[android] source list accessor + example

platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/RuntimeStyleTests.java

@@ -85,6 +85,26 @@ public class RuntimeStyleTests {
     onView(withId(R.id.mapView)).perform(new AddRemoveLayerAction());
   }
 
+  @Test
+  public void testListSources() {
+    ViewUtils.checkViewIsDisplayed(R.id.mapView);
+    onView(withId(R.id.mapView)).perform(new BaseViewAction() {
+
+      @Override
+      public void perform(UiController uiController, View view) {
+        MapboxMap mapboxMap = rule.getActivity().getMapboxMap();
+
+        List<Source> sources = mapboxMap.getSources();
+        assertNotNull(sources);
+        assertTrue(sources.size() > 0);
+        for (Source source: sources) {
+          assertNotNull(source);
+        }
+      }
+
+    });
+  }
+
   @Test
   public void testAddRemoveSource() {
     ViewUtils.checkViewIsDisplayed(R.id.mapView);

platform/android/MapboxGLAndroidSDKTestApp/src/main/java/com/mapbox/mapboxsdk/testapp/activity/style/RuntimeStyleActivity.java

@@ -154,6 +154,9 @@ public class RuntimeStyleActivity extends AppCompatActivity {
       case R.id.action_list_layers:
         listLayers();
         return true;
+      case R.id.action_list_sources:
+        listSources();
+        return true;
       case R.id.action_water_color:
         setWaterColor();
         return true;
@@ -211,6 +214,16 @@ public class RuntimeStyleActivity extends AppCompatActivity {
     Toast.makeText(this, builder.toString(), Toast.LENGTH_LONG).show();
   }
 
+  private void listSources() {
+    List<Source> sources = mapboxMap.getSources();
+    StringBuilder builder = new StringBuilder("Sources:");
+    for (Source source : sources) {
+      builder.append("\n");
+      builder.append(source.getId());
+    }
+    Toast.makeText(this, builder.toString(), Toast.LENGTH_LONG).show();
+  }
+
   private void setLayerInvisible() {
     String[] roadLayers = new String[] {"water"};
     for (String roadLayer : roadLayers) {

platform/android/MapboxGLAndroidSDKTestApp/src/main/res/menu/menu_runtime_style.xml

@@ -6,6 +6,10 @@
         android:id="@+id/action_list_layers"
         android:title="List all layers in the style"
         mapbox:showAsAction="never" />
+    <item
+        android:id="@+id/action_list_sources"
+        android:title="List all sources in the style"
+        mapbox:showAsAction="never" />
     <item
         android:id="@+id/action_water_color"
         android:title="Color the water"