OpenLayers源码学习21:XYZ

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看完WMTS之后, 发现XYZ这个类倒是最简单, 其实应该从XYZ这开始看

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OpenLayers.Layer.XYZ = OpenLayers.Class(OpenLayers.Layer.Grid, {
     
    isBaseLayer: true,
     
    //墨卡托投影 
    sphericalMercator: false,
 
    //zoom全局偏移 
    zoomOffset: 0,
     
    // 服务端分辨率列表
    serverResolutions: null,
 
    // 构造
    initialize: function(name, url, options) {
        if (options && options.sphericalMercator || this.sphericalMercator) {
            options = OpenLayers.Util.extend({//有墨卡托投影, 转成900913_19级
                projection: "EPSG:900913",
                numZoomLevels: 19
            }, options);
        }
        OpenLayers.Layer.Grid.prototype.initialize.apply(this, [
            name || this.name, url || this.url, {}, options
        ]);
    },
     
    //克隆 
    clone: function (obj) {
         
        if (obj == null) {
            obj = new OpenLayers.Layer.XYZ(this.name,
                                            this.url,
                                            this.getOptions());
        }
        //父类构造
        obj = OpenLayers.Layer.Grid.prototype.clone.apply(this, [obj]);
        return obj;
    },    
 
    // 给瓦片用的接口, 产生实际url
    getURL: function (bounds) {
        var xyz = this.getXYZ(bounds);
        var url = this.url;
        if (OpenLayers.Util.isArray(url)) {
            var s = '' + xyz.x + xyz.y + xyz.z;
            url = this.selectUrl(s, url);
        }
         
        return OpenLayers.String.format(url, xyz);
    },
     
    // 瓦片位置计算公式 
    getXYZ: function(bounds) {
        var res = this.getServerResolution();
        var x = Math.round((bounds.left - this.maxExtent.left) /
            (res * this.tileSize.w));// (传入bound - 地图边界最左)/(分辨率x瓦片宽)
        var y = Math.round((this.maxExtent.top - bounds.top) /
            (res * this.tileSize.h));
        var z = this.getServerZoom();
 
        if (this.wrapDateLine) {
            var limit = Math.pow(2, z);
            x = ((x % limit) + limit) % limit;
        }
 
        return {'x': x, 'y': y, 'z': z};
    },
     
    // 设置持有的map对象
    setMap: function(map) {
        OpenLayers.Layer.Grid.prototype.setMap.apply(this, arguments);
        if (!this.tileOrigin) { 
            this.tileOrigin = new OpenLayers.LonLat(this.maxExtent.left,
                                                this.maxExtent.bottom);
        }                                       
    },
 
    CLASS_NAME: "OpenLayers.Layer.XYZ"
});