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Filename : BBOX.js
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/* Copyright (c) 2006-2013 by OpenLayers Contributors (see authors.txt for
 * full list of contributors). Published under the 2-clause BSD license.
 * See license.txt in the OpenLayers distribution or repository for the
 * full text of the license. */

/**
 * @requires OpenLayers/Strategy.js
 * @requires OpenLayers/Filter/Spatial.js
 */

/**
 * Class: OpenLayers.Strategy.BBOX
 * A simple strategy that reads new features when the viewport invalidates
 *     some bounds.
 *
 * Inherits from:
 *  - <OpenLayers.Strategy>
 */
OpenLayers.Strategy.BBOX = OpenLayers.Class(OpenLayers.Strategy, {
    
    /**
     * Property: bounds
     * {<OpenLayers.Bounds>} The current data bounds (in the same projection
     *     as the layer - not always the same projection as the map).
     */
    bounds: null,
    
    /** 
     * Property: resolution 
     * {Float} The current data resolution. 
     */ 
    resolution: null, 
           
    /**
     * APIProperty: ratio
     * {Float} The ratio of the data bounds to the viewport bounds (in each
     *     dimension).  Default is 2.
     */
    ratio: 2,

    /** 
     * Property: resFactor 
     * {Float} Optional factor used to determine when previously requested 
     *     features are invalid.  If set, the resFactor will be compared to the
     *     resolution of the previous request to the current map resolution.
     *     If resFactor > (old / new) and 1/resFactor < (old / new).  If you
     *     set a resFactor of 1, data will be requested every time the
     *     resolution changes.  If you set a resFactor of 3, data will be
     *     requested if the old resolution is 3 times the new, or if the new is
     *     3 times the old.  If the old bounds do not contain the new bounds
     *     new data will always be requested (with or without considering
     *     resFactor). 
     */ 
    resFactor: null, 
    
    /**
     * Property: response
     * {<OpenLayers.Protocol.Response>} The protocol response object returned
     *      by the layer protocol.
     */
    response: null,

    /**
     * Constructor: OpenLayers.Strategy.BBOX
     * Create a new BBOX strategy.
     *
     * Parameters:
     * options - {Object} Optional object whose properties will be set on the
     *     instance.
     */
    
    /**
     * Method: activate
     * Set up strategy with regard to reading new batches of remote data.
     * 
     * Returns:
     * {Boolean} The strategy was successfully activated.
     */
    activate: function() {
        var activated = OpenLayers.Strategy.prototype.activate.call(this);
        if(activated) {
            this.layer.events.on({
                "moveend": this.update,
                "refresh": this.update,
                "visibilitychanged": this.update,
                scope: this
            });
            this.update();
        }
        return activated;
    },
    
    /**
     * Method: deactivate
     * Tear down strategy with regard to reading new batches of remote data.
     * 
     * Returns:
     * {Boolean} The strategy was successfully deactivated.
     */
    deactivate: function() {
        var deactivated = OpenLayers.Strategy.prototype.deactivate.call(this);
        if(deactivated) {
            this.layer.events.un({
                "moveend": this.update,
                "refresh": this.update,
                "visibilitychanged": this.update,
                scope: this
            });
        }
        return deactivated;
    },

    /**
     * Method: update
     * Callback function called on "moveend" or "refresh" layer events.
     *
     * Parameters:
     * options - {Object} Optional object whose properties will determine
     *     the behaviour of this Strategy
     *
     * Valid options include:
     * force - {Boolean} if true, new data must be unconditionally read.
     * noAbort - {Boolean} if true, do not abort previous requests.
     */
    update: function(options) {
        var mapBounds = this.getMapBounds();
        if (mapBounds !== null && ((options && options.force) ||
          (this.layer.visibility && this.layer.calculateInRange() && this.invalidBounds(mapBounds)))) {
            this.calculateBounds(mapBounds);
            this.resolution = this.layer.map.getResolution(); 
            this.triggerRead(options);
        }
    },
    
    /**
     * Method: getMapBounds
     * Get the map bounds expressed in the same projection as this layer.
     *
     * Returns:
     * {<OpenLayers.Bounds>} Map bounds in the projection of the layer.
     */
    getMapBounds: function() {
        if (this.layer.map === null) {
            return null;
        }
        var bounds = this.layer.map.getExtent();
        if(bounds && !this.layer.projection.equals(
                this.layer.map.getProjectionObject())) {
            bounds = bounds.clone().transform(
                this.layer.map.getProjectionObject(), this.layer.projection
            );
        }
        return bounds;
    },

