Fire null Device Motion events only once.

Source/modules/device_orientation/DeviceSensorEventDispatcher.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 16 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "modules/device_orientation/DeviceSensorEventDispatcher.h"
 
 namespace WebCore {
 
 DeviceSensorEventDispatcher::DeviceSensorEventDispatcher()
+    : m_needsPurge(false)
+    , m_isDispatching(false)
 {
 }
 
 DeviceSensorEventDispatcher::~DeviceSensorEventDispatcher()
 {
 }
 
 void DeviceSensorEventDispatcher::addController(DeviceSensorEventController* controller)
 {
     bool wasEmpty = m_controllers.isEmpty();
     if (!m_controllers.contains(controller))
         m_controllers.append(controller);
     if (wasEmpty)
         startListening();
 }
 
 void DeviceSensorEventDispatcher::removeController(DeviceSensorEventController* controller)
 {
-    // Do not actually remove controller from the vector, instead zero them out.
-    // The zeros are removed after didChangeDeviceMotion has dispatched all events.
-    // This is to prevent re-entrancy case when a controller is destroyed while in the
-    // didChangeDeviceMotion method.
+    // Do not actually remove the controller from the vector, instead zero them out.
+    // The zeros are removed in these two cases:
+    // 1. either immediately if we are not dispatching any events,
+    // 2. or after didChangeDeviceMotion/Orientation has dispatched all events.
+    // This is to correctly handle the re-entrancy case when a controller is destroyed
+    // while in the didChangeDeviceMotion/Orientation method.
     size_t index = m_controllers.find(controller);
-    if (index != notFound)
-        m_controllers[index] = 0;
+    if (index == notFound)
+        return;
+
+    m_controllers[index] = 0;
+    m_needsPurge = true;
+
+    if (!m_isDispatching)
+        purgeControllers();
 }
 
 void DeviceSensorEventDispatcher::purgeControllers()
 {
     size_t i = 0;
     while (i < m_controllers.size()) {
         if (!m_controllers[i]) {
             m_controllers[i] = m_controllers.last();
             m_controllers.removeLast();
         } else {
             ++i;
         }
     }
 
+    m_needsPurge = false;

[abarth-chromium, 2013/07/31 00:56:19]

Should we ASSERT(m_needsPurge) at the top of this function?

[timvolodine, 2013/07/31 13:59:00]

Done.

+
     if (m_controllers.isEmpty())
         stopListening();
 }
 
 } // namespace WebCore