Bug 1355161 - provide a scriptable equivalent of NS_DispatchToMainThread, r=froydnj.

xpcom/threads/nsIThreadManager.idl

@@ -8,6 +8,7 @@
 
 [ptr] native PRThread(PRThread);
 
+interface nsIRunnable;
 interface nsIThread;
 
 /**
@@ -79,4 +80,14 @@ interface nsIThreadManager : nsISupports
    * application process.
    */
   readonly attribute boolean isMainThread;
+
+  /**
+   * This queues a runnable to the main thread. It's a shortcut for JS callers
+   * to be used instead of
+   *   .mainThread.dispatch(runnable, Ci.nsIEventTarget.DISPATCH_NORMAL);
+   * or
+   *   .currentThread.dispatch(runnable, Ci.nsIEventTarget.DISPATCH_NORMAL);
+   * C++ callers should instead use NS_DispatchToMainThread.
+   */
+  void dispatchToMainThread(in nsIRunnable event);
 };

xpcom/threads/nsThreadManager.cpp

@@ -348,3 +348,17 @@ nsThreadManager::GetHighestNumberOfThreads()
   OffTheBooksMutexAutoLock lock(mLock);
   return mHighestNumberOfThreads;
 }
+
+NS_IMETHODIMP
+nsThreadManager::DispatchToMainThread(nsIRunnable *aEvent)
+{
+  // Note: C++ callers should instead use NS_DispatchToMainThread.
+  MOZ_ASSERT(NS_IsMainThread());
+
+  // Keep this functioning during Shutdown
+  if (NS_WARN_IF(!mMainThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  return mMainThread->DispatchFromScript(aEvent, 0);
+}