    /**
     * Method: invalidBounds
     * Determine whether the previously requested set of features is invalid. 
     *     This occurs when the new map bounds do not contain the previously 
     *     requested bounds.  In addition, if <resFactor> is set, it will be 
     *     considered.
     *
     * Parameters:
     * mapBounds - {<OpenLayers.Bounds>} the current map extent, will be
     *      retrieved from the map object if not provided
     *
     * Returns:
     * {Boolean} 
     */
    invalidBounds: function(mapBounds) {
        if(!mapBounds) {
            mapBounds = this.getMapBounds();
        }
        var invalid = !this.bounds || !this.bounds.containsBounds(mapBounds);
        if(!invalid && this.resFactor) {
            var ratio = this.resolution / this.layer.map.getResolution();
            invalid = (ratio >= this.resFactor || ratio <= (1 / this.resFactor));
        }
        return invalid;
    },
 
    /**
     * Method: calculateBounds
     *
     * Parameters:
     * mapBounds - {<OpenLayers.Bounds>} the current map extent, will be
     *      retrieved from the map object if not provided
     */
    calculateBounds: function(mapBounds) {
        if(!mapBounds) {
            mapBounds = this.getMapBounds();
        }
        var center = mapBounds.getCenterLonLat();
        var dataWidth = mapBounds.getWidth() * this.ratio;
        var dataHeight = mapBounds.getHeight() * this.ratio;
        this.bounds = new OpenLayers.Bounds(
            center.lon - (dataWidth / 2),
            center.lat - (dataHeight / 2),
            center.lon + (dataWidth / 2),
            center.lat + (dataHeight / 2)
        );
    },
    
    /**
     * Method: triggerRead
     *
     * Parameters:
     * options - {Object} Additional options for the protocol's read method 
     *     (optional)
     *
     * Returns:
     * {<OpenLayers.Protocol.Response>} The protocol response object
     *      returned by the layer protocol.
     */
    triggerRead: function(options) {
        if (this.response && !(options && options.noAbort === true)) {
            this.layer.protocol.abort(this.response);
            this.layer.events.triggerEvent("loadend");
        }
        var evt = {filter: this.createFilter()};
        this.layer.events.triggerEvent("loadstart", evt);
        this.response = this.layer.protocol.read(
            OpenLayers.Util.applyDefaults({
                filter: evt.filter,
                callback: this.merge,
                scope: this
        }, options));
    },
 
    /**
     * Method: createFilter
     * Creates a spatial BBOX filter. If the layer that this strategy belongs
     * to has a filter property, this filter will be combined with the BBOX 
     * filter.
     * 
     * Returns
     * {<OpenLayers.Filter>} The filter object.
     */
    createFilter: function() {
        var filter = new OpenLayers.Filter.Spatial({
            type: OpenLayers.Filter.Spatial.BBOX,
            value: this.bounds,
            projection: this.layer.projection
        });
        if (this.layer.filter) {
            filter = new OpenLayers.Filter.Logical({
                type: OpenLayers.Filter.Logical.AND,
                filters: [this.layer.filter, filter]
            });
        }
        return filter;
    },
   
    /**
     * Method: merge
     * Given a list of features, determine which ones to add to the layer.
     *     If the layer projection differs from the map projection, features
     *     will be transformed from the layer projection to the map projection.
     *
     * Parameters:
     * resp - {<OpenLayers.Protocol.Response>} The response object passed
     *      by the protocol.
     */
    merge: function(resp) {
        this.layer.destroyFeatures();
        if (resp.success()) {
            var features = resp.features;
            if(features && features.length > 0) {
                var remote = this.layer.projection;
                var local = this.layer.map.getProjectionObject();
                if(!local.equals(remote)) {
                    var geom;
                    for(var i=0, len=features.length; i<len; ++i) {
                        geom = features[i].geometry;
                        if(geom) {
                            geom.transform(remote, local);
                        }
                    }
                }
                this.layer.addFeatures(features);
            }
        } else {
            this.bounds = null;
        }
        this.response = null;
        this.layer.events.triggerEvent("loadend", {response: resp});
    },
   
    CLASS_NAME: "OpenLayers.Strategy.BBOX" 
});