title: Detect when fragment becomes visible through tab click
body: <p>I am trying to detect when a fragment becomes visible on a swipe view, in order to update its content when it does become visible. I am doing it like this.</p>
  
  <pre><code>public class MyFragment extends Fragment {
    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
      super.setUserVisibleHint(isVisibleToUser);
      if (isVisibleToUser) { }
      else {  }
    }
  }
  </code></pre>
  
  <p>The thing is when I go to the page be swipping through it works great but when I select the page in the tab it crashes "Attempt to invoke virtual method".</p>
  
  <p>Please help me, thanks</p>
  
  <p>I have a main activity, and then an activity for each fragment.</p>
  
  <p>Main:</p>
  
  <pre><code>public class Principal extends ActionBarActivity implements ActionBar.TabListener {
  
  SectionsPagerAdapter mSectionsPagerAdapter;
  
  /**
   * The {@link ViewPager} that will host the section contents.
   */
  ViewPager mViewPager;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_principal);
  
      // Set up the action bar.
      final ActionBar actionBar = getSupportActionBar();
      actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
  
      // 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.pager);
      mViewPager.setAdapter(mSectionsPagerAdapter);
  
      // When swiping between different sections, select the corresponding
      // tab. We can also use ActionBar.Tab#select() to do this if we have
      // a reference to the Tab.
      mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
          @Override
          public void onPageSelected(int position) {
              actionBar.setSelectedNavigationItem(position);
          }
      });
  
      // For each of the sections in the app, add a tab to the action bar.
      for (int i = 0; i &lt; mSectionsPagerAdapter.getCount(); i++) {
          // Create a tab with text corresponding to the page title defined by
          // the adapter. Also specify this Activity object, which implements
          // the TabListener interface, as the callback (listener) for when
          // this tab is selected.
          actionBar.addTab(
                  actionBar.newTab()
                          .setText(mSectionsPagerAdapter.getPageTitle(i))
                          .setTabListener(this));
      }
  
  
  
  
  
  
  
  }
  
  
  @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_principal, 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);
  }
  
  @Override
  public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
  
  
          mViewPager.setCurrentItem(tab.getPosition());
  
  }
  
  @Override
  public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
  }
  
  @Override
  public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
  }
  
  /**
   * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
   * one of the sections/tabs/pages.
   */
  public class SectionsPagerAdapter extends FragmentPagerAdapter {
  
      public SectionsPagerAdapter(FragmentManager fm) {
          super(fm);
      }
  
      @Override
      public Fragment getItem(int position) {
          Fragment fragment = new Fragment();
          switch (position) {
              case 0:
                  return fragment = new Config();
              case 1:
                  return fragment = new Saidas();
              case 2:
                  return fragment = new Entradas();
              case 3:
                  return fragment = new Enviar();
              case 4:
                  return fragment = new Historico();
              case 5:
                  return fragment = new Status();
  
              default:
                  break;
          }
          return fragment;
  
      }
  
      @Override
      public int getCount() {
          // Show 3 total pages.
          return 6;
      }
  
      @Override
      public CharSequence getPageTitle(int position) {
          Locale l = Locale.getDefault();
          switch (position) {
              case 0:
                  return getString(R.string.title_config).toUpperCase(l);
              case 1:
                  return getString(R.string.title_saidas).toUpperCase(l);
              case 2:
                  return getString(R.string.title_entradas).toUpperCase(l);
              case 3:
                  return getString(R.string.title_enviar).toUpperCase(l);
              case 4:
                  return getString(R.string.title_historico).toUpperCase(l);
              case 5:
                  return getString(R.string.title_status).toUpperCase(l);
          }
          return null;
      }
  }
  
  /**
   * A placeholder fragment containing a simple view.
   */
  public static class PlaceholderFragment extends Fragment {
      /**
       * The fragment argument representing the section number for this
       * fragment.
       */
      private static final String ARG_SECTION_NUMBER = "section_number";
  
      /**
       * 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;
      }
  
      public PlaceholderFragment() {
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
          View rootView = inflater.inflate(R.layout.fragment_principal, container, false);
          return rootView;
      }
  }
  
  }
  </code></pre>
  
  <p>and then the fragments are: </p>
  
  <pre><code>public class Entradas extends android.support.v4.app.Fragment {
  
      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
  
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
  
          return inflater.inflate(R.layout.fragment_entradas, container, false);
  
      }
  
      @Override
      public void setMenuVisibility(final boolean visible) {
          super.setMenuVisibility(visible);
          if (visible) {
               run code here....
      }
  }
  </code></pre>