Question

当我尝试从主要活动中访问片段时，我总是得到NullPointerException。不管我做什么。

问题在于我使用TabsPagerAdapter和ViewPager并且我不知道如何获取膨胀的视图（片段的onCreate()方法已经返回膨胀的视图）。

目标是访问片段内的元素并通过单个后台线程动态隐藏或显示它，这也应该为更多片段执行此操作。

MainActivity.java

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener
{
/* swipe view */
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private android.app.ActionBar actionBar;

// tab titles
private String[] tabs = { "Basic", "Advanced", "Settings"};


@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    /* init swipe views */
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
    viewPager = (ViewPager) findViewById(R.id.pager);
    viewPager.setAdapter(mAdapter);

    actionBar = getActionBar();
    actionBar.setHomeButtonEnabled(false);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    /* addTab returns void, how to geht my fragements and their views???*/
    for (String tabName : tabs)
        actionBar.addTab(actionBar.newTab().setText(tabName).setTabListener(this));

    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) {}

        @Override
        public void onPageScrollStateChanged(int arg0) {}
    });
}

public void testFunction()
{
    FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.layout.fragment_page1);
    GridLayout gridlayout = (GridLayout) fragmentPage1.getRootView().findViewById(R.id.adBannerBasicLayout);
    gridlayout.setVisibility(GridLayout.VISIBLE); /* THATS MY GOAL */
}

FragmentPage1.java

public class FragmentPage1 extends Fragment {

    private View rootView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        rootView = inflater.inflate(R.layout.fragment_page1, container, false);


        /* HERE IT IS WORKING FINE, 
        but later I want to make it visible again 
        from code OUTSIDE FragmentPage1 ??? */

        GridLayout gridlayout = (GridLayout) rootView.findViewById(R.id.adBannerBasicLayout);
        gridlayout.setVisibility(GridLayout.GONE);

        return rootView;
    }    


    /* so I tried this, but also get always NullPointerException */
    public View getRootView()
    {
        return rootView;
    }
}

fragment_page1.xml

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

<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/settings_scroll_view"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    >


    <GridLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:ads="http://schemas.android.com/apk/res-auto"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="#000000"
        android:orientation="vertical"
        android:rowCount="12"
        android:columnCount="5"
        >

        <!-- Some Banner Ads I want to hide show -->           
        <!-- I want to access this from everywhere! --> 
        <GridLayout
            xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:ads="http://schemas.android.com/apk/res-auto"
            android:id="@+id/adBannerBasicLayout"
            android:orientation="vertical"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="fill"
            android:rowCount="3"
            android:columnCount="1"
            android:layout_row="0"
            android:layout_column="0"
            android:layout_columnSpan="5">

            <Space
                android:layout_width="0dp"
                android:layout_height="5dp"
                android:layout_row="0"
                android:layout_column="0"
                />

            <com.google.android.gms.ads.AdView
                android:id="@+id/adBannerBasic"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="fill"
                ads:adSize="BANNER"
                ads:adUnitId="xxxxxxxxxxxxxxxxxxxxxxxxxx"
                android:layout_row="1"
                android:layout_column="0"
                >
            </com.google.android.gms.ads.AdView>

            <Space
                android:layout_width="0dp"
                android:layout_height="5dp"
                android:layout_row="2"
                android:layout_column="0"
                />

        </GridLayout>

        <!-- more stuff... -->
  </GridLayout>
</ScrollView>

请帮帮我，我完全陷入困境！

谢谢！

编辑：

testFunction（）中的第二行抛出NullPointerException：

 GridLayout gridlayout = (GridLayout) fragmentPage1.getRootView().findViewById(R.id.adBannerBasicLayout);

因为getSupportFragmentManager()总是返回null：

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.layout.fragment_page1);

Logcat Outoput

java.lang.IllegalStateException: Could not execute method of the activity
            at android.view.View$1.onClick(View.java:3969)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.reflect.InvocationTargetException
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at android.view.View$1.onClick(View.java:3964)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at org.tzapp.smote.MainActivity.testFuntion(MainActivity.java:393)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at android.view.View$1.onClick(View.java:3964)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

