Google Maps JavaScript API v3

Events

  1. Overview
    1. UI Events
    2. MVC State Changes
  2. Handling Events
    1. Example: Map and Marker Events
    2. Example: Shape Editing and Dragging Events
  3. Accessing Arguments in UI Events
  4. Using Closures in Event Listeners
  5. Getting and Setting Properties within Event Handlers
  6. Listening to DOM Events
  7. Removing Event Listeners

Overview

JavaScript within the browser is event driven, meaning that JavaScript responds to interactions by generating events, and expects a program to listen to interesting events. There are two types of events:

  • User events (such as "click" mouse events) are propagated from the DOM to the Google Maps API. These events are separate and distinct from standard DOM events.
  • MVC state change notifications reflect changes in Maps API objects and are named using a property_changed convention.

Each Maps API object exports a number of named events. Programs interested in certain events will register JavaScript event listeners for those events and execute code when those events are received by registering addListener() event handlers on the google.maps.event namespace.

For a complete list of events, consult the Maps API Reference. Events are listed in a separate section for each object which contains events.

UI Events

Some objects within the Maps API are designed to respond to user events such as mouse or keyboard events. For example, these are some of the user events that a google.maps.Marker object can listen to:

  • 'click'
  • 'dblclick'
  • 'mouseup'
  • 'mousedown'
  • 'mouseover'
  • 'mouseout'

For the full list, see the Marker class. These events may look like standard DOM events, but they are actually part of the Maps API. Because different browsers implement different DOM event models, the Maps API provides these mechanisms to listen for and respond to DOM events without needing to handle the various cross-browser peculiarities. These events also typically pass arguments within the event noting some UI state (such as the mouse position).

MVC State Changes

MVC objects typically contain state. Whenever an object's property changes, the API will fire an event that the property has changed. For example, the API will fire a zoom_changed event on a map when the map's zoom level changes. You can intercept these state changes by registering addListener() event handlers on the event namespace method as well.

User events and MVC state changes may look similar, but you generally wish to treat them differently in your code. MVC events, for example, do not pass arguments within their event. You will want to inspect the property that changed on an MVC state change by calling the appropriate getProperty method on that object.

Handling Events

To register for event notifications, use the addListener() event handler. That method takes an object, an event to listen for, and a function to call when the specified event occurs.

Example: Map and Marker Events

The following code mixes user events with state change events. We attach an event handler to a marker that zooms the map when clicked. We also add an event handler to the map for changes to the center property and pan the map back to the marker after 3 seconds on receipt of the center_changed event:

function initialize() {
  var mapOptions = {
    zoom: 4,
    center: new google.maps.LatLng(-25.363882, 131.044922)
  };

  var map = new google.maps.Map(document.getElementById('map-canvas'),
      mapOptions);

  var marker = new google.maps.Marker({
    position: map.getCenter(),
    map: map,
    title: 'Click to zoom'
  });

  google.maps.event.addListener(map, 'center_changed', function() {
    // 3 seconds after the center of the map has changed, pan back to the
    // marker.
    window.setTimeout(function() {
      map.panTo(marker.getPosition());
    }, 3000);
  });

  google.maps.event.addListener(marker, 'click', function() {
    map.setZoom(8);
    map.setCenter(marker.getPosition());
  });
}

google.maps.event.addDomListener(window, 'load', initialize);

View example (event-simple.html)

Tip: If you're trying to detect a change in the viewport, be sure to use the specific bounds_changed event rather than constituent zoom_changed and center_changed events. Because the Maps API fires these latter events independently, getBounds() may not report useful results until after the viewport has authoritatively changed. If you wish to getBounds() after such an event, be sure to listen to the bounds_changed event instead.

Example: Shape Editing and Dragging Events

When a shape is edited or dragged, an event is fired upon completion of the action. For a list of the events and some code snippets, see Shapes.

View example (rectangle-event.html)

Accessing Arguments in UI Events

UI events within the Google Maps API V3 typically pass an event argument, which can be accessed by the event listener, noting the UI state when the event occurred. For example, a UI 'click' event typically passes a MouseEvent containing a latLng property denoting the clicked location on the map. Note that this behavior is unique to UI events; MVC state changes do not pass arguments in their events.

You can access the event's arguments within an event listener the same way you would access an object's properties. The following example adds an event listener for the map, and creates a marker when the user clicks on the map at the clicked location.

var map;
function initialize() {
  var myLatlng = new google.maps.LatLng(-25.363882,131.044922);
  var mapOptions = {
    zoom: 4,
    center: myLatlng
  }
  map = new google.maps.Map(document.getElementById("map-canvas"), mapOptions);

  google.maps.event.addListener(map, 'click', function(event) {
    placeMarker(event.latLng);
  });
}

function placeMarker(location) {
  var marker = new google.maps.Marker({
      position: location,
      map: map
  });

  map.setCenter(location);
}

View example (event-arguments.html)

Using Closures in Event Listeners

When executing an event listener, it is often advantageous to have both private and persistent data attached to an object. JavaScript does not support "private" instance data, but it does support closures which allows inner functions to access outer variables. Closures are useful within event listeners to access variables not normally attached to the objects on which events occur.

