继续活动-NullPointerException

Question

我在恢复一个活动时遇到了问题，当我打开它时，我的应用程序崩溃了，我得到了一个NullPointerException，这很奇怪，因为我有一个=null条件。

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //setContentView(R.layout.activity_maps);//frag
    setContentView(R.layout.map_view);//linear
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
            .findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);
}
@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    gotoLocation(51.508182, -0.140315, DEFAULT_ZOOM);
    //mMap.setMyLocationEnabled(true);
}
    @Override
protected void onResume(){
    super.onResume();
    MapStateManager mgr = new MapStateManager(this);
    CameraPosition position = mgr.getSavedCameraPosition();
    if(position != null){
        CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
        mMap.moveCamera(update);
    }
}

错误出现在mMap.moveCamera（更新）行；

MapStateManager.java

公共类MapStateManager｛

private static final String LONGITUDE = "longitude";
private static final String LATITUDE = "latitude";
private static final String ZOOM = "zoom";
private static final String BEARING = "bearing";
private static final String TILT = "tilt";
private static final String MAPTYPE = "MAPTYPE";
private static final String PREFS_NAME="mapCameraState";//definition for shared
private SharedPreferences mapStatePrefs;// save data on the device
public MapStateManager(Context context){
    mapStatePrefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
}
public void saveMapState(GoogleMap map){
    SharedPreferences.Editor editor = mapStatePrefs.edit();
    CameraPosition position = map.getCameraPosition();
    editor.putFloat(LATITUDE, (float) position.target.latitude);
    editor.putFloat(LONGITUDE, (float) position.target.longitude);
    editor.putFloat(TILT, position.tilt);
    editor.putFloat(ZOOM, position.zoom);
    editor.putFloat(BEARING, position.bearing);
    editor.commit();
}
public CameraPosition getSavedCameraPosition(){
    double latitude = mapStatePrefs.getFloat(LATITUDE, 0);
    double longitude = mapStatePrefs.getFloat(LONGITUDE, 0);
    if (latitude==0){
        return null;
    }
    LatLng target = new LatLng(latitude, longitude);
    float zoom = mapStatePrefs.getFloat(ZOOM, 0);
    float bearing = mapStatePrefs.getFloat(BEARING, 0);
    float tilt = mapStatePrefs.getFloat(TILT, 0);
    CameraPosition position = new CameraPosition(target, zoom, tilt, bearing);
    return position;
}

}

Answer 1

您检查了postion != null，但mMap可能仍然为null（这就是发生的情况）。

在onResume()方法中，您需要再次获取地图，因为它可能在应用程序处于非活动状态时丢失。

试试这样的东西：

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    //Maybe move this to an else on the if below? I'm not sure exactly what this does
    gotoLocation(51.508182, -0.140315, DEFAULT_ZOOM);
    //Update to the saved position.
    MapStateManager mgr = new MapStateManager(this);
    CameraPosition position = mgr.getSavedCameraPosition();
    if(position != null){
        CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
        mMap.moveCamera(update);
    }
}
@Override
protected void onResume(){
    super.onResume();
    //Get the map asynchronously again.
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);
}

Answer 2

您忘记检查mMap

mMap为null。您也不需要在oncreate中调用map，而是使用接口进行调用。它可以在任何时候出现，在onResume期间不太可能（就像在never中一样）。