findViewById在列表视图上返回null

Question

因为某些未知原因（我也搜索过，检查我是否输入了错误的ID或calling it before setting the view），调用 findViewById 返回null当我尝试获取导航抽屉的列表视图时。我目前也在this tutorial上关注＆＃34; Inicialize the drawer list＆＃34;。

但是，一旦我调用 findViewById ，我就会得到一个 null 引用，当我调用<时，会引起 null引用异常 em> setAdapter 方法。

所以这里是有问题的代码：

public class DrawerActivity extends AppCompatActivity
        implements NavigationDrawerFragment.NavigationDrawerCallbacks, AndroidFragmentApplication.Callbacks {

    [Needless stuff here]

    private DrawerLayout mDrawerLayout;
    private ActionBarDrawerToggle mDrawerToggle;
    private CharSequence mDrawerTitle;
    private ListView mDrawerList;
    private String[] mListItems;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_drawer);


        mListItems = getResources().getStringArray(R.array.list_values);
        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        mDrawerList = (ListView) findViewById(R.id.drawer_list); //this returns null
        //null pointer exception happens next
        mDrawerList.setAdapter(
                new ArrayAdapter<>(
                        this, R.layout.drawer_list_item, mListItems
                )
        );
        [More following, but it isn't needed]

这里是drawer_activity.xml：

<!-- A DrawerLayout is intended to be used as the top-level content view using match_parent for both width and height to consume the full space available. -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:orientation="vertical">

    <!-- placed here as explained @
           http://developer.android.com/training/appbar/setting-up.html-->
    <android.support.v7.widget.Toolbar
        android:id="@+id/my_toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:elevation="4dp"
        android:theme="@style/AppTheme.AppBarOverlay"
        app:popupTheme="@style/AppTheme.PopupOverlay"/>

    <android.support.v4.widget.DrawerLayout
        android:id="@+id/drawer_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context="pt.bluecover.wearable3d.DrawerActivity">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">


            <!-- As the main content view, the view below consumes the entire
         space available using match_parent in both dimensions. -->
            <FrameLayout
                android:id="@+id/container"
                android:layout_width="match_parent"
                android:layout_height="match_parent" />


        </LinearLayout>

        <!--
        This here is the spin bar indicating that the program is loading something
        -->
        <ProgressBar
            android:id="@+id/progressBar1"
            style="?android:attr/progressBarStyle"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="100dip"
            android:layout_marginRight="100dip"
            android:maxHeight="35dip"
            android:minHeight="35dip"
            android:minWidth="7dip"
            android:maxWidth="7dip"
            android:background="@android:color/transparent"
            android:layout_centerHorizontal="true"
            android:indeterminate="true" />

        <!-- The drawer is given a fixed width in dp and extends the full height of
             the container. -->
        <fragment
            android:id="@+id/navigation_drawer"
            android:name="pt.bluecover.wearable3d.NavigationDrawerFragment"
            android:layout_width="@dimen/navigation_drawer_width"
            android:layout_height="match_parent"
            android:layout_gravity="start"
            tools:layout="@layout/drawer_drawer" />



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

至drawer_drawer.xml：

<ListView xmlns:android="http://schemas.android.com/apk/res/android"
    android:theme="@style/AppTheme.NavigationDrawer"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_list"
    android:layout_gravity="start"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:dividerHeight="0dp"
    tools:context="pt.bluecover.wearable3d.NavigationDrawerFragment" />

那么，是什么原因可能导致所述方法返回 null ？

Answer 1

以下行和所有listview实现的代码必须在NavigationDrawerFragment中，而不是在Activity

中

mDrawerList = (ListView) findViewById(R.id.drawer_list);