Fire null Device Motion events only once.

Source/modules/device_orientation/DeviceMotionController.cpp

 /*
  * Copyright 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Samsung Electronics. 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 copyright
@@ skipping 10 matching lines @@
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (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/DeviceMotionController.h"
 
 #include "core/dom/Document.h"
+#include "modules/device_orientation/DeviceMotionData.h"
 #include "modules/device_orientation/DeviceMotionDispatcher.h"
 #include "modules/device_orientation/DeviceMotionEvent.h"
 
 namespace WebCore {
 
 DeviceMotionController::DeviceMotionController(Document* document)
     : DeviceSensorEventController(document)
 {
 }
 
@@ skipping 34 matching lines @@
 void DeviceMotionController::registerWithDispatcher()
 {
     DeviceMotionDispatcher::instance().addDeviceMotionController(this);
 }
 
 void DeviceMotionController::unregisterWithDispatcher()
 {
     DeviceMotionDispatcher::instance().removeDeviceMotionController(this);
 }
 
+bool DeviceMotionController::isNullEvent(Event* event)
+{
+    DeviceMotionEvent* motionEvent = static_cast<DeviceMotionEvent*>(event);

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

How do we know this static_cast is valid?  Can we add some sort of ASSERT to
verify that the type is correct?  For example, we could check the
interfaceName() of the event.

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

The static cast is a bit unfortunate here, but my reasoning for its correctness
is as follows:

The virtual method isNullEvent is only called from
DeviceSensorEventController::dispatchDeviceEvent. For motion,
dispatchDeviceEvent is called from two places, either from
DeviceMotionController::didChangeDeviceMotion or
DeviceSensorEventController::fireDeviceEvent. In the first case the passed event
will be of type motion by definition, in the second case the event will be
fetched by getLastEvent which is an virtual overriden method.

I've added an assert on event->type().

+    DeviceMotionData* data = motionEvent->deviceMotionData();
+    RefPtr<DeviceMotionData::Acceleration> acceleration = data->acceleration();
+    RefPtr<DeviceMotionData::Acceleration> accelerationIncludingGravity = data->accelerationIncludingGravity();
+    RefPtr<DeviceMotionData::RotationRate> rotationRate = data->rotationRate();
+
+    return !acceleration->canProvideX() && !acceleration->canProvideY() && !acceleration->canProvideZ()
+        && !accelerationIncludingGravity->canProvideX() && !accelerationIncludingGravity->canProvideY() && !accelerationIncludingGravity->canProvideZ()
+        && !rotationRate->canProvideAlpha() && !rotationRate->canProvideBeta() && !rotationRate->canProvideGamma();
+}
+
 } // namespace WebCore