谷歌地图：恢复地图活动时如何恢复标记

Question

I am making an app that collects data from a sensor. Every time the sensor sends data, a new marker is placed on a map. When I press back and then resume the map activity all the markers are gone. Is there a way to save and restore each unique marker with its unique MarkerOptions (title,snippet,LatLng) when resuming the activity?

Here is my maps activity

 public class MapsActivity extends FragmentActivity implements OnMapReadyCallback, GoogleApiClient.ConnectionCallbacks, GoogleApiClient.OnConnectionFailedListener, LocationListener
{

private GoogleMap mMap;
GoogleApiClient mGoogleApiClient;
Location mLastLocation;
Marker mCurrLocationMarker;
private int co_mV;
private int no2_mV;
LocationRequest mLocationRequest;
private boolean initiateApp;
String currentTime;

TcpClient mTcpClient;
ArrayList<Marker> markerArrayList;

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

    if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        checkLocationPermission();
    }
    // 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);
    initiateApp = true;
    markerArrayList = new ArrayList<Marker>();

}

/**
 * Manipulates the map once available.
 * This callback is triggered when the map is ready to be used.
 * This is where we can add markers or lines, add listeners or move the camera. In this case,
 * we just add a marker near Sydney, Australia.
 * If Google Play services is not installed on the device, the user will be prompted to install
 * it inside the SupportMapFragment. This method will only be triggered once the user has
 * installed Google Play services and returned to the app.
 */
@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    mMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
    mMap.setMyLocationEnabled(true);


    if(mGoogleApiClient == null) {
    //Initialize Google Play Services
    if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.ACCESS_FINE_LOCATION)
                == PackageManager.PERMISSION_GRANTED) {
            buildGoogleApiClient();

        }
    }
    else {
            buildGoogleApiClient();
        }
    }
}

/* Here we create the infoWindow **/
protected synchronized void buildGoogleApiClient() {
    mGoogleApiClient = new GoogleApiClient.Builder(this)
            .addConnectionCallbacks(this)
            .addOnConnectionFailedListener(this)
            .addApi(LocationServices.API)
            .build();
    mGoogleApiClient.connect();

}

@Override
public void onConnected(Bundle bundle) {

    getNewLocation();
    new ConnectTask().execute("");

}


public void newData(JSONObject d) {
    try {
        co_mV = d.getInt("co_mV");
        no2_mV = d.getInt("no2_mV");
    } catch (JSONException e) {
        e.printStackTrace();
    }

    getNewLocation();
}

public void getTime() {

    Calendar cal = Calendar.getInstance();
    currentTime = new SimpleDateFormat("HH:mm:ss").format(cal.getTime());

}

public void getNewLocation() {
    mLocationRequest = new LocationRequest();
    mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    if (ContextCompat.checkSelfPermission(this,
            Manifest.permission.ACCESS_FINE_LOCATION)
            == PackageManager.PERMISSION_GRANTED) {
        LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, mLocationRequest, this);
    }

}

@Override
public void onConnectionSuspended(int i) {

}

@Override
public void onLocationChanged(Location location) {

    if (markerArrayList.size()>1) {
        if(location.distanceTo(mLastLocation) < 30) {
            markerArrayList.get(markerArrayList.size()-1).remove();
            markerArrayList.remove(markerArrayList.size()-1);
            Toast.makeText(
                    getApplicationContext(),
                    "Reading to close to last reading, replaces last reading", Toast.LENGTH_SHORT).show();
        }
    }


    if (markerArrayList.size() == 8) {
        markerArrayList.get(0).remove();
        markerArrayList.remove(0);
    }

    //Place current location marker
    LatLng latLng = new LatLng(location.getLatitude(), location.getLongitude());

    if (!initiateApp) {
        MarkerOptions markerOptions = new MarkerOptions();
        markerOptions.position(latLng);
        markerOptions.title("Time of reading: " + currentTime);
        markerOptions.snippet("co: " + String.valueOf(co_mV) + " mV, " + "no2: " + String.valueOf(no2_mV) + " mV");
        markerOptions.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_MAGENTA));
        mCurrLocationMarker = mMap.addMarker(markerOptions);
        markerArrayList.add(mCurrLocationMarker);

    }

    mLastLocation = location;


    Log.d("ADebugTag", "Value: " + Double.toString(location.getLatitude()));
    Log.d("ADebugTag", "Value: " + Double.toString(location.getLongitude()));


    //move map camera

    if(initiateApp){
        mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(latLng, 15));
    }

    boolean contains = mMap.getProjection()
            .getVisibleRegion()
            .latLngBounds
            .contains(latLng);

    if(!contains){
        mMap.moveCamera(CameraUpdateFactory.newLatLng(latLng));
    }

    initiateApp = false;
}

@Override
public void onConnectionFailed(ConnectionResult connectionResult) {

}

public static final int MY_PERMISSIONS_REQUEST_LOCATION = 99;
public boolean checkLocationPermission(){
    if (ContextCompat.checkSelfPermission(this,
            Manifest.permission.ACCESS_FINE_LOCATION)
            != PackageManager.PERMISSION_GRANTED) {

        // Asking user if explanation is needed
        if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                Manifest.permission.ACCESS_FINE_LOCATION)) {

            // Show an explanation to the user *asynchronously* -- don't block
            // this thread waiting for the user's response! After the user
            // sees the explanation, try again to request the permission.

