FindViewByID 在片段导航抽屉上返回 null

Question

当我尝试为我的应用按钮设置 onclicklistener 时，出现以下错误。我使用这样的导航抽屉：http://www.androidhive.info/2013/11/android-sliding-menu-using-navigation-drawer/

按钮位于我的应用程序的 Settingsfragment 中。


10-10 16:21:05.506 5641-5641/? E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: eu.twenty1media.smokestop, PID: 5641 ava.lang.RuntimeException: Unable to start activity ComponentInfo{eu.twenty1media.smokestop/eu.twenty1media.smokestop.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

这是在 oncreate 完成之前 Mainactivity 的代码。

public class MainActivity extends Activity {
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;
private ActionBarDrawerToggle mDrawerToggle;

// nav drawer title
private CharSequence mDrawerTitle;

// used to store app title
private CharSequence mTitle;

// slide menu items
private String[] navMenuTitles;
private TypedArray navMenuIcons;

private ArrayList<NavDrawerItem> navDrawerItems;
private NavDrawerListAdapter adapter;

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


    /**
     *  Set onClickListener for Settings Apply Button
     */
    findViewById(R.id.applysettings).setOnClickListener( new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //Inform the user the button has been clicked
            Toast.makeText(MainActivity.this, "Apply clicked.", Toast.LENGTH_SHORT).show();
        }
    });


    //Initalize or create Database
    DatabaseHandler db = new DatabaseHandler(this);
    db.getData();
    //db.addData();
    //db.updateData();



    mTitle = mDrawerTitle = getTitle();

    // load slide menu items
    navMenuTitles = getResources().getStringArray(R.array.nav_drawer_items);

    // nav drawer icons from resources
    navMenuIcons = getResources()
            .obtainTypedArray(R.array.nav_drawer_icons);

    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.list_slidermenu);

    navDrawerItems = new ArrayList<NavDrawerItem>();

    // adding nav drawer items to array
    // Home
    navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));
    // Motivation
    navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));
    // Health
    navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));
    // Statistic
    navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1)));
    // Settings
    navDrawerItems.add(new NavDrawerItem(navMenuTitles[4], navMenuIcons.getResourceId(4, -1)));
     //Welcome
    //navDrawerItems.add(new NavDrawerItem(navMenuTitles[5], navMenuIcons.getResourceId(5, -1)));


    // Recycle the typed array
    navMenuIcons.recycle();

    mDrawerList.setOnItemClickListener(new SlideMenuClickListener());

    // setting the nav drawer list adapter
    adapter = new NavDrawerListAdapter(getApplicationContext(),
            navDrawerItems);
    mDrawerList.setAdapter(adapter);

    // enabling action bar app icon and behaving it as toggle button
    getActionBar().setDisplayHomeAsUpEnabled(true);
    getActionBar().setHomeButtonEnabled(true);

    mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
            R.drawable.ic_drawer, //nav menu toggle icon
            R.string.app_name, // nav drawer open - description for accessibility
            R.string.app_name // nav drawer close - description for accessibility
    ) {
        public void onDrawerClosed(View view) {
            getActionBar().setTitle(mTitle);
            // calling onPrepareOptionsMenu() to show action bar icons
            invalidateOptionsMenu();
        }

        public void onDrawerOpened(View drawerView) {
            getActionBar().setTitle(mDrawerTitle);
            // calling onPrepareOptionsMenu() to hide action bar icons
            invalidateOptionsMenu();
        }
    };
    mDrawerLayout.setDrawerListener(mDrawerToggle);

    if (savedInstanceState == null) {
        // on first time display view for first nav item
        displayView(0);
    }

} </code>

MainActivity.xml

<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- Framelayout to display Fragments -->
    <FrameLayout
        android:id="@+id/frame_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <!-- Listview to display slider menu -->
    <ListView
        android:id="@+id/list_slidermenu"
        android:layout_width="240dp"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:choiceMode="singleChoice"
        android:divider="@color/list_divider"
        android:dividerHeight="1dp"
        android:listSelector="@drawable/list_selector"
        android:background="@color/list_background"/>
</android.support.v4.widget.DrawerLayout>

还有 SettingsFragment.xml

<?xml version="1.0" encoding="utf-8"?>

    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="20dp"
    android:layout_marginBottom="8dp">
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Speichern"
        android:id="@+id/applysettings"
        />

Answer 1

您的applysettings 视图不在activity_main 布局中，因此无法找到。

将findViewById() 移动到您实际扩展包含applysettings 按钮的布局的位置。