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diff --git a/comm/mailnews/base/src/UrlListener.h b/comm/mailnews/base/src/UrlListener.h
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+++ b/comm/mailnews/base/src/UrlListener.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef UrlListener_h__
+#define UrlListener_h__
+
+#include <functional>  // For std::function.
+#include "nsIUrlListener.h"
+class nsIURI;
+
+/**
+ * UrlListener is a small nsIUrlListener implementation which allows
+ * callable objects (including lambdas) to be plugged in instead of deriving
+ * your own nsIUrlListener.
+ *
+ * The aim is to encourage more readable code by allowing the start/stop
+ * notifications of a long-running operation to be handled near to where the
+ * operation was initiated.
+ *
+ * A contrived example:
+ *
+ * void Kick() {
+ *   UrlListener* listener = new UrlListener;
+ *   listener->mStopFn = [](nsIURI* url, nsresult status) -> nsresult {
+ *     // Note that we may get here waaaaaaay after Kick() has returned...
+ *     printf("LongRunningOperation is finished.\n");
+ *     return NS_OK;
+ *   };
+ *   thingService.startLongRunningOperation(listener);
+ *   //...continue doing other stuff while operation is ongoing...
+ * }
+ *
+ * Traditionally, c-c code has tended to use multiple inheritance to add
+ * listener callbacks to the class of the object initiating the operation.
+ * This has a couple of undesirable side effects:
+ *
+ * 1) It separates out the onStopRunningUrl handling into some other
+ *    part of the code, which makes the order of things much harder to follow.
+ * 2) Often the same onStopRunningUrl handler will be used for many different
+ *    kinds of operations (see nsImapMailFolder::OnStopRunningUrl(), for
+ *    example).
+ * 3) It exposes implementation details as part of the public interface
+ *    e.g see all the listener types nsMsgDBFolder derives from to implement
+ *    it's internals. That's all just confusing noise that shouldn't be seen
+ *    from outside the class.
+ *
+ * Just as PromiseTestUtils.jsm brings the Javascript side up from callback
+ * hell to async lovelyness, this can be used to raise the C++ side from
+ * callback-somewhere-else-maybe-in-this-class-but-who-can-really-tell hell
+ * up to normal callback hell :-)
+ *
+ */
+class UrlListener : public nsIUrlListener {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIURLLISTENER
+
+  UrlListener() {}
+  /**
+   * mStartFn and mStopFn are the OnStartRunning() and OnStopRunningUrl()
+   * handlers. It's fine for them to be null (often you'll only need mStopFn).
+   */
+  std::function<nsresult(nsIURI*)> mStartFn;
+  std::function<nsresult(nsIURI*, nsresult)> mStopFn;
+
+ protected:
+  virtual ~UrlListener() {}
+};
+
+#endif  // UrlListener_h__