            //Prompt the user once explanation has been shown
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.ACCESS_FINE_LOCATION},
                    MY_PERMISSIONS_REQUEST_LOCATION);


        } else {
            // No explanation needed, we can request the permission.
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.ACCESS_FINE_LOCATION},
                    MY_PERMISSIONS_REQUEST_LOCATION);
        }
        return false;
    } else {
        return true;
    }
}

@Override
public void onRequestPermissionsResult(int requestCode,
                                       String permissions[], int[] grantResults) {
    switch (requestCode) {
        case MY_PERMISSIONS_REQUEST_LOCATION: {
            // If request is cancelled, the result arrays are empty.
            if (grantResults.length > 0
                    && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                // permission was granted. Do the
                // contacts-related task you need to do.
                if (ContextCompat.checkSelfPermission(this,
                        Manifest.permission.ACCESS_FINE_LOCATION)
                        == PackageManager.PERMISSION_GRANTED) {

                    if (mGoogleApiClient == null) {
                        buildGoogleApiClient();
                    }
                    mMap.setMyLocationEnabled(true);
                }

            } else {

                // Permission denied, Disable the functionality that depends on this permission.
                Toast.makeText(this, "permission denied", Toast.LENGTH_LONG).show();
            }
            return;
        }

        // other 'case' lines to check for other permissions this app might request.
        // You can add here other case statements according to your requirement.
    }
}

ublic JSONObject getNewJSON(JSONObject json) {
    try {

        int humidity = json.getInt("humidity_ppm");
        int pressure = json.getInt("pressure_Pa");
        int noise = json.getInt("noise_dB");
        double lat = mLastLocation.getLatitude();
        double lng = mLastLocation.getLongitude();
        long time = System.currentTimeMillis() / 1000L;

       JSONObject c = new JSONObject();
        c.put("time",time);
        c.put("lat",lat);
        c.put("long",lng);
        c.put("humidity",humidity);
        c.put("pressure",pressure);
        c.put("noise_dB",noise);
        return c;

    } catch (JSONException e) {
        e.printStackTrace();
    }

return null;
}


public class ConnectTask extends AsyncTask<String, String, TcpClient> {

    @Override
    protected TcpClient doInBackground(String... message) {

        //we create a TCPClient object
        mTcpClient = new TcpClient(new TcpClient.OnMessageReceived() {
            @Override
            //here the messageReceived method is implemented
            public void messageReceived(String message) {
                //this method calls the onProgressUpdate
                publishProgress(message);
            }
        });
        mTcpClient.run();

        return null;
    }

    @Override
    protected void onProgressUpdate(String... values) {
        super.onProgressUpdate(values);


        try {
            JSONObject parser = new JSONObject(values[0]);
            JSONObject d = parser.getJSONObject("d");
            JSONObject cloudData = getNewJSON(d);

            if (mLastLocation != null) {
                newData(d);
                getTime();
            }

            System.out.println(cloudData);
        } catch (JSONException e) {
            e.printStackTrace();
        }
        //process server response here....
    }

}

Answer 1

You said

When I press back and then resume the map

right? Not closing your app.

Simplest way is to override the onBackPressed method, like this:

@Override
public void onBackPressed() {
    moveTaskToBack(true);
}

This won't trigger onDestroy() and onCreate() again, so your would leave your Activity instance unmodified. Simple enough, huh?

Answer 2

You can make a function that populates with fresh data from your server. Then you can call that function as soon as the server sends the JSON data. To answer your question, you simply call the same function in onResume. For more information about activity state read this .

The other approach is to save it in a savedInstanceState bundle and do check in onSaveInstanceState just like this SO accepted answer if that bundle is not null, if so populate the map with the List(last known locations of the markers, if that works with your project). Otherwise just load the map.

Answer 3

I found this answer which provides an elegant one-line solution. This will restore markers when the user rotates the phone, but not when he presses the Back button (I know this is not what OP asked but Google will lead people here)

Add the following in onCreate :

// mapFragment is a SupportMapFragment 
mapFragment.setRetainInstance(true);

Answer 4

In manifest file, you can add this line:

    <activity android:name=".MapsActivity"
              android:configChanges="orientation|screenSize">

It was one of the simplest solution that worked for me in tandem with

@Override
public void onBackPressed() {
moveTaskToBack(true);
}

, and works for simple enough apps, though onSaveInstanceState() method is recommended too. What I found from my research is it's tough to serialize and deserialize marker lists, and onSaveInstanceState() only works with simple objects like strings and such. If anyone found a better solution, please let me know too :)