無法在Action欄中設置Android導航欄抽屜

Question

我正在嘗試設置具有操作欄的Android導航欄抽屜。我很難爲導航抽屜設置圖標並生成所有列表項目。下面是我的代碼，我目前產生一個空指針異常：

崩潰：

                    --------- beginning of crash 
04-20 18:02:03.690 13170-13170/com.sourcey.materialloginexample E/AndroidRuntime: FATAL EXCEPTION: main 
                        Process: com.sourcey.materialloginexample, PID: 13170 
                        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sourcey.materialloginexample/com.troychuinard.fanpolls.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325) 
                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
                         at android.app.ActivityThread.access$800(ActivityThread.java:151) 
                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
                         at android.os.Handler.dispatchMessage(Handler.java:102) 
                         at android.os.Looper.loop(Looper.java:135) 
                         at android.app.ActivityThread.main(ActivityThread.java:5254) 
                         at java.lang.reflect.Method.invoke(Native Method) 
                         at java.lang.reflect.Method.invoke(Method.java:372) 
                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
                        Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
                         at com.troychuinard.fanpolls.MainActivity.onCreate(MainActivity.java:71) 
                         at android.app.Activity.performCreate(Activity.java:5990) 
                         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) 
                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278) 
                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)  
                         at android.app.ActivityThread.access$800(ActivityThread.java:151)  
                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)  
                         at android.os.Handler.dispatchMessage(Handler.java:102)  
                         at android.os.Looper.loop(Looper.java:135)  
                         at android.app.ActivityThread.main(ActivityThread.java:5254)  
                         at java.lang.reflect.Method.invoke(Native Method)  
                         at java.lang.reflect.Method.invoke(Method.java:372)  
                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)  
                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 

MainActivity：

public class MainActivity extends AppCompatActivity implements HomePollsFragment.OnFragmentInteractionListener { 

    private Toolbar toolbar; 

    private Firebase mPollsRef; 
    private ViewPager mPager; 

    private ScreenSlidePagerAdapter mPagerAdapter; 
    private DateFormat mDateFormat; 
    private Date mDate; 
    private String mCurrentDateString; 
    private ValueEventListener v; 
    private String[] mPlanetTitles; 
    private DrawerLayout mDrawerLayout; 
    private ListView mDrawerList; 
    private ActionBarDrawerToggle mDrawerToggle; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     toolbar = (Toolbar) findViewById(R.id.action_tool_bar); 
     setSupportActionBar(toolbar); 
     Firebase.setAndroidContext(this); 


     mPollsRef = FirebaseUtil.FIREBASE.child("Polls"); 

     mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout); 
     mDrawerToggle = new ActionBarDrawerToggle(
       this,     /* host Activity */ 
       mDrawerLayout,   /* DrawerLayout object */ 
       toolbar, /* nav drawer icon to replace 'Up' caret */ 
       R.string.drawer_open, /* "open drawer" description */ 
       R.string.drawer_close /* "close drawer" description */) { 

     }; 
     getActionBar().setDisplayHomeAsUpEnabled(true); 
     getActionBar().setHomeButtonEnabled(true); 

     mDrawerList = (ListView) findViewById(R.id.left_drawer); 
     ArrayList<String> drawerTitleArray = new ArrayList<>(); 
     drawerTitleArray.add(0, "TEST"); 
     drawerTitleArray.add(1, "TEST 1"); 
     // Set the adapter for the list view 
     mDrawerList.setAdapter(new ArrayAdapter<String>(this, 
       R.layout.drawer_list_item, drawerTitleArray)); 


     // TODO: Add Fragment Code to check if savedInstanceState == null; add at Activity Level? 
     // Check that the activity is using the layout version with 
     // the fragment_container FrameLayout 


     //set up viewpager for current day 
     mPager = (ViewPager) findViewById(R.id.home_polls_viewpager_fragment_container); 
     mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager()); 
     mPager.setAdapter(mPagerAdapter); 


     //get current date to apply to Viewpager 
     mDateFormat = new SimpleDateFormat("MM-dd-yyyy"); 
     mDate = new Date(); 
     mCurrentDateString = mDateFormat.format(mDate); 

     // TODO: Checkn if AuthStateListenerNecessary 
     //Determine whether necessary to use an AuthStateListener here 
//  mUserRef.addAuthStateListener(new Firebase.AuthStateListener() { 
//   @Override 
//   public void onAuthStateChanged(AuthData authData) { 
//    if (authData == null) { 
//     Intent backToSignIn = new Intent(getApplication(), SignupActivity.class); 
//     startActivity(backToSignIn); 
//     finish(); 
//    } 
//   } 
//  }) 

