Android：导航抽屉 NullPointerException

Question

我正在使用开发者网站 (http://developer.android.com/training/implementing-navigation/nav-drawer.html) 中的代码来安装导航抽屉，但是，我在这个方法中得到了 NullPointerException：

/* Called whenever we call invalidateOptionsMenu() */
@Override
public boolean onPrepareOptionsMenu(Menu menu) {
    // If the nav drawer is open, hide action items related to the content view
    boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
    menu.findItem(R.id.action_search).setVisible(!drawerOpen);
    menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
    return super.onPrepareOptionsMenu(menu);
}

这是错误

04-14 20:51:39.720: D/dalvikvm(826): GC_FOR_ALLOC freed 93K, 6% free 3119K/3284K, paused 95ms, total 96ms
04-14 20:51:39.970: I/Choreographer(826): Skipped 69 frames!  The application may be doing too much work on its main thread.
04-14 20:51:40.360: I/Choreographer(826): Skipped 256 frames!  The application may be doing too much work on its main thread.
04-14 20:51:40.440: D/gralloc_goldfish(826): Emulator without GPU emulation detected.
04-14 20:51:40.730: D/AndroidRuntime(826): Shutting down VM
04-14 20:51:40.730: W/dalvikvm(826): threadid=1: thread exiting with uncaught exception (group=0xb2ac7ba8)
04-14 20:51:40.740: E/AndroidRuntime(826): FATAL EXCEPTION: main
04-14 20:51:40.740: E/AndroidRuntime(826): Process: com.example.myfirstapp, PID: 826
04-14 20:51:40.740: E/AndroidRuntime(826): java.lang.NullPointerException
04-14 20:51:40.740: E/AndroidRuntime(826):  at com.example.myfirstapp.MainActivity.onPrepareOptionsMenu(MainActivity.java:139)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.app.Activity.onPreparePanel(Activity.java:2556)
04-14 20:51:40.740: E/AndroidRuntime(826):  at com.android.internal.policy.impl.PhoneWindow.preparePanel(PhoneWindow.java:464)
04-14 20:51:40.740: E/AndroidRuntime(826):  at com.android.internal.policy.impl.PhoneWindow.doInvalidatePanelMenu(PhoneWindow.java:800)
04-14 20:51:40.740: E/AndroidRuntime(826):  at com.android.internal.policy.impl.PhoneWindow$1.run(PhoneWindow.java:221)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.view.Choreographer.doCallbacks(Choreographer.java:574)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.view.Choreographer.doFrame(Choreographer.java:543)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.os.Handler.handleCallback(Handler.java:733)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.os.Handler.dispatchMessage(Handler.java:95)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.os.Looper.loop(Looper.java:136)
04-14 20:51:40.740: E/AndroidRuntime(826):  at android.app.ActivityThread.main(ActivityThread.java:5017)
04-14 20:51:40.740: E/AndroidRuntime(826):  at java.lang.reflect.Method.invokeNative(Native Method)
04-14 20:51:40.740: E/AndroidRuntime(826):  at java.lang.reflect.Method.invoke(Method.java:515)
04-14 20:51:40.740: E/AndroidRuntime(826):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-14 20:51:40.740: E/AndroidRuntime(826):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-14 20:51:40.740: E/AndroidRuntime(826):  at dalvik.system.NativeStart.main(Native Method)

这是主要活动的其余部分

package com.example.myfirstapp;

import java.util.Locale;

import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.SearchManager;
import android.content.Intent;
import android.content.res.Configuration;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.Toast;

public class MainActivity extends Activity {
    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    private DrawerLayout mDrawerLayout;
    private ListView mDrawerList;
    private ActionBarDrawerToggle mDrawerToggle;

    private CharSequence mDrawerTitle;
    private CharSequence mTitle;
    private String[] mPlanetTitles;

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

        mTitle = mDrawerTitle = getTitle();
        mPlanetTitles = getResources().getStringArray(R.array.navigation_drawer_options);
        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        mDrawerList = (ListView) findViewById(R.id.left_drawer);

        // set a custom shadow that overlays the main content when the drawer opens
        mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
        // set up the drawer's list view with items and click listener
        mDrawerList.setAdapter(new ArrayAdapter<String>(this,
                R.layout.drawer_list_item, mPlanetTitles));
        mDrawerList.setOnItemClickListener(new DrawerItemClickListener());

        // enable ActionBar app icon to behave as action to toggle nav drawer
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);

        // ActionBarDrawerToggle ties together the the proper interactions
        // between the sliding drawer and the action bar app icon
        mDrawerToggle = new ActionBarDrawerToggle(
                this,                  /* host Activity */
                mDrawerLayout,         /* DrawerLayout object */
                R.drawable.ic_drawer,  /* nav drawer image to replace 'Up' caret */
                R.string.drawer_open,  /* "open drawer" description for accessibility */
                R.string.drawer_close  /* "close drawer" description for accessibility */
                ) {
            public void onDrawerClosed(View view) {
                getActionBar().setTitle(mTitle);
                invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
            }

            public void onDrawerOpened(View drawerView) {
                getActionBar().setTitle(mDrawerTitle);
                invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
            }
        };
        mDrawerLayout.setDrawerListener(mDrawerToggle);

        if (savedInstanceState == null) {
            selectItem(0);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.main, menu);
        return super.onCreateOptionsMenu(menu);
    }

    /* Called whenever we call invalidateOptionsMenu() */
    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        // If the nav drawer is open, hide action items related to the content view
        boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
        menu.findItem(R.id.action_search).setVisible(!drawerOpen);
        menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
        return super.onPrepareOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
         // The action bar home/up action should open or close the drawer.
         // ActionBarDrawerToggle will take care of this.
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        // Handle action buttons
//        switch(item.getItemId()) {
//        case R.id.action_websearch:
//            // create intent to perform web search for this planet
//            Intent intent = new Intent(Intent.ACTION_WEB_SEARCH);
//            intent.putExtra(SearchManager.QUERY, getActionBar().getTitle());
//            // catch event that there's no activity to handle intent
//            if (intent.resolveActivity(getPackageManager()) != null) {
//                startActivity(intent);
//            } else {
//                Toast.makeText(this, R.string.app_not_available, Toast.LENGTH_LONG).show();
//            }
//            return true;
//        default:
            return super.onOptionsItemSelected(item);
//        }
    }

    /* The click listner for ListView in the navigation drawer */
    private class DrawerItemClickListener implements ListView.OnItemClickListener {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            selectItem(position);
        }
    }

    private void selectItem(int position) {
    }

    @Override
    public void setTitle(CharSequence title) {
        mTitle = title;
        getActionBar().setTitle(mTitle);
    }

    /**
     * When using the ActionBarDrawerToggle, you must call it during
     * onPostCreate() and onConfigurationChanged()...
     */

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        // Pass any configuration change to the drawer toggls
        mDrawerToggle.onConfigurationChanged(newConfig);
    }
}

菜单 xml

<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    tools:context="com.example.myfirstapp.MainActivity" >

    <item
        android:id="@+id/action_settings"
        android:orderInCategory="100"
        android:title="@string/action_settings"
        app:showAsAction="never"/>

</menu>

Answer 1

您的 menu.xml 只有一次 id= action_settings。您应该为搜索添加另一个菜单项。

示例： 您可以使用搜索视图

<item
 android:id="@+id/action_search"
 android:icon="@drawable/ic_action_search"
 android:title="@string/action_search"
 app:actionViewClass="android.support.v7.widget.SearchView"
 app:showAsAction="ifRoom|collapseActionView" />