MapView的getMap()方法可能返回null。我知道这是Google有意为之的行为。
是否有人可以提供一个明确的描述，说明什么情况下以及在何种情况下getMap()方法会返回null?
我知道如果给定设备上无法使用Google服务，getMap()将返回null。这种情况已经相对较好地记录下来了。我更关心的是一种模糊的情况，即即使在设备上安装了Google服务，getMap()仍然可能返回null。
到目前为止，我的假设是，在底层地图系统初始化期间，您的代码可能会执行并获取空地图。
我的假设正确吗？
是否有特定的Activity或Fragment生命周期的某个特定位置，我们可以确定地获取非空的GoogleMap（如果我们假定已安装Google服务）？
我提出这个问题的目的是为了避免在我的代码中散布大量的if(mapView.getMap()!= null)检查。此外，这个问题似乎仍然经常在StackOverflow上出现，我想看看我们能否弄清楚MapView和getMap()实际上发生了什么。
getMap()并保存返回的地图对象。遵循地图示例使用的基本模式，在onCreate方法和onResume方法中调用setupmapifneeded，当地图对象不为空时调用setupmap。/*
* Copyright (C) 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.gosylvester.testapp;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
/**
* This shows how to create a simple activity with a map and a marker on the map.
* <p>
* Notice how we deal with the possibility that the Google Play services APK is not
* installed/enabled/updated on a user's device.
*/
public class BasicMapActivity extends FragmentActivity {
/**
* Note that this may be null if the Google Play services APK is not available.
*/
private GoogleMap mMap;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.basic_demo);
setUpMapIfNeeded();
}
@Override
protected void onResume() {
super.onResume();
setUpMapIfNeeded();
}
/**
* Sets up the map if it is possible to do so (i.e., the Google Play services APK is correctly
* installed) and the map has not already been instantiated.. This will ensure that we only ever
* call {@link #setUpMap()} once when {@link #mMap} is not null.
* <p>
* If it isn't installed {@link SupportMapFragment} (and
* {@link com.google.android.gms.maps.MapView MapView}) will show a prompt for the user to
* install/update the Google Play services APK on their device.
* <p>
* A user can return to this FragmentActivity after following the prompt and correctly
* installing/updating/enabling the Google Play services. Since the FragmentActivity may not have been
* completely destroyed during this process (it is likely that it would only be stopped or
* paused), {@link #onCreate(Bundle)} may not be called again so we should call this method in
* {@link #onResume()} to guarantee that it will be called.
*/
private void setUpMapIfNeeded() {
// Do a null check to confirm that we have not already instantiated the map.
if (mMap == null) {
// Try to obtain the map from the SupportMapFragment.
mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map))
.getMap();
// Check if we were successful in obtaining the map.
if (mMap != null) {
setUpMap();
}
}
}
/**
* This is where we can add markers or lines, add listeners or move the camera. In this case, we
* just add a marker near Africa.
* <p>
* This should only be called once and when we are sure that {@link #mMap} is not null.
*/
private void setUpMap() {
mMap.addMarker(new MarkerOptions().position(new LatLng(0, 0)).title("Marker"));
}
}
Google有一种更方便的方法来获取地图，可以使用以下方法，因为它可能依赖于设备来获取准备好的地图！在地图准备就绪时，请使用以下方法来获取地图。
myMapFragment.getMapAsync(new OnMapReadyCallback) {
@Override
public void onMapReady(GoogleMap googleMap) {
myMap = googleMap;
}
});
我有一台运行Android 4.2.2的Android Tab3，我所要做的就是更新谷歌地图，它就可以解决问题了。