Xamarin.Forms片段在ViewRenderer中不可见

Question

Click here for Screenshot我正在尝试使用Xamarin.Forms为andriod显示谷歌地图。我想在屏幕的一半显示地图，所以我将ContentView添加到我的ContentPage。

 public partial class FacilitiesProductivity : ContentPage
{
    public FacilitiesProductivity()
    {            
        InitializeComponent();
        Content = new ContentView { Content = new ProductivityMapPage(new Managers.Facility()) };
    }
}

ProductivityMapPage.xaml

    <?xml version="1.0" encoding="utf-8" ?>
<local:ProductivityMapCustomContentView xmlns="http://xamarin.com/schemas/2014/forms" 
                  xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
                  xmlns:maps="clr-namespace:Xamarin.Forms.Maps;assembly=Xamarin.Forms.Maps" 
                  xmlns:local="clr-namespace:RadLocWF;assembly=RadLocWF" 
                  x:Class="RadLocWF.views.ProductivityMapPage"/>

ProductivityMapCustomContentView.cs

  public class ProductivityMapCustomContentView : ContentView
{
}

ProductivityMapRenderer.cs

    [assembly: ExportRenderer(typeof(ProductivityMapCustomContentView), typeof(ProductivityMapRenderer))]
namespace RadLoc.Droid.Renderer
{
    class ProductivityMapRenderer : ViewRenderer, IOnMapReadyCallback
    {
        Location _location;
        Android.Views.View _view;
        GoogleMap _map;
        Marker currentLocationMarker;

        public ProductivityMapRenderer()
        {
            SetUpMap();
        }

        public void SetUpMap()
        {
                var activity = this.Context as Activity;
                MapFragment mapFrag = (MapFragment)activity.FragmentManager.FindFragmentById(Resource.Id.maps);
                mapFrag.GetMapAsync(this);    
        }
        public void OnMapReady(GoogleMap googleMap)
        {
            _map = googleMap;
            _map.MyLocationEnabled = true;
            _map.MapType = GoogleMap.MapTypeNormal;
            _map.UiSettings.CompassEnabled = true;
            _map.UiSettings.MyLocationButtonEnabled = true;
            _map.UiSettings.ZoomControlsEnabled = true;
            double lat = 0, lng = 0;

            LatLng location = new LatLng(lat, lng);
            CameraPosition.Builder builder = CameraPosition.InvokeBuilder();
            builder.Target(location);
            builder.Zoom(12f);
            builder.Bearing(90);
            builder.Tilt(40);
            CameraPosition cameraPosition = builder.Build();
            CameraUpdate cameraUpdate = CameraUpdateFactory.NewCameraPosition(cameraPosition);
            _map?.AnimateCamera(cameraUpdate);
        }

        protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.View> e)
        {  
                base.OnElementChanged(e);

                var activity = this.Context as Activity;                
                _view = activity.LayoutInflater.Inflate(Resource.Layout.ProductivityMapLayout, this, false);                
                var mapFragment = activity.FragmentManager.FindFragmentById<MapFragment>(Resource.Id.maps);

                AddView(_view);         
        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            if (!isMapReady)
            {
                base.OnLayout(changed, l, t, r, b);
                var msw = MeasureSpec.MakeMeasureSpec(r - l, MeasureSpecMode.Exactly);
                var msh = MeasureSpec.MakeMeasureSpec(b - t, MeasureSpecMode.Exactly);
                _view.Measure(msw, msh);
                _view.Layout(0, 0, r - l, b - t);
            }
        }
        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);           
            var activity = this.Context as Activity;
            var mapFragment = activity.FragmentManager.BeginTransaction().Remove(activity.FragmentManager.FindFragmentById<MapFragment>(Resource.Id.maps)).Commit();
        }
    }

ProductivityMapLayout.axml

   <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <View
        android:layout_height="0dp"
        android:layout_width="fill_parent"
        android:layout_weight="1" />
    <fragment
        class="com.google.android.gms.maps.MapFragment"
        android:id="@+id/maps"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_marginRight="0.0dp"
        android:layout_marginTop="0.0dp"
        android:layout_marginBottom="0.0dp" />
</LinearLayout>

代码没有给出任何错误，但显示黑屏。如果单独的屏幕上出现Google标记可能是Google API Key问题，但我只是白屏。因此，渲染必定存在问题。任何人都可以帮我解决问题。谢谢！