refactor: add check for css-loader

Author: vakrilov

Diff for: apply-css-loader.js

@@ -1,18 +1,43 @@
+const cssLoaderWarning = "The apply-css-loader expects the file to be pre-processed by css-loader. It might not work properly. Please check your webpack configuration";
+
+function checkForCssLoader(loaders, loaderIndex) {
+    if (!loaders) {
+        return false;
+    }
+
+    for (var i = loaderIndex + 1; i < loaders.length; i++) {
+        const loader = loaders[i];
+        if (loader.path && loader.path.indexOf("css-loader") > 0) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 module.exports = function (content, map) {
     if (this.request.match(/\/app\.(css|scss|less|sass)$/)) {
         return content;
     }
+
+    // Emit a warning if the file was not processed by the css-loader.
+    if (!checkForCssLoader(this.loaders, this.loaderIndex)) {
+        this.emitWarning(new Error(cssLoaderWarning));
+    }
+
     content += `
     const application = require("tns-core-modules/application");
     require("tns-core-modules/ui/styling/style-scope");
 
-    exports.forEach(cssExport => {
-        if (cssExport.length > 1 && cssExport[1]) {
-            // applying the second item of the export as it contains the css contents
-            application.addCss(cssExport[1]);
-        }
-    });
-    `;
+    if (typeof exports.forEach === "function") {
+        exports.forEach(cssExport => {
+            if (cssExport.length > 1 && cssExport[1]) {
+                // applying the second item of the export as it contains the css contents
+                application.addCss(cssExport[1]);
+            }
+        });
+    }
+`;
 
     this.callback(null, content, map);
-}
+}