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.
答案 0 :(得分：1)
onLocationChanged 和 onMapReady 方法是侦听器。它们几乎表现异步，并在完成后为您更新UI。一旦发生事件，它们也会被触发，即如果设备的位置发生变化，则会触发 onLocationChanged 方法并提供要使用的新位置。使用 onMapReady 方法也会发生这种情况，一旦地图准备就绪，顾名思义，此方法已激活，并随GoogleMap对象一起提供。 onCreate 方法是在创建活动时执行的常规方法。如果您使用谷歌地图API查找设备位置，而不是使用设备GPS实用程序，那么您需要有一个 GoogleApiClient 对象来确定设备的当前位置。您可以从 onConnected 侦听器中检索当前位置，并在地图上放置一个标记以指示该位置。
这是运行谷歌地图片段活动的示例代码：
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) {
}
}
，XML代码只是一个片段：
<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" />
希望这有帮助：）