Android FragmentActivity zwraca wartość null w getActionBar()

Question

Zaimportowałem przykładowy projekt Android o nazwie HorizontalPaging w Android Studio i działa poprawnie po uruchomieniu. Ale kiedy skopiowałem kod do mojego kodu, otrzymuję wyjątek wskaźnika NULL w getActionBar().

Czytałem o tych problemach przez cały dzień, ale nie mogę go uruchomić. Próbowałem skopiować cały kod z MainActivity próbki do mojego projektu, ale bez ulepszeń, więc domyślam się, że problem jest w innym pliku, jak manifest, style itp.

MainActivity.java skopiowany z przykładowego projektu

import android.app.ActionBar; 
import android.app.FragmentTransaction; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.support.v4.app.FragmentActivity; 
import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentPagerAdapter; 
import android.support.v4.view.ViewPager; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.TextView; 

import java.util.Locale; 


public class View_Tabs extends FragmentActivity implements ActionBar.TabListener { 


SectionsPagerAdapter mSectionsPagerAdapter; 


ViewPager mViewPager; 


@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    // Load the UI from res/layout/activity_main.xml 
    setContentView(R.layout.view_tabs); 


    final ActionBar actionBar = getActionBar(); 
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 


    // Create the adapter that will return a fragment for each of the three primary sections 
    // of the app. 
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager()); 

    // Set up the ViewPager with the sections adapter. 
    mViewPager = (ViewPager) findViewById(R.id.pager); 
    mViewPager.setAdapter(mSectionsPagerAdapter); 

    // When swiping between different sections, select the corresponding tab. We can also use 
    // ActionBar.Tab#select() to do this if we have a reference to the Tab. 

    mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() { 
     @Override 
     public void onPageSelected(int position) { 
      actionBar.setSelectedNavigationItem(position); 
     } 
    }); 



    // For each of the sections in the app, add a tab to the action bar. 
    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) { 
     // Create a tab with text corresponding to the page title defined by the adapter. Also 
     // specify this Activity object, which implements the TabListener interface, as the 
     // callback (listener) for when this tab is selected. 
     actionBar.addTab(
       actionBar.newTab() 
         .setText(mSectionsPagerAdapter.getPageTitle(i)) 
         .setTabListener(this)); 
    } 

} 


@Override 
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { 
    // When the given tab is selected, tell the ViewPager to switch to the corresponding page. 
    mViewPager.setCurrentItem(tab.getPosition()); 
} 



@Override 
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { 
} 


@Override 
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { 
} 


public class SectionsPagerAdapter extends FragmentPagerAdapter { 


    public SectionsPagerAdapter(FragmentManager fm) { 
     super(fm); 
    } 



    @Override 
    public Fragment getItem(int position) { 
     // getItem is called to instantiate the fragment for the given page. 
     // Return a DummySectionFragment (defined as a static inner class 
     // below) with the page number as its lone argument. 
     Fragment fragment = new DummySectionFragment(); 
     Bundle args = new Bundle(); 
     args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1); 
     fragment.setArguments(args); 
     return fragment; 
    } 


    @Override 
    public int getCount() { 
     // Show 3 total pages. 
     return 3; 
    } 


    @Override 
    public CharSequence getPageTitle(int position) { 
     Locale l = Locale.getDefault(); 
     switch (position) { 
      case 0: 
       return getString(R.string.tab_list).toUpperCase(l); 
      case 1: 
       return getString(R.string.tab_map).toUpperCase(l); 
      case 2: 
       return getString(R.string.tab_list).toUpperCase(l); 
     } 
     return null; 
    } 

} 

/** 
* A dummy fragment representing a section of the app, but that simply displays dummy text. 
* This would be replaced with your application's content. 
*/ 
public static class DummySectionFragment extends Fragment { 
    /** 
    * The fragment argument representing the section number for this 
    * fragment. 
    */ 
    public static final String ARG_SECTION_NUMBER = "section_number"; 

    public DummySectionFragment() { 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
          Bundle savedInstanceState) { 
     View rootView = inflater.inflate(R.layout.list_tab_fragment, container, false); 
     TextView dummyTextView = (TextView) rootView.findViewById(R.id.section_label); 
     dummyTextView.setText(Integer.toString(getArguments().getInt(ARG_SECTION_NUMBER))); 
     return rootView; 
    } 
} 



} 

mój manifest

<uses-permission android:name="android.permission.INTERNET"/> 
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/> 
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> 
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/> 

<!-- TABS: While ViewPager will work on API 4 or above, tabs require an ActionBar. ActionBar is only 
available in API 11 or above. --> 
<!-- Min/target SDK versions (<uses-sdk>) managed by build.gradle --> 

<application 
    android:allowBackup="true" 
    android:icon="@mipmap/ic_launcher" 
    android:label="@string/app_name" 
    android:theme="@style/AppTheme" > 



    <meta-data android:name="com.google.android.gms.version" 
     android:value="@integer/google_play_services_version" /> 

    <!-- 
    <meta-data android:name="com.google.android.gms.version" 
     android:value="@integer/google_play_services_version" /> 
    --> 

    <meta-data 
     android:name="com.google.android.maps.v2.API_KEY" 
     android:value="xxxxx"/> 

    <activity 
     android:name=".MainActivity" 
     android:label="@string/app_name" 
     android:screenOrientation="portrait" 
     > 
     <intent-filter> 
      <action android:name="android.intent.action.MAIN" /> 

      <category android:name="android.intent.category.LAUNCHER" /> 
     </intent-filter> 
    </activity> 

.... 

    <activity 
     android:name=".View_Tabs" 
     android:label="@string/app_name" 
     android:screenOrientation="portrait" 
     > 
    </activity> 
</application> 

mój styles.xml

<resources> 

    <!-- Base application theme. --> 
    <!-- BEFORE <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar"> --> 
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar"> 
     <!-- Customize your theme here. --> 
     <item name="android:windowActionBar">true</item> 
    </style> 

    <style name="Theme.Base" parent="android:Theme.Light" /> 




</resources> 

mój Gradle

apply plugin: 'com.android.application' 

android { 
    compileSdkVersion 21 
    buildToolsVersion "21.1.2" 

    defaultConfig { 
     applicationId "xxxx" 
     minSdkVersion 14 
     targetSdkVersion 21 
     versionCode 1 
     versionName "1.0" 
    } 
    buildTypes { 
     release { 
      minifyEnabled false 
      proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro' 
     } 
    } 
} 

dependencies { 
    compile fileTree(dir: 'libs', include: ['*.jar']) 
    compile 'com.android.support:appcompat-v7:21.0.3' 
    compile 'com.google.android.gms:play-services:6.5.87' 
    //compile 'com.google.android.gms:play-services:7.0.0' 
    compile files('lib/gson-2.3.jar') 
} 

Answer 1

Musisz rozszerzyć ActionBarActivity zamiast FragmentActivity, aby mieć pasek czynności z fragmentami.

Jeśli używasz biblioteki v7 appcompat, Twoja aktywność powinna zamiast tego rozszerzyć ActionBarActivity, która jest podklasą FragmentActivity (więcej informacji znajdziesz w Dodawanie paska akcji).

Wciąż można spróbować tego,

ActionBar actionBar = (ActionBarActivity)getActivity().getSupportActionBar(); 

Więcej szczegółów można znaleźć tutaj. http://developer.android.com/training/basics/fragments/creating.html

Answer 2

getActionBar() zwróci poprawną wartość tylko wtedy, gdy czynność z paska tytułowego. Upewnij się, że w pliku manifestu ustawiłeś motyw z paskiem tytułu dla swojej aktywności.