Question

我有一个带ViewPager的简单片段。

我正在使用最新的支持库，v4 rev18！

如果我第一次显示它，一切正常，如果我回去再显示一下，应用程序会崩溃并出现以下异常：

java.lang.NullPointerException
at android.support.v4.app.FragmentManagerImpl.getFragment(FragmentManager.java:569)
at android.support.v4.app.FragmentStatePagerAdapter.restoreState(FragmentStatePagerAdapter.java:211)
at android.support.v4.view.ViewPager.onRestoreInstanceState(ViewPager.java:1281)
at android.view.View.dispatchRestoreInstanceState(View.java:12043)
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:2688)
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:2694)
at android.view.View.restoreHierarchyState(View.java:12021)
at android.support.v4.app.Fragment.restoreViewState(Fragment.java:425)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:949)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1460)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:440)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4800)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:798)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:565)
at dalvik.system.NativeStart.main(Native Method)

我的简单课程看起来像下面一样，实际上应该很好......

public class RoutineDayFragment extends BaseFragment
{
    private RoutinesActivity mParent = null;

    @InjectView(R.id.pager)
    MyViewPager pager = null;
    @InjectView(R.id.indicator)
    PagerSlidingTabStrip indicator = null;

    private FragmentStatePagerAdapter mAdapter = null;

    @Override
    public void onAttach(Activity activity)
    {
        super.onAttach(activity);
        mParent = ((RoutinesActivity) getBaseActivity());
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        View v = inflater.inflate(R.layout.view_pager, container, false);
        Views.inject(this, v);

        mAdapter = new FragmentStatePagerAdapter(getChildFragmentManager())
        {
            @Override
            public int getCount()
            {
                if (mParent.getSharedData().selectedRoutine == -1)
                    return 0;
                return mParent.getSharedData().routines.get(mParent.getSharedData().selectedRoutine).getRWorkoutDay().size();
            }

            @Override
            public Fragment getItem(int pos)
            {
                return DayFragment.newInstance(pos);
            }

            @Override
            public CharSequence getPageTitle(int pos)
            {
                return Common.getString(R.string.day) + (pos + 1);
            }
        };

        pager.setAdapter(mAdapter);
        indicator.setViewPager(pager);
        return v;
    }
}

Answer 1

使用 AsyncTask 设置ViewPagerAdapter：

private class SetAdapterTask extends AsyncTask<Void, Void, Void> {
        protected Void doInBackground(Void... params) {
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {

            if(mAdapter != null) mViewPager.setAdapter(mAdapter);
        }
    }

这样称呼：

mAdapter = new PageAdapter(getChildFragmentManager());
new SetAdapterTask().execute();

并且重置片段的onResume(）方法中的适配器。

更新 - 如何将ViewPager嵌套在片段中？

好吧，好吧。我修改了 Effective Navigation 的Google示例，以满足您的需求。

我创造了什么？

我创建了一个简单的应用程序，其中包含一个带有的MainActivity ViewPager和3个标签。
这些标签中的每一个都由包含a的片段表示 ViewPager也是如此。
Fragment中的ViewPager包含10页。
所以我们有3个“主”标签/片段，每个片段/片段包含10个片段。
出于演示目的，我制作了顶级 ViewPager不可刷卡，因此您必须使用标签在主要片段之间切换（我创建了一个CustomViewPager并进行了一些更改以删除ViewPagers滑动功能）
主片段内的ViewPager可以滑动，因此您可以滑动以在子片段之间切换，并按下标签在主片段之间切换
如果单击子片段，则会启动新的活动。
如果您在新活动关闭时返回旧活动，片段和ViewPager的状态被保留。
切换主片段时，也会保留其状态

这是 Main Activity.java

它包含两个适配器，一个用于主片段，一个用于子片段。此外，它们是两个Fragment类，一个是包含ViewPager的Fragment（主片段），另一个是子片段（嵌套的ViewPager内部）

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {

    AppSectionsPagerAdapter mAppSectionsPagerAdapter;
    NonSwipeableViewPager mViewPager;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mAppSectionsPagerAdapter = new AppSectionsPagerAdapter(getSupportFragmentManager());

        // Set up the action bar.
        final ActionBar actionBar = getActionBar();
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Set up the ViewPager, attaching the adapter and setting up a listener for when the
        // user swipes between sections.
        mViewPager = (NonSwipeableViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mAppSectionsPagerAdapter);
        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 < mAppSectionsPagerAdapter.getCount(); i++) {
            actionBar.addTab(
                    actionBar.newTab()
                            .setText(mAppSectionsPagerAdapter.getPageTitle(i))
                            .setTabListener(this));
        }
    }

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

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        // When the given tab is selected, switch to the corresponding page in the ViewPager.
        mViewPager.setCurrentItem(tab.getPosition());
    }

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

    public static class AppSectionsPagerAdapter extends FragmentPagerAdapter {

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

        @Override
        public Fragment getItem(int i) {
            switch (i) {

                default:
                    // The other sections of the app are dummy placeholders.
                    Fragment fragment = new ViewPagerContainerFragment();
                    return fragment;
            }
        }

        @Override
        public int getCount() {
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return "Tab " + (position +1);
        }
    }

    public static class ViewPagerAdapter extends FragmentPagerAdapter {

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

        @Override
        public Fragment getItem(int i) {
            switch (i) {

                default:
                    // The other sections of the app are dummy placeholders.
                    Fragment fragment = new ViewPagerFragment();
                    Bundle b = new Bundle();
                    b.putString("key", "I am fragment nr " + i);
                    fragment.setArguments(b);
                    return fragment;
            }
        }

        @Override
        public int getCount() {
            return 10;
        }
    }    

    /**
     * THIS FRAGMENT CONTAINS A VIEWPAGER
     */
    public static class ViewPagerContainerFragment extends Fragment {

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

            ViewPager pager = (ViewPager) rootView.findViewById(R.id.nestedViewPager);

            pager.setAdapter(new ViewPagerAdapter(getChildFragmentManager()));

            return rootView;
        }
    }

    /**
     * THIS FRAGMENT IS INSIDE THE VIEWPAGER
     */
    public static class ViewPagerFragment extends Fragment {

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

            ((TextView) rootView.findViewById(R.id.tv1)).setText(getArguments().getString("key"));
            ((TextView) rootView.findViewById(R.id.tv1)).setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    Intent intent = new Intent(getActivity(), CollectionDemoActivity.class);
                    startActivity(intent);
                }
            });

            return rootView;
        }
    }
}

<强> activity_main.xml中

<com.example.android.effectivenavigation.NonSwipeableViewPager 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<强> main_fragment.xml

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/nestedViewPager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:textSize="24sp" />

<强> sub_fragment.xml

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/tv1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:textSize="50sp" />

最终结果如下：

我们有3个顶级片段，每个片段包含一个包含10个片段的ViewPager。

enter image description here

当Fragment被重新显示时，Fragment中的ViewPager会丢失它的内容

1 个答案:

更新 - 如何将ViewPager嵌套在片段中？