diff -r 000000000000 -r 6474c204b198 mobile/android/base/util/EventCallback.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mobile/android/base/util/EventCallback.java	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,31 @@
+package org.mozilla.gecko.util;
+
+/**
+ * Callback interface for Gecko requests.
+ *
+ * For each instance of EventCallback, exactly one of sendResponse, sendError, or sendCancel
+ * must be called to prevent observer leaks. If more than one send* method is called, or if a
+ * single send method is called multiple times, an {@link IllegalStateException} will be thrown.
+ */
+public interface EventCallback {
+    /**
+     * Sends a success response with the given data.
+     *
+     * @param response The response data to send to Gecko. Can be any of the types accepted by
+     *                 JSONObject#put(String, Object).
+     */
+    public void sendSuccess(Object response);
+
+    /**
+     * Sends an error response with the given data.
+     *
+     * @param response The response data to send to Gecko. Can be any of the types accepted by
+     *                 JSONObject#put(String, Object).
+     */
+    public void sendError(Object response);
+
+    /**
+     * Cancels the request, preventing any Gecko-side callbacks from being executed.
+     */
+    public void sendCancel();
+}