您使用自动生成的R文件中的错误静态值！

不使用引用XML资源的 R.layout.fragment_page1 ，而应使用 R.id.fragment_page1 ，它应该是 /res/layout/activity_main.xml中的R.layout.activity_main XML文件

R.layout 引用XML布局文件
R.id 引用各个XML节点（视图，片段等）

简而言之，改变：

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.layout.fragment_page1);

要：

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.id.fragment_page1);

并确保/res/layout/activity_main.xml中的片段ID设置为 R.id.fragment_page1 ，如下所示：

<fragment android:name="com.example.yourpackage.FragmentPage1"
              android:id="@+id/fragment_page1"
              android:layout_weight="1"
              android:layout_width="0dp"
              android:layout_height="match_parent" />

Answer 2

使用getSupportFragmentManager（）来获取片段。请仔细检查FragmentPage1是否扩展了android.support.v4.app.Fragment; 不是 android.Fragment;

Answer 3

使用 import android.support.v4.app.Fragment 代替import android.app.Fragment

请在您的gradle依赖项中添加此内容

compile 'com.android.support:support-v4:22.1.1'

编辑：请使用 findFragmentById（R.id.fragment_page1）代替R.layout

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.id.fragment_page1);

Answer 4

更新SDK和存储库后，出现了新的问题。我不能再建造了，我不想在第23级编译:(

   match(n:Label)
   with n limit 10000
   delete n

Answer 5

我也找到了另一个答案，我认为最令人满意的是：）

我忘了提到还有另一堂课。这一切都来自于swipe Views的一段愚蠢的示例代码，可以在developers.google.com上找到...

每当我尝试使用stupid getItem（int i）方法通过我的TabPagerAdapter类访问FragmentPages时，我创建了一个新的Fragment（）： - /非常烦人！

来自糟糕谷歌示例的类TabsPagerAdapter

public class TabsPagerAdapter extends FragmentPagerAdapter {    

    public TabsPagerAdapter(FragmentManager fm, MainActivity mainActivity){
        super(fm);
    }

    // page index, fragment selector
    @Override
    public Fragment getItem(int index) {

        switch (index)
        {
            case 0: return new FragmentPage1(); // bad practice
            case 1: return new FragmentPage2(); // why should one do that?
            case 2: return new FragmentPage3(); 
        }
        return null;
    }

    @Override
    public int getCount() {
        return 3;
    }
}

多么废话^^我只是复制了它然后完全忘记了它的存在;）

将片段的instanciation移动到构造函数并保留它们供以后使用。现在工作得很好。

优化后的TabsPagerAdapter类

    public class TabsPagerAdapter extends FragmentPagerAdapter {

    // hosted fragments
    private FragmentPage fragmentPageBasic;
    private FragmentPage fragmentPageAdvanced;
    private FragmentPage fragmentPageSettings;

    //constructor
    public TabsPagerAdapter(FragmentManager fm, MainActivity mainActivity){
        super(fm);

        fragmentPageBasic = new FragmentPage();
        fragmentPageBasic.setMainActivity(mainActivity);
        fragmentPageBasic.setLayoutResource(R.layout.fragment_page1);
        fragmentPageBasic.setAdBannerResource(R.id.adBannerBasic);
        fragmentPageBasic.setAdBannerLayoutResource(R.id.adBannerBasicLayout);

        fragmentPageAdvanced = new FragmentPage();
        fragmentPageAdvanced.setMainActivity(mainActivity);
        fragmentPageAdvanced.setLayoutResource(R.layout.fragment_page2);
        fragmentPageAdvanced.setAdBannerResource(R.id.adBannerAdvanced);
        fragmentPageAdvanced.setAdBannerLayoutResource(R.id.adBannerAdvancedLayout);

        fragmentPageSettings = new FragmentSettingsPage();
        fragmentPageSettings.setMainActivity(mainActivity);
        fragmentPageSettings.setLayoutResource(R.layout.fragment_page3);
        fragmentPageSettings.setAdBannerResource(R.id.adBannerSettings);
        fragmentPageSettings.setAdBannerLayoutResource(R.id.adBannerSettingsLayout);
    }

