SupportMapFragment 地图为空

我使用以下代码在 Xamarin.Android 中显示地图：

private SupportMapFragment mapFragment;
private GoogleMap map;

protected override void OnCreate (Bundle bundle)
        {
            base.OnCreate (bundle);

            SetContentView (Resource.Layout.Main);

            mapFragment = SupportFragmentManager.FindFragmentByTag ("map") as SupportMapFragment;
            if (mapFragment == null) {
                GoogleMapOptions options = new GoogleMapOptions ()
                    .InvokeCompassEnabled (true)
                    .InvokeMapType (GoogleMap.MapTypeNone)
                    .InvokeZoomControlsEnabled (false);

                FragmentTransaction frx = SupportFragmentManager.BeginTransaction ();
                mapFragment = SupportMapFragment.NewInstance (options);
                frx.Add (Resource.Id.map,mapFragment,"map");
                frx.Commit ();
            }


if (map == null)
                map = mapFragment.Map;

            CircleOptions circle = new CircleOptions ();
            circle.InvokeCenter (new LatLng(18.5203,73.8567));
            circle.InvokeRadius (1000);
            map.AddCircle (circle);

我的 AXML 是

<fragment xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/map"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    class="com.google.android.gms.maps.SupportMapFragment" />

在设备上成功部署后，在线出现异常map.AddCircle (circle); as System.NullReferenceException已被抛出Object Reference is not set to an instance of an Object.

我究竟做错了什么？是不是有什么东西需要初始化？

这行：

SupportFragmentManager.FindFragmentByTag ("map") as SupportMapFragment;

当您尝试投射时不应该起作用View来到外面Mono.Android。因此你必须使用：

SupportFragmentManager.FindFragmentByTag<SupportMapFragment>("map");

or

SupportFragmentManager.FindFragmentByTag ("map").JavaCast<SupportMapFragment>();

您还试图通过以下方式找到片段Tag。然而，你并没有给出你的Fragment a Tag在你的XML中，所以你需要通过以下方式找到它Id反而：

var mapFragment = SupportFragmentManager.FindFragmentById<SupportMapFragment>(Resource.Id.map);

实际上，从您粘贴的代码中可以看出：

SupportFragmentManager.FindFragmentByTag ("map") as SupportMapFragment;

将始终为空，因为您实际上完全忽略了布局，并且您将始终使用手动创建的SupportMapFragment in the if (mapFragment == null)健康）状况...

另外，上次我检查时，FindFragmentByXXX不支持泛型类型，所以你可能必须这样做JavaCast<T>();

因此，我会修改我的代码，使其看起来更像：

private SupportMapFragment mapFragment;
private GoogleMap map;

protected override void OnCreate (Bundle bundle)
{
    base.OnCreate (bundle);

    SetContentView (Resource.Layout.Main);

    SetUpMapIfNeeded();
}

public override void OnResume()
{
    base.OnResume();
    SetUpMapIfNeeded();
}

private void SetUpMapIfNeeded()
{
    if(null != map) return;

    mapFragment = SupportFragmentManager.FindFragmentById(Resource.Id.map).JavaCast<SupportMapFragment>();
    if (mapFragment != null)
    {
        map = mapFragment.Map;

        if (map == null)
        {
            // throw error here, should never happen though...
            return;
        }

        // set up map here, i.e.:
        map.UiSettings.CompassEnabled = true;
        map.MapType = GoogleMap.MapTypeHybrid;
        map.MyLocationChange += MapOnMyLocationChange;

        CircleOptions circle = new CircleOptions ();
        circle.InvokeCenter (new LatLng(18.5203,73.8567));
        circle.InvokeRadius (1000);
        map.AddCircle (circle);
    }
}