onCreate，onMapReady和onLocationChanged

Question

我正在開發一個使用（谷歌地圖）的應用程序，但是我無法理解這三種方法中的哪一種首先開始運行？ 還是onLocationChanged等待onCreate完成執行然后開始運行？並且，如果它只是並行運行，如何強制它等待onCreate完成執行。 這可能是一個荒謬的問題，但是文檔並沒有幫助我。 謝謝。

Answer 1

onLocationChanged和onMapReady方法是偵聽器 。 它們幾乎表現為異步，並在完成后為您更新UI。 事件發生時也會觸發它們，即，如果更改設備的位置，則將觸發onLocationChanged方法並將提供要使用的新位置。 onMapReady方法也會發生這種情況，一旦地圖准備就緒（顧名思義，該方法便會被激活並隨GoogleMap對象一起提供）。 onCreate方法是創建活動時執行的常規方法。 如果您使用的是Google Maps API而不是設備的GPS實用程序來查找設備的位置，則需要有一個GoogleApiClient對象來確定設備的當前位置。 您可以從onConnected偵聽器中檢索當前位置，然后在地圖上放置一個指示該位置的標記。
這是運行Google地圖片段活動的示例代碼：

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代碼只是一個片段：

<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" />

希望對您有所幫助：）