android - findFragmentById 始终返回 null

Question

我在 mainActivity 的导航模式下使用操作栏。不同的选项卡有不同的布局，在其中一个选项卡上，我嵌入了一个内部有 MapFragment 的布局。使用以下代码，应用程序运行良好，mapFragments 运行良好，选项卡运行良好。 但是，一旦我想访问嵌入在 myFragment 中的 mapFragment 使用 getFragmentManager().findFragmentById(R.id.map_add)，我总是得到 null。

我已经尝试使用 FragmentManager 和 FragmentTransaction 添加 myfragment 和 MapFragment，但仍然从 findFragmentById(R.id.map_add) 中得到 null。请帮助...

MainActivity 初始化如下:

protected void onCreate(Bundle bundle) {
    super.onCreate(bundle);
    setContentView(R.layout.main);

    // ActionBar gets initiated
    ActionBar actionbar = getActionBar();
    // Tell the ActionBar we want to use Tabs.
    actionbar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    // initiating the tabs and set text to it.
    ActionBar.Tab myTab = actionbar.newTab().setText("hey");

    // create the fragments we want to use for display content
    Fragment myFragment = new myFragment();

    // set the Tab listener. Now we can listen for clicks.
    myTab.setTabListener(new MyTabsListener(myFragment));


    // add the tabs to the actionbar
    actionbar.addTab(myTab);
    map=((MapFragment)getFragmentManager().findFragmentById(R.id.map_add)).getMap();//line 103

}

MainActivity 的布局，main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >
</RelativeLayout>

tablistener的代码:

    class MyTabsListener implements ActionBar.TabListener {
    public Fragment fragment;

    public MyTabsListener(Fragment fragment) {
        this.fragment = fragment;
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {

    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        ft.replace(R.id.fragment_container, fragment);           
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        ft.remove(fragment);
    }

}

myFragment的代码:

public static class AddAliasFragment extends Fragment {

    private static View view;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (view != null) {
            ViewGroup parent = (ViewGroup) view.getParent();
            if (parent != null)
                parent.removeView(view);
        }
        try {
            view = inflater.inflate(R.layout.myfragment_layout, container, false);
        } catch (InflateException e) {
            /* map is already there, just return view as it is */
        }
        return view;
    }

    public MapFragment getMapFragment(){
        return (MapFragment)getFragmentManager().findFragmentById(R.id.map_add);
    } 

}

myFragment的布局:myfragment_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/map_add_layout"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<EditText
    android:id="@+id/search"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:hint="@string/searchhint"
    />

<Button
    android:id="@+id/searchbtn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/search"
    android:layout_toRightOf="@id/search"
    android:text="@string/go" />

<fragment 
      android:id="@+id/map_add"
      android:tag="add"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:layout_below="@id/search"
      android:name="com.google.android.gms.maps.MapFragment"/>

Logcat输出:空指针异常是指第103行，也就是getMap()行，我在上述代码后面有注释

04-02 21:23:12.122: E/AndroidRuntime(3672): FATAL EXCEPTION: main
04-02 21:23:12.122: E/AndroidRuntime(3672): java.lang.RuntimeException: Unable to start     activity ComponentInfo{com.test.test/com.test.test.MainActivity}:     java.lang.NullPointerException
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2357)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.ActivityThread.access$600(ActivityThread.java:153)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.os.Looper.loop(Looper.java:137)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.ActivityThread.main(ActivityThread.java:5226)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at java.lang.reflect.Method.invokeNative(Native Method)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at java.lang.reflect.Method.invoke(Method.java:511)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at dalvik.system.NativeStart.main(Native Method)
04-02 21:23:12.122: E/AndroidRuntime(3672): Caused by: java.lang.NullPointerException
04-02 21:23:12.122: E/AndroidRuntime(3672):     at     com.test.test.MainActivity.onCreate(MainActivity.java:103)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.Activity.performCreate(Activity.java:5104)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
04-02 21:23:12.122: E/AndroidRuntime(3672):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261)
04-02 21:23:12.122: E/AndroidRuntime(3672):     ... 11 more

Answer 1

发生这种情况是因为 MapFragment 尚未完成加载谷歌地图对象。要解决此问题，请尝试在创建 map fragment 时覆盖 onActivityCreated。然后从您对 onActivityCreated 的实现进行所有 map 访问和修改。

myMapFragment=new SupportMapFragment(){
    @Override
    public void onActivityCreated(Bundle savedInstanceState){
        //modify your map here
    }
}