[medusa-interfaces] : Adds decorator functionality to BaseService (#39)

Plugins and projects can add decorators in services. E.g. if a plugin needs to load some additional information on carts the plugin can register a decorator via: `cartService.addDecorator(someFunc)` which will be available later through `cartService.runDecorators()`.

packages/medusa-interfaces/src/base-service.js

@@ -2,5 +2,35 @@
  * Common functionality for Services
  * @interface
  */
-class BaseService {}
+class BaseService {
+  constructor() {
+    this.decorators_ = []
+  }
+
+  /**
+   * Adds a decorator to a service. The decorator must be a function and should
+   * return a decorated object.
+   * @param {function} fn - the decorator to add to the service
+   */
+  addDecorator(fn) {
+    if (typeof fn !== "function") {
+      throw Error("Decorators must be of type function")
+    }
+
+    this.decorators_.push(fn)
+  }
+
+  /**
+   * Runs the decorators registered on the service. The decorators are run in
+   * the order they have been registered in. Failing decorators will be skipped
+   * in order to ensure deliverability in spite of breaking code.
+   * @param {object} obj - the object to decorate.
+   * @return {object} the decorated object.
+   */
+  runDecorators_(obj) {
+    return this.decorators_.reduce(async (acc, next) => {
+      return acc.then(res => next(res)).catch(() => acc)
+    }, Promise.resolve(obj))
+  }
+}
 export default BaseService