onCreate, onMapReady and onLocationChanged

Question

I'm developing an app that utilizes (google map) but I'm having trouble understanding which method of the three starts running first? or does onLocationChanged wait for onCreate to finish executing then it starts running?, and if it just runs in parallel how can I force it to wait for onCreate to finish executing. It may be a ridiculous question but the documentation didn't help me. Thanks.

Answer 1

onLocationChanged and onMapReady methods are listeners. They are almost behaving asynchronously and updating the UI for you once they completes. They are, also, fired once an event is occurred i.e if the location of the device is changed, then onLocationChanged method is fired and will provide the new location to use. This is also happening with onMapReady method, once the map is ready, as its name suggests, this method is activated and provided with the GoogleMap object to use. onCreate method is a normal method executed when the activity is created. If you are using google maps API to find the device location, not using the device GPS utility, then you need to have a GoogleApiClient object to determine the current location of the device. you may retrieve the current location from onConnected listener and put a marker on the map indicating that location.
This is a sample code to run a google map fragment activity:

import android.Manifest;
import android.content.pm.PackageManager;
import android.location.Location;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.ActivityCompat;
import android.support.v4.app.FragmentActivity;
import android.support.v4.content.ContextCompat;
import android.widget.Toast;

import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.location.LocationListener;
import com.google.android.gms.location.LocationRequest;
import com.google.android.gms.location.LocationServices;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;

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

    private GoogleMap myMap;
    private GoogleApiClient googleClient;
    private Location lastLocation;
    private LocationRequest locationRequest;
    private Marker currentLocationMarker;
    public static final int REQUEST_LOCATION_CODE = 99;

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

        // this code is to check whether the app gains the necessary permissions to get device location
        if(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);
    }


    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch(requestCode){
            case REQUEST_LOCATION_CODE:
                if(grantResults.length>0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                    // Premission is granted
                    if(ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED){
                        if(googleClient == null){
                            buildGoogleApiClient();
                        }
                        myMap.setMyLocationEnabled(true);
                    }
                }
                else{ // Permission is not granted
                    Toast.makeText(this, "Permission denied", Toast.LENGTH_LONG).show();
            }
            return;
        }

    }

       /**
     * 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) {
        myMap = googleMap;

        if(ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED){
            //    ActivityCompat#requestPermissions
            // here to request the missing permissions, and then overriding
            //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
            //                                          int[] grantResults)
            // to handle the case where the user grants the permission. See the documentation
            // for ActivityCompat#requestPermissions for more details.
            buildGoogleApiClient();
            myMap.setMyLocationEnabled(true);
            //return;
        }
    }

    protected synchronized void buildGoogleApiClient(){
           googleClient = new GoogleApiClient.Builder(this)
                   .addConnectionCallbacks(this)
                   .addOnConnectionFailedListener(this)
                   .addApi(LocationServices.API)
                   .build();
        googleClient.connect();

    }

    @Override
    public void onLocationChanged(Location location) {
        lastLocation = location;

        if(currentLocationMarker != null){
            currentLocationMarker.remove();
        }

        LatLng latlng = new LatLng(location.getLatitude(), location.getLongitude());
        MarkerOptions markerOptions = new MarkerOptions();
        markerOptions.position(latlng);
        markerOptions.title("your current location");
        markerOptions.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE));
        currentLocationMarker = myMap.addMarker(markerOptions);

        myMap.moveCamera(CameraUpdateFactory.newLatLng(latlng));
        myMap.animateCamera(CameraUpdateFactory.zoomBy(10));

        if(googleClient != null){
            LocationServices.FusedLocationApi.removeLocationUpdates(googleClient,this);
        }
    }

    @Override
    public void onConnected(@Nullable Bundle bundle) {
        locationRequest = new LocationRequest();
        locationRequest.setInterval(1000);
        locationRequest.setFastestInterval(1000);
        locationRequest.setPriority(LocationRequest.PRIORITY_BALANCED_POWER_ACCURACY);
        if(ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED) {
            LocationServices.FusedLocationApi.requestLocationUpdates(googleClient, locationRequest, this);
        }

    }

    public boolean checkLocationPermission(){
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, REQUEST_LOCATION_CODE);
            return  false;
        }
        return  true;
    }

    @Override
    public void onConnectionSuspended(int i) {

    }

    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

    }
}

and the XML code is only a single fragment:

<fragment xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/map"
    android:name="com.google.android.gms.maps.SupportMapFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="so.tweet.client.MapsActivity" />

hope this help:)