The following example uses a function closure in the event listener to assign a secret message to a set of markers. Clicking on each marker will review a portion of the secret message, which is not contained within the marker itself.

var map;
function initialize() {
  var myLatlng = new google.maps.LatLng(-25.363882,131.044922);
  var mapOptions = {
    zoom: 4,
    center: myLatlng
  }

  map = new google.maps.Map(document.getElementById("map-canvas"), mapOptions);

  // Add 5 markers to the map at random locations.
  var southWest = new google.maps.LatLng(-31.203405,125.244141);
  var northEast = new google.maps.LatLng(-25.363882,131.044922);
  var bounds = new google.maps.LatLngBounds(southWest,northEast);
  map.fitBounds(bounds);
  var lngSpan = northEast.lng() - southWest.lng();
  var latSpan = northEast.lat() - southWest.lat();
  for (var i = 0; i < 5; i++) {
    var location = new google.maps.LatLng(southWest.lat() + latSpan * Math.random(),
        southWest.lng() + lngSpan * Math.random());
    var marker = new google.maps.Marker({
        position: location,
        map: map
    });
    var j = i + 1;
    marker.setTitle(j.toString());
    attachSecretMessage(marker, i);
  }
}

// The five markers show a secret message when clicked
// but that message is not within the marker's instance data.
function attachSecretMessage(marker, number) {
  var message = ["This","is","the","secret","message"];
  var infowindow = new google.maps.InfoWindow(
      { content: message[number],
        size: new google.maps.Size(50,50)
      });
  google.maps.event.addListener(marker, 'click', function() {
    infowindow.open(map,marker);
  });
}

View example (event-closure.html)

Getting and Setting Properties within Event Handlers

None of the MVC state change events in the Maps API event system pass arguments when the event is triggered. (User events do pass arguments which can be inspected.) If you need to inspect a property on an MVC state change, you should explicitly call the appropriate getProperty() method on that object. This inspection will always retrieve the current state of the MVC object, which may not be the state when the event was first fired.

Note: Explicitly setting a property within an event handler which responds to a state change of that particular property may produce unpredictable and/or unwanted behavior. Setting such a property will trigger a new event, for example, and if you always set a property within this event handler, you may end up creating an infinite loop.

In the example below, we set up an event handler to respond to zoom events by bringing up an info window displaying that level.

function initialize() {
  var myLatLng = new google.maps.LatLng(-25.363882, 131.044922);
  var mapOptions = {
    zoom: 4,
    center: myLatLng
  };

  var map = new google.maps.Map(document.getElementById('map-canvas'),
      mapOptions);

  var infowindow = new google.maps.InfoWindow({
    content: 'Change the zoom level',
    position: myLatLng
  });
  infowindow.open(map);

  google.maps.event.addListener(map, 'zoom_changed', function() {
    var zoomLevel = map.getZoom();
    map.setCenter(myLatLng);
    infowindow.setContent('Zoom: ' + zoomLevel);
  });
}

google.maps.event.addDomListener(window, 'load', initialize);

View example (event-properties.html)

Listening to DOM Events

The Google Maps JavaScript API event model creates and manages its own custom events. However, the DOM (Document Object Model) within the browser also creates and dispatches its own events, according to the particular browser event model in use. If you wish to capture and respond to these events, the Maps API provides the addDomListener() static method to listen to and bind to DOM events.

This convenience method has a signature as shown below:

addDomListener(instance:Object, eventName:string, handler:Function)

where instance may be any DOM element supported by the browser, including:

  • Hierarchical members of the DOM such as window or document.body.myform
  • Named elements such as document.getElementById("foo")

Note that addDomListener() simply passes the indicated event to the browser, which handles it according to the browser's DOM event model; however, almost all modern browsers at least support DOM Level 2. (For more information on DOM level events, see the Mozilla DOM Levels reference.)

If you've been reading the documentation this far, you're probably already familiar with one DOM event: the window.onload event, which we've handled within the <body> tag. We use this event to trigger the initial JavaScript code once an HTML page is fully loaded, as shown below:

<script>
  function initialize() {

    // Map initialization

  }
</script>
<body onload="initialize()">
  <div id="map-canvas"></div>
</body>

Although this event is attached to the <body> element here, this event is really a window event indicating that the DOM hierarchy below the window element has been fully built out and rendered.

Although easy to understand, having an onload event within a <body> tag mixes content with behavior. Generally, it is good practice to separate your content code (HTML) from your behavioral code (JavaScript) and provide your presentation code (CSS) separately as well. You can do so by replacing the inline onload event handler with a DOM listener within your Maps API JavaScript code like so:

<script>
  function initialize() {

    // Map initialization

  }

  google.maps.event.addDomListener(window, 'load', initialize);
</script>
<body>
  <div id="map-canvas"></div>
</body>

View example (event-domListener.html)

Although the above code is Maps JavaScript API code, the addDomListener() method binds to the window object of the browser and allows the API to communicate with objects outside of the API's normal domain.

Removing Event Listeners

To remove a specific event listener, it must have been assigned to a variable. You can then call removeListener(), passing the variable name to which the listener was assigned.

var listener1 = google.maps.event.addListener(marker, 'click', aFunction);

google.maps.event.removeListener(listener1);

To remove all listeners from a particular instance, call clearInstanceListeners(), passing the instance name.

var listener1 = google.maps.event.addListener(marker, 'click', aFunction);
var listener2 = google.maps.event.addListener(marker, 'mouseover', bFunction);

// Remove listener1 and listener2 from marker instance.
google.maps.event.clearInstanceListeners(marker);

To remove all listeners for a specific event type for a specific instance, call clearListeners(), passing the instance name and the event name.

marker.addListener('click', aFunction);
marker.addListener('click', bFunction);
marker.addListener('click', cFunction);

// Remove all click listeners from marker instance.
google.maps.event.clearListeners(marker, 'click');

For more information, refer to the reference documentation for the google.maps.event namespace.

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