MapView 的getMap() 方法可以返回null。我知道这是 Google 的预期行为。

有人可以提供关于 getMap() 方法何时以及在何种情况下返回 null 的明确描述吗？

我知道如果 Google 服务在给定设备上不可用，getMap() 将返回 null。这种可能性是有据可查的。我更关心其他模糊的情况，即使设备上安装了 Google 服务，getMap() 仍然可以返回 null。

到目前为止，我的假设是底层 map 系统有一些初始化，在此期间您的代码可能会执行并获得一个空 map 。

我的假设是否正确？

在 Activity 或 Fragment 生命周期中是否有任何特定位置可以明确获得非空 GoogleMap(如果我们假设已安装 Google 服务)？

我提出这个问题的目的是防止在我的代码中乱扔一连串 if(mapView.getMap() != null) 检查。此外，这个问题似乎仍然在 StackOverflow 上定期出现，我想看看我们是否可以充实 MapView 和 究竟发生了什么背后的真相getMap()

最佳答案

Google 处理的场景 如果未安装 google play apk，您将获得空值。但它以某种方式内置以从 Playstore 下载它。但是可能没有安装 playstore，所以你得到一个空 map 。如果互联网不可用并且已安装 playstore 但未安装 google play 服务，则您将获得 null，因为无法安装 google play 服务。但是这些场景的编码已经为您完成了。

您的代码处理的场景。 这就是您必须编码的内容。如果您偏离 map 示例中概述的模式，您将获得一个空 map 。每个样本都以相同的方式进行。因此，如果 onCreate 中的第一次调用失败，仍有第二次机会在 onResume 中获取 map 。您的解决方案是仅在一处调用 getMap() 并保存返回的 map 对象。遵循 map 示例使用的基本模式。在 onCreate 和 onResume 方法中，您调用 setupmapifneeded 本地图对象不为 null 时又调用 setupmap。

这是第一个 map 示例。

/*
 * 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"));
    }
}

关于Android MapView getMap() 返回 null，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/22471766/