Reduced number of UpdateDeviceList calls

ui/events/platform/x11/x11_event_source.cc

Index: ui/events/platform/x11/x11_event_source.cc
diff --git a/ui/events/platform/x11/x11_event_source.cc b/ui/events/platform/x11/x11_event_source.cc
index 337273f189ed4abb2d15bf270e5031434208116f..0729ce713d95952bfe18cf5e4feb1eb201d1c5dd 100644
--- a/ui/events/platform/x11/x11_event_source.cc
+++ b/ui/events/platform/x11/x11_event_source.cc
@@ -206,9 +206,25 @@ void X11EventSource::ExtractCookieDataDispatchEvent(XEvent* xevent) {
 }
 
 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);

[bokan, 2016/07/13 14:45:50]

Is it guaranteed that there won't ever be a new reason code added?

[Will Shackleton, 2016/07/14 18:11:10]

Hah. Not in the current X11 spec but I'll make this an else if just in case.

+        ui::DeviceDataManagerX11::GetInstance()->InvalidateScrollClasses(
+            xev->sourceid);
+      }
+    }
+  }
+
+  if (should_update_device_list) {
     UpdateDeviceList();
     hotplug_event_handler_->OnHotplugEvent();
   }
@@ -217,7 +233,7 @@ void X11EventSource::PostDispatchEvent(XEvent* xevent) {
       xevent->xcrossing.detail != NotifyInferior &&
       xevent->xcrossing.mode != NotifyUngrab) {
     // Clear stored scroll data
-    ui::DeviceDataManagerX11::GetInstance()->InvalidateScrollClasses();
+    ui::DeviceDataManagerX11::GetInstance()->InvalidateScrollClasses(-1);

[bokan, 2016/07/13 14:45:50]

Make the -1 an Enum (e.g. ALL_DEVICES) for readability (update the comment in
the declaration to use that instead of -1 too).

[Will Shackleton, 2016/07/14 18:11:09]

Done.

   }
 }