Add metrics for the number of attached displays.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

Index: chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc
diff --git a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc
index 64c01a71f77087d0d41986768434f9542925975b..f859ccff1083bc9bbce388d450ed3f2ebafac087 100644
--- a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc
+++ b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc
@@ -24,6 +24,7 @@
 #include "ui/base/touch/touch_device.h"
 #include "ui/base/ui_base_switches.h"
 #include "ui/events/event_switches.h"
+#include "ui/gfx/screen.h"
 
 #if defined(OS_LINUX) && !defined(OS_CHROMEOS)
 #include <gnu/libc-version.h>
@@ -219,9 +220,13 @@ void RecordTouchEventState() {
 }  // namespace
 
 ChromeBrowserMainExtraPartsMetrics::ChromeBrowserMainExtraPartsMetrics() {
+  display_count_ = gfx::Screen::GetNativeScreen()->GetNumDisplays();
+  UMA_HISTOGRAM_COUNTS_100("DisplayCount.StartUp", display_count_);

[Ilya Sherman, 2014/10/14 06:34:22]

nit: Please name this "Display.Count.StartUp" (or, even better IMO,
"Display.Count.AtStartUp".  The important part here is to have the group name be
"Display" rather than "DisplayCount", so that there's room to add additional
display metrics if they become useful over time.

[erikchen, 2014/10/14 23:09:38]

Went with "Hardware.Display.Count.OnStartup".

+  gfx::Screen::GetNativeScreen()->AddObserver(this);
 }
 
 ChromeBrowserMainExtraPartsMetrics::~ChromeBrowserMainExtraPartsMetrics() {
+  gfx::Screen::GetNativeScreen()->RemoveObserver(this);
 }
 
 void ChromeBrowserMainExtraPartsMetrics::PreProfileInit() {
@@ -250,6 +255,29 @@ void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {
       base::TimeDelta::FromSeconds(kStartupMetricsGatheringDelaySeconds));
 }
 
+void ChromeBrowserMainExtraPartsMetrics::OnDisplayAdded(
+    const gfx::Display& new_display) {
+  EmitDisplaysChangedMetric();
+}
+
+void ChromeBrowserMainExtraPartsMetrics::OnDisplayRemoved(
+    const gfx::Display& old_display) {
+  EmitDisplaysChangedMetric();
+}
+
+void ChromeBrowserMainExtraPartsMetrics::OnDisplayMetricsChanged(
+    const gfx::Display& display,
+    uint32_t changed_metrics) {
+}
+
+void ChromeBrowserMainExtraPartsMetrics::EmitDisplaysChangedMetric() {
+  int display_count = gfx::Screen::GetNativeScreen()->GetNumDisplays();
+  if (display_count != display_count_) {
+    display_count_ = display_count;
+    UMA_HISTOGRAM_COUNTS_100("DisplayCount.Changed", display_count_);

[Ilya Sherman, 2014/10/14 06:34:22]

nit: And correspondingly, please name this "Display.Count.Changed" (or
"Display.Count.OnChange").

[erikchen, 2014/10/14 23:09:38]

Went with "Hardware.Display.Count.OnChange".

+  }
+}
+
 namespace chrome {
 
 void AddMetricsExtraParts(ChromeBrowserMainParts* main_parts) {