Add unit test

packages/google_maps_flutter/google_maps_flutter/android/src/main/java/io/flutter/plugins/googlemaps/GoogleMapController.java

@@ -16,6 +16,7 @@
 import android.view.View;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
+import androidx.annotation.VisibleForTesting;
 import androidx.lifecycle.DefaultLifecycleObserver;
 import androidx.lifecycle.Lifecycle;
 import androidx.lifecycle.LifecycleOwner;
@@ -110,6 +111,11 @@ public View getView() {
     return mapView;
   }
 
+  @VisibleForTesting
+  /*package*/ void setView(MapView view) {
+    mapView = view;
+  }
+
   void init() {
     lifecycleProvider.getLifecycle().addObserver(this);
     mapView.getMapAsync(this);
@@ -272,19 +278,23 @@ public void onSnapshotReady(Bitmap bitmap) {
           // To workaround this limitation, wait two frames.
           // This ensures that at least the frame budget (16.66ms at 60hz) have passed since the
           // drawing operation was issued.
-          googleMap.setOnMapLoadedCallback(
-              new GoogleMap.OnMapLoadedCallback() {
-                @Override
-                public void onMapLoaded() {
-                  postFrameCallback(
-                      () -> {
-                        postFrameCallback(
-                            () -> {
-                              mapView.invalidate();
-                            });
-                      });
-                }
-              });
+          if (googleMap != null) {
+            googleMap.setOnMapLoadedCallback(
+                new GoogleMap.OnMapLoadedCallback() {
+                  @Override
+                  public void onMapLoaded() {
+                    postFrameCallback(
+                        () -> {
+                          postFrameCallback(
+                              () -> {
+                                if (mapView != null) {
+                                  mapView.invalidate();
+                                }
+                              });
+                        });
+                  }
+                });
+          }
 
           result.success(null);
           break;

packages/google_maps_flutter/google_maps_flutter/android/src/test/java/io/flutter/plugins/googlemaps/GoogleMapControllerTest.java

@@ -4,6 +4,8 @@
 
 package io.flutter.plugins.googlemaps;
 
+import static org.mockito.Mockito.verify;
+import static org.mockito.ArgumentMatchers.any;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 
@@ -12,15 +14,21 @@
 import androidx.activity.ComponentActivity;
 import androidx.test.core.app.ApplicationProvider;
 import com.google.android.gms.maps.GoogleMap;
+import com.google.android.gms.maps.MapView;
 import io.flutter.plugin.common.BinaryMessenger;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import org.mockito.ArgumentCaptor;
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 import org.robolectric.Robolectric;
 import org.robolectric.RobolectricTestRunner;
 import org.robolectric.annotation.Config;
+import java.util.HashMap;
+import io.flutter.plugin.common.MethodChannel;
+import io.flutter.plugin.common.MethodCall;
+import static org.mockito.Mockito.mock;
 
 @RunWith(RobolectricTestRunner.class)
 @Config(sdk = Build.VERSION_CODES.P)
@@ -58,4 +66,19 @@ public void OnDestroyReleaseTheMap() throws InterruptedException {
     googleMapController.onDestroy(activity);
     assertNull(googleMapController.getView());
   }
+
+  @Test
+  public void InvalidateMapAfterMarkersUpdate() throws InterruptedException {
+    googleMapController.onMapReady(mockGoogleMap);
+    MethodChannel.Result result = mock(MethodChannel.Result.class);
+    googleMapController.onMethodCall(new MethodCall​("markers#update", new HashMap<String, Object>()), result);
+
+    ArgumentCaptor<GoogleMap.OnMapLoadedCallback> argument = ArgumentCaptor.forClass(GoogleMap.OnMapLoadedCallback.class);
+    verify(mockGoogleMap).setOnMapLoadedCallback(argument.capture());
+
+    MapView mapView = mock(MapView.class);
+    googleMapController.setView(mapView);
+    argument.getValue().onMapLoaded();
+    verify(mapView).invalidate();
+  }
 }