Reduced number of UpdateDeviceList calls

ui/events/platform/x11/x11_event_source.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/events/platform/x11/x11_event_source.h"
 
 #include <X11/Xatom.h>
 #include <X11/XKBlib.h>
 #include <X11/Xlib.h>
 
@@ skipping 190 matching lines @@
     if (time_difference < 0 || time_difference > (UINT32_MAX >> 1))
       last_seen_server_time_ = event_time;
   }
   delegate_->ProcessXEvent(xevent);
   PostDispatchEvent(xevent);
   if (have_cookie)
     XFreeEventData(xevent->xgeneric.display, &xevent->xcookie);
 }
 
 void X11EventSource::PostDispatchEvent(XEvent* xevent) {
-  if (xevent->type == GenericEvent &&
-      (xevent->xgeneric.evtype == XI_HierarchyChanged ||
-       xevent->xgeneric.evtype == XI_DeviceChanged)) {
+  bool should_update_device_list = false;
+
+  if (xevent->type == GenericEvent) {
+    if (xevent->xgeneric.evtype == XI_HierarchyChanged) {
+      should_update_device_list = true;
+    } else if (xevent->xgeneric.evtype == XI_DeviceChanged) {
+      XIDeviceChangedEvent* xev =
+          static_cast<XIDeviceChangedEvent*>(xevent->xcookie.data);
+      if (xev->reason == XIDeviceChange) {
+        should_update_device_list = true;
+      } else {
+        DCHECK_EQ(XISlaveSwitch, xev->reason);
+        ui::DeviceDataManagerX11::GetInstance()->InvalidateScrollClasses(
+            xev->sourceid);

[sadrul, 2016/06/27 14:49:21]

Can |xev->sourceid| here be really -1?

[Will Shackleton, 2016/07/12 16:05:04]

There are other places |InvalidateScrollClasses| is called - see new change now
default parameter doesn't exist.

+      }
+    }
+  }
+
+  if (should_update_device_list) {
     UpdateDeviceList();
     hotplug_event_handler_->OnHotplugEvent();
   }
 
   if (xevent->type == EnterNotify &&
       xevent->xcrossing.detail != NotifyInferior &&
       xevent->xcrossing.mode != NotifyUngrab) {
     // Clear stored scroll data
     ui::DeviceDataManagerX11::GetInstance()->InvalidateScrollClasses();
   }
 }
 
 void X11EventSource::StopCurrentEventStream() {
   continue_stream_ = false;
 }
 
 void X11EventSource::OnDispatcherListChanged() {
   if (!hotplug_event_handler_) {
     hotplug_event_handler_.reset(new X11HotplugEventHandler());
     // Force the initial device query to have an update list of active devices.
     hotplug_event_handler_->OnHotplugEvent();
   }
 }
 
 }  // namespace ui