android - 检测 fragment 何时通过选项卡单击变得可见

Question

我正在尝试检测 fragment 何时在滑动 View 中变得可见，以便在它变得可见时更新其内容。我是这样做的。

public class MyFragment extends Fragment {
  @Override
  public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);
    if (isVisibleToUser) { }
    else {  }
  }
}

问题是，当我转到页面时，它可以快速浏览，但当我在选项卡中选择页面时，它会崩溃“尝试调用虚拟方法”。

请帮帮我，谢谢

我有一个主要 Activity ，然后是每个 fragment 的 Activity 。

主要内容:

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 < 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;
    }
}

}

然后 fragment 是:

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....
    }
}

Answer 1

有两种方法可以做到这一点:

从 fragment 中你可以调用isVisible();

Return true if the fragment is currently visible to the user. This means it: (1) has been added, (2) has its view attached to the window, and (3) is not hidden.

您可以从父 Activity 中执行以下操作:

public boolean checkIsFragVisible() {
        Fragment yourFragment = getSupportFragmentManager().findFragmentById(FRAG_HOLDER_ID);
        return yourFragment != null && yourFragment.isVisible();
    }