android - Android应用程序中抽屉导航上的空指针异常

Question

我正在学习本教程: http://codetheory.in/android-navigation-drawer/

当应用程序启动时，我得到: java.lang.RuntimeException: 无法启动 Activity ComponentInfo{pakage/pakage.MainActivity}: java.lang.NullPointerException ... 引起原因:java.lang.NullPointerException 在 pakage.MainActivity.onPostCreate(MainActivity.java:138)

第138行标有注释//第138行

代码:

package net.udazz.udazz;

import android.app.Fragment;
import android.app.FragmentManager;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.RelativeLayout;
import android.widget.TextView;

import java.util.ArrayList;


public class MainActivity extends AppCompatActivity {

    ListView mDrawerList;
    RelativeLayout mDrawerPane;
    private ActionBarDrawerToggle mDrawerToggle;
    private DrawerLayout mDrawerLayout;

    ArrayList<NavItem> mNavItems = new ArrayList<NavItem>();

    private WebView mWebView;

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

        mNavItems.add(new NavItem("Home", "Meetup destination", R.mipmap.ic_action_home));
        mNavItems.add(new NavItem("Preferences", "Change your preferences", R.mipmap.ic_action_settings));
        mNavItems.add(new NavItem("About", "Get to know about us", R.mipmap.ic_action_about));

        // DrawerLayout
        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout);

        // Populate the Navigtion Drawer with options
        mDrawerPane = (RelativeLayout) findViewById(R.id.drawerPane);
        mDrawerList = (ListView) findViewById(R.id.navList);
        DrawerListAdapter adapter = new DrawerListAdapter(this, mNavItems);
        mDrawerList.setAdapter(adapter);

        // Drawer Item click listeners
        mDrawerList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                selectItemFromDrawer(position);
            }
        });
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        mWebView = (WebView) findViewById(R.id.activity_main_webview);

        // Enable Javascript
        WebSettings webSettings = mWebView.getSettings();
        webSettings.setJavaScriptEnabled(true);
        mWebView.setOverScrollMode(WebView.OVER_SCROLL_NEVER);
        mWebView.getSettings().setLoadWithOverviewMode(true);
        mWebView.getSettings().setUseWideViewPort(true);

        mWebView.loadUrl("http://page/");

        // Stop local links and redirects from opening in browser instead of WebView
        mWebView.setWebViewClient(new MyAppWebViewClient());
    }

    @Override
    public void onBackPressed() {
        if(mWebView.canGoBack()) {
            mWebView.goBack();
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Pass the event to ActionBarDrawerToggle
        // If it returns true, then it has handled
        // the nav drawer indicator touch event
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }

        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
    /*
    * Called when a particular item from the navigation drawer
    * is selected.
    * */
    private void selectItemFromDrawer(int position) {
        Fragment fragment = new PreferencesFragment();

        FragmentManager fragmentManager = getFragmentManager();
        fragmentManager.beginTransaction()
                .replace(R.id.mainContent, fragment)
                .commit();

        mDrawerList.setItemChecked(position, true);
        setTitle(mNavItems.get(position).mTitle);

        // Close the drawer
        mDrawerLayout.closeDrawer(mDrawerPane);
    }
    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        mDrawerToggle.syncState(); // line 138
    }
}
class NavItem {
    String mTitle;
    String mSubtitle;
    int mIcon;

    public NavItem(String title, String subtitle, int icon) {
        mTitle = title;
        mSubtitle = subtitle;
        mIcon = icon;
    }
}
class DrawerListAdapter extends BaseAdapter {

    Context mContext;
    ArrayList<NavItem> mNavItems;

    public DrawerListAdapter(Context context, ArrayList<NavItem> navItems) {
        mContext = context;
        mNavItems = navItems;
    }

    @Override
    public int getCount() {
        return mNavItems.size();
    }

    @Override
    public Object getItem(int position) {
        return mNavItems.get(position);
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View view;

        if (convertView == null) {
            LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = inflater.inflate(R.layout.drawer_item, null);
        }
        else {
            view = convertView;
        }

        TextView titleView = (TextView) view.findViewById(R.id.title);
        TextView subtitleView = (TextView) view.findViewById(R.id.subTitle);
        ImageView iconView = (ImageView) view.findViewById(R.id.icon);

        titleView.setText(mNavItems.get(position).mTitle);
        subtitleView.setText(mNavItems.get(position).mSubtitle);
        iconView.setImageResource(mNavItems.get(position).mIcon);

        return view;
    }
}

Answer 1

您忘记在onCreate() 方法中初始化mDrawerToggle:

mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
                R.string.drawer_open,
                R.string.drawer_close) {

                public void onDrawerClosed(View view) {
                    super.onDrawerClosed(view);
                    invalidateOptionsMenu();
                }

                public void onDrawerOpened(View drawerView) {
                    super.onDrawerOpened(drawerView);
                    invalidateOptionsMenu();
                }
};

参数:

> this                      The Activity hosting the drawer
> mDrawerLayout             The DrawerLayout to link to the given Activity's ActionBar
> R.string.drawer_open      A String resource to describe the "open drawer" action for accessibility
> R.string.drawer_close     A String resource to describe the "close drawer" action for accessibility