findFragmentById 始终返回 null [英] findFragmentById always returns a null

问题描述

我在 mainActivity 中使用导航模式下的操作栏.不同的选项卡具有不同的布局，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.

I'm using an actionbar in navigation mode in the mainActivity. Different tabs has different layouts, and on one of these tabs, I embed a layout which has a MapFragment inside. With the following code, the app works well, the mapFragments works fine, the tabs works fine. However, once I want to get access the mapFragment embeded in myFragment use getFragmentManager().findFragmentById(R.id.map_add), I always get 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.请帮忙...

I've tried using FragmentManager and FragmentTransaction to add myfragment and the MapFragment, but still get null from findFragmentById(R.id.map_add). Please help...

MainActivity 初始化如下:

The MainActivity initialized like this:

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

the layout of the MainActivity, 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:

the code of the tablistener:

    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:

the code of myFragment:

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

the layout of myFragment: 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:nullpointer 异常指的是第 103 行，也就是 getMap() 行，我在前面提到的代码中在它后面有注释

the Logcat output: the nullpointer exception refers to line 103, which is the getMap() line, I have comment after it in the aforementioned code

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

推荐答案

发生这种情况是因为 MapFragment 尚未完成加载 google 地图对象.要解决此问题，请尝试在创建地图片段时覆盖 onActivityCreated.从 onActivityCreated 的实现开始，然后执行所有地图访问和修改.

This happens because the MapFragment has not finished loading the google map object. To fix this try overriding onActivityCreated when you create the map fragment. From your implementation of onActivityCreated then do all of your map accesses and modifications.

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

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