Android getSupportFragmentManager Fragment中的NULL指针异常

问题描述

单击导航抽屉项目后，我正在尝试使用 TabBar 和 View Pager 显示新片段.

i'm trying to display new fragment with TabBar and View Pager after click on the Navigation drawer item.

问题是新创建的 Fragment 导致 NULL 指针异常

Problem is that newly created Fragment is causing NULL pointer exception on

viewPager.setAdapter(mAdapter);

我被困在 Fragment 中带有标签的可滑动视图几个小时但没有运气，有人可以帮我解决这个问题吗?

I'm stucked on swipeable-views with tabs in Fragment a several hours but without luck, can somebody helps me with this problem please?

对于可滑动视图，我遵循了本教程:

For swipable views i followed this tutorial:

http://www.androidhive.info/2013/10/android-tab-layout-with-swipeable-views-1/

片段代码在这里:

import android.app.ActionBar;
import android.support.v4.app.Fragment;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import eu.com.mycompany.adapter.TabsPagerAdapter;

public class Fragment_AboutApplication extends Fragment implements ActionBar.TabListener {

    public Fragment_AboutApplication(){}
    private ViewPager viewPager;
    private TabsPagerAdapter mAdapter;
    private ActionBar actionBar;
    // Tab titles
    private String[] tabs = { "Top Rated", "Games", "Movies" };

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_about_app, container, false);


        // Initilization
        viewPager = (ViewPager) getActivity().findViewById(R.id.pager);
        actionBar = getActivity().getActionBar();
        mAdapter = new TabsPagerAdapter(getActivity().getSupportFragmentManager());

        /*HERE IS THROWED NULL POINTER EXCEPTION*/ 
        viewPager.setAdapter(mAdapter);
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);


        // Adding Tabs
        for (String tab_name : tabs) {
            actionBar.addTab(actionBar.newTab().setText(tab_name)
                    .setTabListener(this));
        }
        return rootView;
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {

    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {

    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {

    }
}

类 TabPagerAdapter:

public class TabsPagerAdapter extends FragmentPagerAdapter {

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

    @Override
    public Fragment getItem(int index) {

        switch (index) {
            case 0:
                // Top Rated fragment activity
                return new Fragment_TopRated();
            case 1:
                // Games fragment activity
                return new Fragment_TopRated();
            case 2:
                // Movies fragment activity
                return new Fragment_TopRated();
        }

        return null;
    }

    @Override
    public int getCount() {
        // get item count - equal to number of tabs
        return 3;
    }

}

使用 View Pager 的主要片段布局

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </android.support.v4.view.ViewPager>

    <TextView
        android:id="@+id/txtLabel"
         android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:textSize="16dp"
        android:text="Settings"/>

    <ImageView android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/txtLabel"
        android:src="@drawable/ic_communities"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dp"/>    

</RelativeLayout>

例外情况是:

E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
    at android.support.v4.app.Fragment.performCreateView(Fragment.java:1504)
                at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:942)
                at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1121)
                at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
                at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1484)
                at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:450)
                at android.os.Handler.handleCallback(Handler.java:725)
                at android.os.Handler.dispatchMessage(Handler.java:92)
                at android.os.Looper.loop(Looper.java:153)
                at android.app.ActivityThread.main(ActivityThread.java:5330)
                at java.lang.reflect.Method.invokeNative(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:511)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:608)
                at dalvik.system.NativeStart.main(Native Method)

推荐答案

更改:

mAdapter = new TabsPagerAdapter(getActivity().getSupportFragmentManager())

到:

 mAdapter = new TabsPagerAdapter(getChildFragmentManager())

当您在另一个片段中使用片段时，您必须使用 ChildFragmentManager 而不是 FragmentManager.

when you are using fragment inside another fragment you must use ChildFragmentManager not FragmentManager.

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