openlayers3中应用proj4js

要在openlayers3中应用proj4js，需要在html中引用proj4js，然后在引用所需要的projection的js定义，如 http://epsg.io/21781-1753.js
然后在openlayers中就会支持这种EPSG:21781的坐标转换。

<script src="http://cdnjs.cloudflare.com/ajax/libs/proj4js/2.2.1/proj4.js" type="text/javascript"></script>
<script src="http://epsg.io/21781-1753.js" type="text/javascript"></script>

http://epsg.io/21781-1753.js会返回一个js，这个js一旦执行就会给proj4添加一个支持的projection，如下：

proj4.defs("EPSG:21781","+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=600000 +y_0=200000 +ellps=bessel +towgs84=660.077,13.551,369.344,2.484,1.783,2.939,5.66 +units=m +no_defs");

下面是openlayer的实现

/**
* Fetches a Projection object for the code specified.
*
* @param {ol.proj.ProjectionLike} projectionLike Either a code string which is
*     a combination of authority and identifier such as "EPSG:4326", or an
*     existing projection object, or undefined.
* @return {ol.proj.Projection} Projection object, or null if not in list.
* @api stable
*/
ol.proj.get = function(projectionLike) {
var projection;
if (projectionLike instanceof ol.proj.Projection) {
projection = projectionLike;
} else if (goog.isString(projectionLike)) {
var code = projectionLike;
var projections = ol.proj.projections_;
projection = projections[code];

//判断proj4js被引入进来了

if (ol.ENABLE_PROJ4JS && !goog.isDef(projection) && typeof proj4 == 'function') {
//如果需要的code=EPSG被引入进来了，就会proj4.defs（code）返回定义，并加入到openlayers中
var def = proj4.defs(code);
if (goog.isDef(def)) {
var units = def.units;
if (!goog.isDef(units)) {
if (goog.isDef(def.to_meter)) {
units = def.to_meter.toString();
ol.proj.METERS_PER_UNIT[units] = def.to_meter;
}
}
projection = new ol.proj.Projection({
code: code,
units: units,
axisOrientation: def.axis
});
ol.proj.addProjection(projection);
var currentCode, currentDef, currentProj, proj4Transform;
for (currentCode in projections) {
currentDef = proj4.defs(currentCode);
if (goog.isDef(currentDef)) {
currentProj = ol.proj.get(currentCode);
if (currentDef === def) {
ol.proj.addEquivalentProjections([currentProj, projection]);
} else {
proj4Transform = proj4(currentCode, code);
ol.proj.addCoordinateTransforms(currentProj, projection,
proj4Transform.forward, proj4Transform.inverse);
}
}
}
} else {
goog.asserts.assert(goog.isDef(projection));
projection = null;
}
}
} else {
projection = null;
}
return projection;