    // page index, fragment selector
    @Override
    public Fragment getItem(int index) {

        switch (index)
        {
            case 0: return fragmentPageBasic;
            case 1: return fragmentPageAdvanced;
            case 2: return fragmentPageSettings;
        }
        return null;
    }

    @Override
    public int getCount() {
        return 3;
    }

    // GETTERS
    public FragmentPage getFragmentPageBasic() {return fragmentPageBasic;}
    public FragmentPage getFragmentPageAdvanced() {return fragmentPageAdvanced;}
    public FragmentPage getFragmentPageSettings() {return fragmentPageSettings;}

}

而不是拥有冗余的FragmentPage1，Fragment2，FragmentPage3类，当然现在只有FragmentPage和FragmentSettingsPage（扩展FragmentPage来覆盖OnCreateView方法），这样可以产生更清晰，更易理解的代码。

Class FragmentPage

public class FragmentPage extends Fragment {

    protected MainActivity mainActivity;

    // layout resource
    protected int layoutResource;

    // view
    protected View rootView;

    //Admob
    protected AdView adBanner;
    protected int adBannerResource;
    protected int adBannerLayoutResource;


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        onCreate(inflater, container);

        return rootView;
    }

    // manage common onCreate things
    protected void onCreate(LayoutInflater inflater, ViewGroup container)
    {
        rootView = inflater.inflate(layoutResource, container, false);

        // admob banner
        adBanner = (AdView) rootView.findViewById(adBannerResource);
        adBanner.setAdListener(new AdListener() {

            @Override
            public void onAdLoaded() {
            }

            @Override
            public void onAdOpened() {
            }

            @Override
            public void onAdClosed() {
                newAdBannerRequest();
            }

            @Override
            public void onAdFailedToLoad(int errorCode) {
                newAdBannerRequest();
            }

            @Override
            public void onAdLeftApplication() {
            }
        });

        if(mainActivity.showAdBanners())
        {
            newAdBannerRequest();
            showAdBanner();
        }
        else
            hideAdBanner();
    }


    // ADMOB
    public void newAdBannerRequest()
    {
        AdRequest request = new AdRequest.Builder()
                .addTestDevice(AdRequest.DEVICE_ID_EMULATOR)        // All emulators
                .addTestDevice("XXXXXXXXXXXXXXXXXXXXXXXXXX")  // My Galaxy Nexus test phone
                .build();
        adBanner.loadAd(request);
    }

    public void showAdBanner()
    {
        GridLayout adBannerLayout = (GridLayout) rootView.findViewById(adBannerLayoutResource);
        if(adBannerLayout.getVisibility() == GridLayout.GONE)
            adBannerLayout.setVisibility(GridLayout.VISIBLE);
    }

    public void hideAdBanner()
    {
        GridLayout adBannerLayout = (GridLayout) rootView.findViewById(adBannerLayoutResource);
        if(adBannerLayout.getVisibility() == GridLayout.VISIBLE)
            adBannerLayout.setVisibility(GridLayout.GONE);
    }


    // GETTERS
    public AdView getAdBanner() { return adBanner;}
    public View getRootView() { return rootView;}


    // SETTERS
    public void setMainActivity(MainActivity mainActivity) {this.mainActivity = mainActivity;}
    public void setLayoutResource(int layoutResource){this.layoutResource = layoutResource;}
    public void setAdBannerResource(int adBannerResource){this.adBannerResource = adBannerResource;}
    public void setAdBannerLayoutResource(int adBannerLayoutResource){this.adBannerLayoutResource = adBannerLayoutResource;}
}

Class FragmentSettingsPage

public class FragmentSettingsPage extends FragmentPage {

    // Preferences
    public static final String PREFS_NAME = "Preferences";

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        super.onCreate(inflater, container);

        SharedPreferences preferences = mainActivity.getSharedPreferences(PREFS_NAME, 0);
        // do some other top secret stuff here ;)


        return rootView;
    }
}

有任何想法如何进一步改善这一点吗？

Android - 如何从FragmentActivity

5 个答案: