Android：在Google地图中保存地图状态

之前我使用过这段代码,但现在我在恢复地图状态时遇到了一些错误.我从David Gassner的教程中得到了这段代码,现在我想再次使用它.我有点困惑,我把一些抛出的消息,以便追踪. LogCat报告指出RESUME中的错误从此行开始任何人都知道如何解决这个问题？

map.animateCamera(update);          
map.setMapType(mgr.getSavedMapType())

继承守则：

@Override
protected void onStop() {
    super.onStop();
    MapStateManager mgr = new MapStateManager(this);
    mgr.saveMapState(mMap);
    Toast.makeText(this,"Map State has been save?",Toast.LENGTH_SHORT).show();
}

@Override
protected void onResume() {
    super.onResume();
    MapStateManager mgr = new MapStateManager(this);
    CameraPosition position = mgr.getSavedCameraPosition();
    if (position != null) {
        CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
        Toast.makeText(this,"entering Resume State",Toast.LENGTH_SHORT).show();
        mMap.moveCamera(update);

        mMap.setMapType(mgr.getSavedMapType());
    }
}
public class 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";

    private SharedPreferences mapStatePrefs;

    public MapStateManager(Context context) {
        mapStatePrefs = context.getSharedPreferences(PREFS_NAME,Context.MODE_PRIVATE);
    }

    public void saveMapState(GoogleMap mapMie) {
        SharedPreferences.Editor editor = mapStatePrefs.edit();
        CameraPosition position = mapMie.getCameraPosition();

        editor.putFloat(LATITUDE,(float) position.target.latitude);
        editor.putFloat(LONGITUDE,(float) position.target.longitude);
        editor.putFloat(ZOOM,position.zoom);
        editor.putFloat(TILT,position.tilt);
        editor.putFloat(BEARING,position.bearing);
        editor.putInt(MAPTYPE,mapMie.getMapType());
        editor.commit();
    }

    public CameraPosition getSavedCameraPosition() {
        double latitude = mapStatePrefs.getFloat(LATITUDE,0);
        if (latitude == 0) {
            return null;
        }
        double longitude = mapStatePrefs.getFloat(LONGITUDE,0);
        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;
    }

    public int getSavedMapType() {
        return mapStatePrefs.getInt(MAPTYPE,GoogleMap.MAP_TYPE_NORMAL);
    }
}

这是logcat的错误.我只包括红线.

> 01-07 12:51:46.145: E/AndroidRuntime(15770): FATAL EXCEPTION: main
01-07 12:51:46.145: E/AndroidRuntime(15770): java.lang.RuntimeException: Unable to resume activity {com.ourThesis.junieNegentien2015/com.ourThesis.junieNegentien2015.MainActivity}: java.lang.NullPointerException
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2936)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2965)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2400)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread.access$600(ActivityThread.java:162)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.os.Handler.dispatchMessage(Handler.java:107)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.os.Looper.loop(Looper.java:194)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread.main(ActivityThread.java:5371)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at java.lang.reflect.Method.invokeNative(Native Method)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at java.lang.reflect.Method.invoke(Method.java:525)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:608)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at dalvik.system.NativeStart.main(Native Method)
01-07 12:51:46.145: E/AndroidRuntime(15770): Caused by: java.lang.NullPointerException
01-07 12:51:46.145: E/AndroidRuntime(15770):    at com.ourThesis.junieNegentien2015.MainActivity.onResume(MainActivity.java:183)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1190)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.Activity.performResume(Activity.java:5213)
01-07 12:51:46.145: E/AndroidRuntime(15770):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2923)
01-07 12:51:46.145: E/AndroidRuntime(15770):    ... 12 more

@Tim Castelijns,下面是修改后的模式,但仍然不能正常工作你能看看吗？

private SupportMapFragment mapFragment;
    private GoogleMap map;  
    private Location mCurrentLocation;
    private GoogleApiClient mGoogleApiClient;

    @Override
    protected void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main); 
        setupMapIfNeeded();
        mToolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(mToolbar);

    }

    private void setupMapIfNeeded() {
        mapFragment = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map));             
        if (mapFragment != null) {          
            mapFragment.getMapAsync(new OnMapReadyCallback()
            {
                @Override
                public void onMapReady(GoogleMap map) 
                {
                    loadMap(map);                                       
                }
            });
        } else 
        {
            Toast.makeText(this,"Error - Map Fragment was null!!",Toast.LENGTH_SHORT).show();
        }
    }

    protected void loadMap(GoogleMap googleMap) {
        map = googleMap;
        if (map != null) {
            // Map is ready
            Toast.makeText(this,"Map Fragment was loaded properly!",Toast.LENGTH_LONG).show();
            initListeners();            

            // map is loaded,trace your location
            mGoogleApiClient = new GoogleApiClient.Builder(this)
                    .addApi(LocationServices.API)               
                    .addConnectionCallbacks(this)
                    .addOnConnectionFailedListener(this)
                    .build();
            connectClient();

            //calling resume state
            resumeState(map);

        } else {
            Toast.makeText(this,"Error - Map was null!!",Toast.LENGTH_SHORT).show();
        }
    }

    protected void connectClient() {
        // Connect the client.
        if (isGooglePlayServicesAvailable() && mGoogleApiClient != null) {
            mGoogleApiClient.connect();
            Toast.makeText(this,"Google API Client successfully connected",Toast.LENGTH_LONG).show();
        }
    }

    /*
     * Called when the Activity becomes visible.
    */
    @Override
    protected void onStart() {
        super.onStart();
        connectClient();
        }

    public void resumeState(GoogleMap googleMap) {      
        map = googleMap;
        MapStateManager mgr = new MapStateManager(this);
        CameraPosition position = mgr.getSavedCameraPosition();
        if (position != null) {
            CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
            Toast.makeText(this,Toast.LENGTH_LONG).show();
            map.moveCamera(update);

            map.setMapType(mgr.getSavedMapType());
        }
    }

    /*
     * Called when the Activity is no longer visible.
     */    
    @Override
    protected void onPause() {
        super.onPause();
        MapStateManager mgr = new MapStateManager(this);
        mgr.saveMapState(map);
        Toast.makeText(this,Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onResume() {     
        super.onResume();       
        setupMapIfNeeded();
    }

    @Override
    public void onConnected(Bundle dataBundle) {
        // Display the connection status
        Location location = LocationServices.FusedLocationApi.getLastLocation(mGoogleApiClient);
        if (location != null) {
            Toast.makeText(this,"GPS location was found!",Toast.LENGTH_SHORT).show();
            LatLng latLng = new LatLng(location.getLatitude(),location.getLongitude());
            CameraUpdate cameraUpdate = CameraUpdateFactory.newLatLngZoom(latLng,16);
            map.animateCamera(cameraUpdate);
            /*startLocationUpdates();*/
        } else {
            Toast.makeText(this,"Current location was null,enable GPS on your mobile!",Toast.LENGTH_SHORT).show();
        }
    }

    private void initListeners() {
        map.setMapType(GoogleMap.MAP_TYPE_HYBRID);
        map.getUiSettings().setZoomControlsEnabled(true);
        map.setMyLocationEnabled(true);
        map.setOnMarkerClickListener(null);        
        map.setOnMapLongClickListener(null);
        map.setOnInfoWindowClickListener(null);
        map.setOnMapClickListener(null);
        map.setTrafficEnabled(true);
        map.setOnMapLongClickListener(this);
        map.setOnMapClickListener(this);
        map.setPadding(0,80,0);

    }

解决方法

问题是当你恢复时你的mMap为null,然后你不能使用mMap.moveCamera();例如,这会导致您的应用崩溃.

解决方法是在尝试再次使用之前检查是否需要设置地图.

我稍微修改了你的代码,以检查是否需要在onCreate和onResume中设置地图.请注意,我使用onPause而不是onStop,因为它与onResume相对应.

private GoogleMap mMap;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_maps);
    setupMapIfNeeded();
}

private void setupMapIfNeeded() {
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    if (mMap == null) {
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
    }
}
@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;

    MapStateManager mgr = new MapStateManager(this);
    CameraPosition position = mgr.getSavedCameraPosition();
    if (position != null) {
        CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
        Toast.makeText(this,Toast.LENGTH_SHORT).show();
        mMap.moveCamera(update);

        mMap.setMapType(mgr.getSavedMapType());
    }
}

@Override
protected void onPause() {
    super.onPause();
    MapStateManager mgr = new MapStateManager(this);
    mgr.saveMapState(mMap);
    Toast.makeText(this,Toast.LENGTH_SHORT).show();
}

@Override
protected void onResume() {
    super.onResume();
    setupMapIfNeeded();
}

MapstateManager完全一样.

public class 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";

    private SharedPreferences mapStatePrefs;

    public MapStateManager(Context context) {
        mapStatePrefs = context.getSharedPreferences(PREFS_NAME,GoogleMap.MAP_TYPE_NORMAL);
    }
}

原文链接：https://www.f2er.com/android/309002.html

Android：在Google地图中保存地图状态

解决方法

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