Android - getSupportFragmentManager() 返回 null

Question

我知道这里有很多类似的问题，但到目前为止没有一个对我有用，所以我希望通过在这里发布我的（略微编辑的）课程，有人可以帮助我。这是一个使用 4 个片段作为页面的 ViewPager。

所以我的问题是 getFragmentByPosition 方法不起作用，因为 getSupportFragmentManager() 返回 null。在我的 onCreate 中，如果我尝试从中调用任何内容，mSectionsPagerAdapter 也会返回 null，但是我的选项卡都显示正常，我只是无法从我需要的片段中访问数据。

目前我真的不知道问题出在哪里，因此感谢您提供任何帮助。我之前发布了一些其他问题，并尝试了这些建议无济于事，所以现在我只是发布我的整个班级。谢谢！

package com.me.testapp;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;

import android.net.Uri;
import android.support.v4.app.FragmentActivity;
import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.CheckBox;
import android.widget.TextView;

import com.me.testapp.CheckListFragments.Thing1Fragment;
import com.me.testapp.CheckListFragments.Thing2Fragment;
import com.me.testapp.CheckListFragments.Thing3Fragment;
import com.me.testapp.CheckListFragments.Thing4Fragment;
import com.me.testapp.Data.ClientDAO;

import io.karim.MaterialTabs;


public class CheckList extends AppCompatActivity
        implements Thing1Fragment.OnFragmentInteractionListener,
        Thing2Fragment.OnFragmentInteractionListener,
        Thing3Fragment.OnFragmentInteractionListener,
        Thing4Fragment.OnFragmentInteractionListener {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    /**
     * The {@link ViewPager} that will host the section contents.
     */

    public SectionsPagerAdapter mSectionsPagerAdapter;
    public ViewPager pager;
    private ClientDAO clientDAO;

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        TextView toolbarName = (TextView) findViewById(R.id.toolbar_name);
        TextView toolbarFloor = (TextView) findViewById(R.id.toolbar_floor);
        TextView toolbarOffice = (TextView) findViewById(R.id.toolbar_office);

        toolbarName.setText(getIntent().getStringExtra("Name"));
        toolbarRoom.setText(": " + getIntent().getStringExtra("Office"));

        toolbarFloor.setVisibility(View.GONE);

        // Initialize the ViewPager and set an adapter
        pager = (ViewPager) findViewById(R.id.pager);
        mSectionsPagerAdapter = new SectionsPagerAdapter(this.getSupportFragmentManager());
        pager.setAdapter(mSectionsPagerAdapter);
        pager.setOffscreenPageLimit(3);

        // Bind the tabs to the ViewPager
        MaterialTabs tabs = (MaterialTabs) findViewById(R.id.tabs);
        tabs.setViewPager(pager);

        clientDAO = new ClientDAO(getApplicationContext());

    }


    @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_checklist, 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 {
        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            switch(position) {
                case 0:
                    Thing1Fragment thing1 = new Thing1Fragment();
                    return thing1;
                case 1:
                    Thing2Fragment thing2 = new Thing2Fragment();
                    return thing2;
                case 2:
                    Thing3Fragment thing3 = new Thing3Fragment();
                    return thing3;
                case 3:
                    Thing4Fragment thing4 = new Thing4Fragment();
                    return thing4;
                default:
                    thing1 = new Thing1Fragment();
                    return thing1;
            }
        }

        @Override
        public int getCount() {
            // Show 4 total pages.
            return 4;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
                case 0:
                    return getString(R.string.title_section1).toUpperCase(l);
                case 1:
                    return getString(R.string.title_section2).toUpperCase(l);
                case 2:
                    return getString(R.string.title_section3).toUpperCase(l);
                case 3:
                    return getString(R.string.title_section4).toUpperCase(l);
            }
            return null;
        }
    }

    public void onFragmentInteraction(Uri uri){
        //you can leave it empty
    }

    public int booleanToInt(boolean bool) {
        return bool ? 1 : 0;
    }

    public Fragment getFragmentByPosition(int pos) {
        String tag = "android:switcher:" + R.id.pager + ":" + pos;
        return getSupportFragmentManager().findFragmentByTag(tag);
    }

    public int getCheckboxState(int id, String page) {

        switch (page) {
            case "thing1":
                return booleanToInt(((CheckBox) getFragmentByPosition(0).getView().findViewById(id)).isChecked());
            case "thing2":
                return booleanToInt(((CheckBox) getFragmentByPosition(1).getView().findViewById(id)).isChecked());
            case "thing3":
                return booleanToInt(((CheckBox) getFragmentByPosition(2).getView().findViewById(id)).isChecked());
            case "thing4":
                return booleanToInt(((CheckBox) getFragmentByPosition(3).getView().findViewById(id)).isChecked());
            default:
                return booleanToInt(((CheckBox) getFragmentByPosition(0).getView().findViewById(id)).isChecked());
        }
    }
}

Answer 1

我遇到了同样的问题。

我对活动生命周期的推断是，在 onCreate 方法中，活动并未完全创建，因此当您尝试将 getSupportFragmentManager 传递给其 null 时。

我要解决这个问题，就是将这行代码放在onStart 中。问题是每次暂停活动并再次运行时，Fragment 将重新启动。