问调用NullPointerException ()获取片段的视图时
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Stack Overflow用户

提问于 2013-09-04 13:18:21

回答 2查看 3.8K关注 0票数 5

我正在尝试设置一个FragmentStatePagerAdapter，以便在我的一个类似于这个例子的片段上创建一个可滚动的选项卡接口。在我的活动的onCreate中

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

     setVersionString();
     setupActionBar();
     setupNavigationDrawer();
     setupFragments();
}

我将内容视图设置为以下布局(R.layout.base)：

<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".BaseActivity" >

    <!-- The main content view -->
    <FrameLayout
        android:id="@+id/main_frame"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <!-- The navigation drawer -->
    <ListView
        android:id="@+id/nav_drawer"
        android:layout_width="240dp"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:choiceMode="singleChoice"
        android:divider="@android:color/transparent"
        android:dividerHeight="0dp"
        android:background="#FFF" />

</android.support.v4.widget.DrawerLayout>

然后调用以下函数：

private void setupFragments() {
    // Check that the activity is using the correct layout
    if (findViewById(R.id.main_frame) != null) {
        // Initialize the first fragment
        MainFragment firstFrag = new MainFragment();

        // Pass any extras from an intent to the Fragment
        firstFrag.setArguments(getIntent().getExtras());

        // Add the first Fragment to the screen
        getSupportFragmentManager().beginTransaction()
            .add(R.id.main_frame, firstFrag).commit();

        tabsAdapter = firstFrag.getAdapter();
        Log.d("Base.LOG", "tabsAdapter = " + tabsAdapter);
        tabsAdapter.addTab(DummyFragment.class, null);
    }
}

在尝试将FragmentStatePagerAdapter设置为片段布局(R.layout.main)中视图的适配器时：

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

    <android.support.v4.view.PagerTitleStrip
        android:id="@+id/pager_title_strip"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="top"
        android:background="#33b5e5"
        android:textColor="#fff"
        android:paddingTop="4dp"
        android:paddingBottom="4dp" />
</android.support.v4.view.ViewPager>

我的片段代码看起来是这样的：

private View mView;
private TabbedPagerAdapter mTabPageAdapter;
private ViewPager mTabPager;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    Log.d("Main.LOG", "onCreate called");
    mView = inflater.inflate(R.layout.main, container, false);
    Log.d("Main.LOG", "Layout inflated");
    setupTabs();
    Log.d("Main.LOG", "Tabs setup");

    return mView;
}

@Override
public void onResume() {
    super.onResume();
    Log.d("Main.LOG", "onResume called");
}

/** Set up the ViewPager and Adapter to handle the primary tabs */
private void setupTabs() {
    // Initialize the adapter
    mTabPageAdapter = new TabbedPagerAdapter(getActivity(),
            getActivity().getSupportFragmentManager());

    // Initialize the view pager
    mTabPager = (ViewPager) mView.findViewById(R.id.main_page);
    Log.d("Main.LOG", "mTabPager = " + mTabPager);
    Log.d("Main.LOG", "mTabPageAdapter = " + mTabPageAdapter);
    mTabPager.setAdapter(mTabPageAdapter);
}

/* Accessor for TabbedPagerAdapter */
public TabbedPagerAdapter getAdapter() {
    return mTabPageAdapter;
}

当我尝试添加带有NullPointerException的选项卡时，应用程序强制关闭。日志中的调试消息显示：

D/Base.LOG(27173)：tabsAdapter = null

显示在任何片段的调试消息之前。在尝试访问它的任何视图或子类之前，如何确保布局是膨胀的？

android-fragments

findviewbyid

android

android-layout

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回答 2

Stack Overflow用户

回答已采纳

发布于 2013-09-05 07:51:54

据我的评论

public class MainFragment extends Fragment
{
    private MainInterface mInterface = null;
    //make sure to set the above somehow

    public interface MainInterface{
        public void onFragmentReady(.....);
    }

    ... //Do regular stuff here

    public void onResume()
    {
        //Do what you need to do in onResume
        if(mInterface != null)
        {
            mInteface.onFragmentReady();
        }
    }
}

这将使您能够知道片段何时准备好，然后可以执行getTabAdapter调用。

票数 2

Stack Overflow用户

发布于 2013-09-04 13:44:20

在调用setContentView(...)之前，在活动的onCreate(...)方法中调用setupFragments(...)吗？

(如果是：)：您要分配给活动的布局是否包含ViewPager？

My建议:由于您显然是在片段中使用ViewPager，所以在片段onCreateView(...)方法中进行ViewPager的初始化。

示例：

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

    // ASSUMING your layout.main contains the ViewPager
    View v = inflater.inflate(R.layout.main, container, false);

    ViewPager pager = (ViewPager) v.findViewById(R.id.pager);
    // and so on ...

    return v;
}