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POBackbone.js with Google Maps - problems with this and listeners
 

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  1. POBackbone.js with Google Maps - problems with this and listeners
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    <p>I have a module I created for Google Maps v3 that I'm trying to convert into a Backbone.js view constructor.</p> <p>Here's my view module so far: I'll explain the problems I'm having after the code:</p> <pre><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); } }); </code></pre> <p>The problem I'm having is with the Google Maps event listeners and how "this" is handled.</p> <p>I originally didn't have the dragMarker and dragMap methods and instead these two in the initialize block:</p> <pre><code>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); }); </code></pre> <p>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().</p> <p>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");</p> <p>The problem with this approach is that I then had to write the listeners in the event block like so:</p> <pre><code>google.maps.event.addListener(this.marker, "dragend", this.dragMarker()); google.maps.event.addListener(this.map, "dragend", this.dragMap()); </code></pre> <p>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. </p> <p>No problem I thought and then wrapped those up in anonymous functions like so:</p> <pre><code>google.maps.event.addListener(this.marker, "dragend", function() { this.dragMarker(); }); google.maps.event.addListener(this.map, "dragend", function() { this.dragMap(); }); </code></pre> <p>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.</p> <p>I'm completely stuck here. Anyone have any ideas on how I solve this?</p>
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    1. USAndrew De Andrade
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      1. POBackbone.js with Google Maps - problems with this and listeners
      1. USAlan David Garcia
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      1. VTUpMod
    1. COThe correct way to attach event handler is like this - `google.maps.event.addListener(this.marker, "dragend", this.dragMarker);` (without the parentheses). But this won't solve your context problem.
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      1. POBackbone.js with Google Maps - problems with this and listeners
      1. USChetan Sastry
 

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