Android - fragment 中的 MapView

Question

我在这里学习了很多教程并阅读了很多问题，也许我有些困惑。

我正在开发一个应用程序，它显示一个带有操作栏的 MainActivity，将 Activity 分成 3 个选项卡(每个选项卡都是一个 fragment )。在第一个选项卡上，我想显示一个 Google map (带有 MapView，我猜它比 MapFragment 更容易)，但我在实例化 map 时遇到 NullPointerException。这是日志:

05-25 06:14:26.740: E/AndroidRuntime(918): FATAL EXCEPTION: main
05-25 06:14:26.740: E/AndroidRuntime(918): Process: com.example.mypackage, PID: 918
05-25 06:14:26.740: E/AndroidRuntime(918): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mypackage/com.example.mypackage.activities.MainActivity}: java.lang.NullPointerException
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.os.Handler.dispatchMessage(Handler.java:102)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.os.Looper.loop(Looper.java:136)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.ActivityThread.main(ActivityThread.java:5017)
05-25 06:14:26.740: E/AndroidRuntime(918):  at java.lang.reflect.Method.invokeNative(Native Method)
05-25 06:14:26.740: E/AndroidRuntime(918):  at java.lang.reflect.Method.invoke(Method.java:515)
05-25 06:14:26.740: E/AndroidRuntime(918):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-25 06:14:26.740: E/AndroidRuntime(918):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-25 06:14:26.740: E/AndroidRuntime(918):  at dalvik.system.NativeStart.main(Native Method)
05-25 06:14:26.740: E/AndroidRuntime(918): Caused by: java.lang.NullPointerException
05-25 06:14:26.740: E/AndroidRuntime(918):  at com.example.mypackage.fragments.MapFragment.onCreateView(MapFragment.java:30)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:927)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1467)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:570)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.Activity.performStart(Activity.java:5241)
05-25 06:14:26.740: E/AndroidRuntime(918):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168)
05-25 06:14:26.740: E/AndroidRuntime(918):  ... 11 more

这是 MapFragment

public class MapFragment extends Fragment {

    MapView mapView;
    GoogleMap map;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_map, container, false);
        // Gets the MapView from the XML layout and creates it
        mapView = (MapView) v.findViewById(R.id.map);
        mapView.onCreate(savedInstanceState);
        // Gets to GoogleMap from the MapView and does initialization stuff
        map = mapView.getMap();
        map.getUiSettings().setMyLocationButtonEnabled(false);
        map.setMyLocationEnabled(true);

        // Needs to call MapsInitializer before doing any CameraUpdateFactory calls

        MapsInitializer.initialize(this.getActivity());

        // Updates the location and zoom of the MapView
        CameraUpdate cameraUpdate = CameraUpdateFactory.newLatLngZoom(new LatLng(0, 0), 10);
        map.animateCamera(cameraUpdate);
        return v;
    }

    @Override
    public void onResume() {
        mapView.onResume();
        super.onResume();
    }
    @Override
    public void onDestroy() {
        super.onDestroy();
        mapView.onDestroy();
    }
    @Override
    public void onLowMemory() {
        super.onLowMemory();
        mapView.onLowMemory();
    }
}

所以产生错误的调用(第30行)是

map.getUiSettings().setMyLocationButtonEnabled(false);

最后，这是 fragment_map.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <com.google.android.gms.maps.MapView
        android:id="@+id/map"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" />

</LinearLayout>

如果有任何帮助，这是我的 list :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.mypackage"
    android:versionCode="1"
    android:versionName="1.0" >

    <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="com.google.android.providers.gsf.permission.READ_GSERVICES" />
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

    <uses-sdk
        android:minSdkVersion="11"
        android:targetSdkVersion="19" />

    <uses-feature
        android:glEsVersion="0x00020000"
        android:required="true" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/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.maps.v2.API_KEY"
            android:value="my_api_key" />

        <activity
            android:name="com.example.mypackage.activities.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

有人知道为什么吗？谢谢。

Answer 1

在 android 文档中，它是关于 MapView.getMap() 的:

如果 map View 尚未准备好，则为空。当 Google Play 服务不可用时，可能会发生这种情况。如果之后 Google Play 服务可用，再次调用此方法将初始化并返回 GoogleMap。

您是否在启动应用的设备上安装了 google play 服务应用？