Question

I have a module I created for Google Maps v3 that I’m trying to convert into a Backbone.js view constructor.

Here’s my view module so far: I’ll explain the problems I’m having after the code:

pg.views.CreateMap = Backbone.View.extend({

  tagName:  "div",
  className: "map",

  events: {},

  latitude:   "-23.56432",
  longitude:  "-46.65183", 

  initialize: function() {
    _.bindAll(this, 'render', 'dragMarker', 'dragMap');

    this.latlng = new google.maps.LatLng(this.latitude, this.longitude);
    var myOptions = {
      zoom: 16,
      center: this.latlng,
      mapTypeId: google.maps.MapTypeId.ROADMAP
    };
    this.map = new google.maps.Map($(this.el)[0], myOptions);
    this.marker = new google.maps.Marker({
      map: this.map,
      position: this.latlng, 
      draggable: true
    });

    google.maps.event.addListener(this.marker, "dragend", this.dragMarker());

    google.maps.event.addListener(this.map, "dragend", this.dragMap());

  },

  render: function() {
    return this;
  },

  dragMarker: function() {
    this.latlng = this.marker.getPosition();
    this.map.panTo(this.latlng);
  },

  dragMap: function() {
    this.latlng = this.map.getCenter();
    this.marker.setPosition(this.latlng);
  }

});

The problem I’m having is with the Google Maps event listeners and how “this” is handled.

I originally didn’t have the dragMarker and dragMap methods and instead these two in the initialize block:

google.maps.event.addListener(this.marker, "dragend", function() {
  this.latlng = this.marker.getPosition();
  this.map.panTo(this.latlng);
});

google.maps.event.addListener(this.map, "dragend", function() {
  this.latlng = this.map.getCenter();
  this.marker.setPosition(this.latlng);
});

The problem I encountered with this first approach is that “this” inside those anonymous functions referred to “this.marker” and “this.map” respectively. The problem with this first approach was that in the first listener, I had no way of referring to “this.map” and therefore could not perform a panTo(). With the second listener, I had no way of referring to “this.marker” and therefore could not recenter the the map around that marker using setPosition().

I then thought that I could pull out the anonymous functions in the listeners and declare them as methods of the view, which I would then perform a _.bindAll(this, “dragMarker”, “dragMap”);

The problem with this approach is that I then had to write the listeners in the event block like so:

google.maps.event.addListener(this.marker, "dragend", this.dragMarker());

google.maps.event.addListener(this.map, "dragend", this.dragMap());

This meant that when I called the constructor with newmap = new pg.views.CreateMap; that the “this.dragMarker()” and “this.dragMap()” were evaluated immediately instead of being evaluated as a callback when the “dragend” event is triggered.

No problem I thought and then wrapped those up in anonymous functions like so:

google.maps.event.addListener(this.marker, "dragend", function() {
  this.dragMarker();
});

google.maps.event.addListener(this.map, "dragend", function() {
  this.dragMap();
});

Unfortunately this also brings me back to an earlier problem that the “this” in “this.dragMarker” no longer refers to the parent object I constructed, but instead refer to “this.marker” again. The same problem occurs with the second listener.

I’m completely stuck here. Anyone have any ideas on how I solve this?

Answer 1

Take the anonymous functions called on dragend and bind explicitly.

_.bindAll(this, 'dragMarker', 'dragMap');
google.maps.event.addListener(this.marker, "dragend", this.dragMarker);
/* etc ... */

This way this will always be tied to CreateMap even if called out of context.