Add utility function resolveUrl on manager

This is needed to be able to resolve file URLs relative to the widget's
notebook.

packages/base/src/manager-base.ts

@@ -481,6 +481,16 @@ abstract class ManagerBase<T> {
  this._models[i].then(model => { model.comm_live = false; });
  });
  }
+
+ /**
+ * Resolve a URL relative to the current notebook location.
+ *
+ * The default implementation just returns the original url.
+ */
+ resolveUrl(url: string): Promise<string> {
+ return Promise.resolve(url);
+ }
+
  /**
  * The comm target name to register
  */

packages/jupyterlab-manager/src/manager.ts

@@ -156,6 +156,13 @@ class WidgetManager extends ManagerBase<Widget> implements IDisposable {
  this._context = null;
  }
 
+ /**
+ * Resolve a URL relative to the current notebook location.
+ */
+ resolveUrl(url: string): Promise<string> {
+ return this.context.resolveUrl(url);
+ }
+
  /**
  * Load a class and return a promise to the loaded object.
  */