    } 


    @Override 
    protected void onStart() { 
     super.onStart(); 

     v = new ValueEventListener() { 
      //testing methodology of adding children 
      @Override 
      public void onDataChange(DataSnapshot dataSnapshot) { 
       int pollsAvailable = (int) dataSnapshot.child(mCurrentDateString).getChildrenCount(); 
       mPagerAdapter.setPollsAvailable(pollsAvailable); 
       Log.i("TAG", "There are " + String.valueOf(pollsAvailable) + " children in today's poll count."); 
      } 

      @Override 
      public void onCancelled(FirebaseError firebaseError) { 

      } 
     }; 

     mPollsRef.addValueEventListener(v); 

    } 

    @Override 
    protected void onStop() { 
     super.onStop(); 
     mPollsRef.removeEventListener(v); 

    } 

    @Override 
    public void onFragmentInteraction(Uri uri) { 

    } 


    class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter { 
     private int pollsAvailable = 0; 

     //fragment adapter constructor 
     public ScreenSlidePagerAdapter(FragmentManager fm) { 
      super(fm); 
     } 

     @Override 
     public Fragment getItem(int position) { 
      return HomePollsFragment.newInstance(position); 
     } 

     @Override 
     public int getCount() { 
      return pollsAvailable; 
     } 

     public void setPollsAvailable(int pollsAvailable) { 
      this.pollsAvailable = pollsAvailable; 
      this.notifyDataSetChanged(); 
     } 
    } 





    @Override 
    public void onBackPressed() { 
     super.onBackPressed(); 
     if (mPager.getCurrentItem() == 0) { 
      // If the user is currently looking at the first step, allow the system to handle the 
      // Back button. This calls finish() on this activity and pops the back stack. 
      super.onBackPressed(); 
     } else { 
      // Otherwise, select the previous step. 
      mPager.setCurrentItem(mPager.getCurrentItem() - 1); 
     } 
    } 
} 

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" 
    android:orientation="vertical"> 

    <LinearLayout 
     android:id="@+id/content_frame" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent" 
     android:orientation="vertical"> 

     <android.support.v7.widget.Toolbar 
      android:id="@+id/action_tool_bar" 
      android:layout_width="match_parent" 
      android:layout_height="wrap_content" 
      android:background="@color/actionRed"> 

     </android.support.v7.widget.Toolbar> 

     <android.support.v4.view.ViewPager 
      android:id="@+id/home_polls_viewpager_fragment_container" 
      android:layout_width="match_parent" 
      android:layout_height="match_parent"> 
     </android.support.v4.view.ViewPager> 

    </LinearLayout> 

    <!-- The navigation drawer --> 
    <ListView 
     android:id="@+id/left_drawer" 
     android:layout_width="240dp" 
     android:layout_height="match_parent" 
     android:layout_gravity="start" 
     android:background="#111" 
     android:choiceMode="singleChoice" 
     android:divider="@android:color/transparent" 
     android:dividerHeight="0dp" /> 
</android.support.v4.widget.DrawerLayout> 

Answer 1

我不喜歡這樣：

android.support.v7.app.ActionBar actionBar = getSupportActionBar(); 
     if(actionBar != null){ 
      actionBar.setDisplayHomeAsUpEnabled(true); 
      actionBar.setHomeButtonEnabled(true); 
     } 

這應該解決java.lang.NullPointerException。

「和生成所有列表中的項目以及」

我不能看到什麼問題在這裏。你能解釋一下嗎？

的圖標 - 三條水平線在封閉的模式，在開模式下的後退箭頭，一個不錯的斯穆特動畫：

drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout); 
     drawerToggle = new ActionBarDrawerToggle(
       this,     /* host Activity */ 
       drawerLayout,   /* DrawerLayout object */ 
       toolbar, /* nav drawer icon to replace 'Up' caret */ 
       R.string.nav, /* "open drawer" description */ 
       R.string.nav /* "close drawer" description */ 
     ) { 

      /** Called when a drawer has settled in a completely closed state. */ 
      public void onDrawerClosed(View view) { 
       super.onDrawerClosed(view); 
       if(extra == null){ 
        toolbar.setTitle(title); 
       }else{ 
        toolbar.setTitle(title + " - " + extra); 
       } 
       isOpen = false; 
      } 

      /** Called when a drawer has settled in a completely open state. */ 
      public void onDrawerOpened(View drawerView) { 
       super.onDrawerOpened(drawerView); 
       actionBar.setTitle(R.string.nav); 
       isNavReady = true; 
       isOpen = true; 
      } 
     }; 
     drawerLayout.addDrawerListener(drawerToggle); 

@Override 
    public boolean onOptionsItemSelected(MenuItem item){ 
     return drawerToggle.onOptionsItemSelected(item) || super.onOptionsItemSelected(item); 
    } 


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

    @Override 
    public void onConfigurationChanged(Configuration newConfig) { 
     super.onConfigurationChanged(newConfig); 
     drawerToggle.onConfigurationChanged(newConfig); 
    }