add a ol.layer.wms factory

src/api/layer/wms.js

@@ -0,0 +1,41 @@
+goog.provide('ol.layer.wms');
+
+goog.require('ol.layer.WMS');
+
+/**
+ * @export
+ * @param {Object} opt_arg Config object.
+ * @return {ol.layer.WMS}
+ */
+ol.layer.wms = function(opt_arg) {
+    if (opt_arg instanceof ol.layer.WMS) {
+        return opt_arg;
+    }
+
+    /** @type {string} */
+    var url;
+    /** @type {Array.<string>} */
+    var layers;
+    /** @type {string} */
+    var format;
+
+    if (goog.isObject(opt_arg)) {
+        ol.base.checkKeys(opt_arg, ['url', 'layers', 'format']);
+        url = opt_arg['url'];
+        layers = opt_arg['layers'];
+        format = opt_arg['format'];
+    }
+
+    var msg;
+    if (!goog.isDef(url)) {
+        msg = 'Cannot create WMS layer; option "url" is missing';
+        ol.error(msg);
+    }
+    if (!goog.isArray(layers)) {
+        msg = 'Cannot create WMS layer; option "layers" is missing, ' +
+              'or is not an array';
+        ol.error(msg);
+    }
+
+    return new ol.layer.WMS(url, layers, format);
+};