feat(utils): add triggerEvent api

* Closes #161

src/GoogleMap.js

@@ -102,6 +102,7 @@ Use "GoogleMapLoader" instead. See https://github.com/tomchentw/react-google-map
 
       return (
         <GoogleMapLoader
+          ref="loader"
           containerElement={React.createElement(realContainerTagName, containerProps)}
           googleMapElement={
             <GoogleMap ref="delegate" containerTagName={USE_NEW_BEHAVIOR_TAG_NAME} {...mapProps}>

src/utils/index.js

@@ -0,0 +1,31 @@
+import {
+  default as invariant,
+} from "invariant";
+
+import {
+  default as GoogleMap,
+} from "../GoogleMap";
+
+export function __getInstance (component) {
+  const {constructor} = component;
+  if (GoogleMap === constructor) {
+    if (component.props.map) {
+      return component.props.map;
+    } else {
+      // Deprecated
+      component = component.refs.loader;
+    }
+  }
+  // Otherwise, every other instance type exists in component state
+  const key = Object.keys(component.state)[0];
+  return component.state[key];
+}
+
+export function triggerEvent (component, ...args) {
+  const instance = __getInstance(component);
+  invariant(instance,
+`The react-google-maps component %s is not mounted, hence we can't find an
+ associated Google Maps JavaScript API v3 instance with it.`, component);
+
+  return google.maps.event.trigger(instance, ...args);
+}