViewPager.setAdapter(mSectionsPagerAdapter); add your first fragment(the PlaceHolder in your case). (or do it in the SectionPagerAdapter 's constructor).

And then in your OnClickListener call the SectionPagerAdapter 's addFragment method.

EDIT: In MainActivity:

mSectionsPagerAdapter.addFragment(PlaceholderFragment.newInstance(0));
mSectionsPagerAdapter.setAdapter(mSectionsPagerAdapter);