设备离线时，谷歌地图屏幕显示为空白

Question

我正在为我的应用程序使用xamarin表单。

我正在将以下代码用于Google地图。该代码可以在线正常运行。当我杀死该应用程序并在离线状态下打开该应用程序时，地图将显示为附件图像。 谁能帮我解决这个问题。

formmap.xaml：

<local:CustomMap x:Name="customMap" Grid.Row="0" Grid.Column="0" Margin="0" MapType="Street" HasZoomEnabled="True" IsShowingUser="True" WidthRequest="{x:Static default:App.ScreenWidth}" HeightRequest="{x:Static default:App.ScreenHeight}" VerticalOptions="FillAndExpand" />

formmap.xaml.cs

customMap.MoveToRegion(MapSpan.FromCenterAndRadius(new Position(13.008677, 80.208194), Xamarin.Forms.Maps.Distance.FromMiles(6)));

androidmenifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="internalOnly" package="myapp" android:versionName="1.0.1" android:versionCode="6">
 <uses-sdk android:minSdkVersion="20" android:targetSdkVersion="28" />
 <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
 <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
 <uses-permission android:name="android.permission.INTERNET" />
 <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
 <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
 <uses-permission android:name="android.permission.ACCESS_LOCATION_EXTRA_COMMANDS" />
 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
 <uses-permission android:name="android.permission.WAKE_LOCK" />
 <uses-permission android:name="android.permission.ACCESS_LOCATION_EXTRA_COMMANDS" /> 
 <uses-permission android:name="android.permission.ACCESS_MOCK_LOCATION" />
 <uses-permission android:name="android.permission.CALL_PHONE" android:required="false" />
 <application android:label="myapp" android:largeHeap="true" android:icon="@drawable/Icon" android:theme="@style/MyTheme">
 <uses-library android:name="org.apache.http.legacy" android:required="false"/>
 <meta-data android:name="com.google.android.geo.API_KEY" android:value="mykey" />
 <provider
 android:name="android.support.v4.content.FileProvider"
 android:authorities="myapp.fileprovider"
 android:exported="false"
 android:grantUriPermissions="true">
 <meta-data
 android:name="android.support.FILE_PROVIDER_PATHS"
 android:resource="@xml/file_paths" />
 </provider>
 </application>
</manifest>

CustomMap.cs：

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Text;
using Xamarin.Forms;
using Xamarin.Forms.Maps;

namespace myprj.CustomControls
{
 public class CustomMap : Map
 {
 public static readonly BindableProperty RouteCoordinatesProperty =
 BindableProperty.Create<CustomMap, ObservableCollection<CustomPosition>>(p => p.RouteCoordinates, new ObservableCollection<CustomPosition>());


 public static readonly BindableProperty CustomPinsProperty =
 BindableProperty.Create<CustomMap, ObservableCollection<CustomPin>>(p => p.CustomPins, new ObservableCollection<CustomPin>());

 public ObservableCollection<CustomPosition> RouteCoordinates
 {
 get { return (ObservableCollection<CustomPosition>)GetValue(RouteCoordinatesProperty); }
 set { SetValue(RouteCoordinatesProperty, value); }
 }

 public ObservableCollection<CustomPin> CustomPins
 {
 get { return (ObservableCollection<CustomPin>)GetValue(CustomPinsProperty); }
 set { SetValue(CustomPinsProperty, value); }
 }
 public CustomMap()
 {
 RouteCoordinates = new ObservableCollection<CustomPosition>();
 }
 public CustomMap(MapSpan region) : base(region)
 {

 }

 }
}

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