cros: use callback to record TapDragging started uma

changes:
Instead of using ScopedUmaRecorder, which is essentially implementing
in a "callback" way. This CL replaces it with base::OnceCallback, which
makes code more concise.

Bug: 817643
Test: covered by tests
Change-Id: Id99e9c08e8fe49886ca7c30a5901fb4128b04a42
Reviewed-on: https://chromium-review.googlesource.com/957816
Commit-Queue: Qiang Xu <warx@google.com>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542546}

ash/touch/touch_devices_controller.cc

@@ -4,6 +4,8 @@
 
 #include "ash/touch/touch_devices_controller.h"
 
+#include <utility>
+
 #include "ash/accessibility/accessibility_controller.h"
 #include "ash/public/cpp/accessibility_types.h"
 #include "ash/public/cpp/ash_pref_names.h"
@@ -39,23 +41,6 @@ PrefService* GetActivePrefService() {
 
 }  // namespace
 
-// Used to record pref started UMA. It is created on user session added and
-// destroyed on active user pref changed, which is a point of pref started.
-class TouchDevicesController::ScopedUmaRecorder {
- public:
-  ScopedUmaRecorder() = default;
-  ~ScopedUmaRecorder() {
-    PrefService* prefs = GetActivePrefService();
-    if (!prefs)
-      return;
-    UMA_HISTOGRAM_BOOLEAN("Touchpad.TapDragging.Started",
-                          prefs->GetBoolean(prefs::kTapDraggingEnabled));
-  }
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(ScopedUmaRecorder);
-};
-
 // static
 void TouchDevicesController::RegisterProfilePrefs(PrefRegistrySimple* registry,
                                                   bool for_test) {
@@ -131,7 +116,10 @@ void TouchDevicesController::SetTouchscreenEnabled(
 }
 
 void TouchDevicesController::OnUserSessionAdded(const AccountId& account_id) {
-  scoped_uma_recorder_ = std::make_unique<ScopedUmaRecorder>();
+  uma_record_callback_ = base::BindOnce([](PrefService* prefs) {
+    UMA_HISTOGRAM_BOOLEAN("Touchpad.TapDragging.Started",
+                          prefs->GetBoolean(prefs::kTapDraggingEnabled));
+  });
 }
 
 void TouchDevicesController::OnSigninScreenPrefServiceInitialized(
@@ -141,7 +129,8 @@ void TouchDevicesController::OnSigninScreenPrefServiceInitialized(
 
 void TouchDevicesController::OnActiveUserPrefServiceChanged(
     PrefService* prefs) {
-  scoped_uma_recorder_.reset();
+  if (uma_record_callback_)
+    std::move(uma_record_callback_).Run(prefs);
   ObservePrefs(prefs);
 }
 

ash/touch/touch_devices_controller.h

@@ -9,6 +9,7 @@
 
 #include "ash/ash_export.h"
 #include "ash/session/session_observer.h"
+#include "base/callback.h"
 
 class AccountId;
 class PrefChangeRegistrar;
@@ -52,8 +53,6 @@ class ASH_EXPORT TouchDevicesController : public SessionObserver {
   void SetTouchscreenEnabled(bool enabled, TouchscreenEnabledSource source);
 
  private:
-  class ScopedUmaRecorder;
-
   // Overridden from SessionObserver:
   void OnUserSessionAdded(const AccountId& account_id) override;
   void OnSigninScreenPrefServiceInitialized(PrefService* prefs) override;
@@ -83,8 +82,10 @@ class ASH_EXPORT TouchDevicesController : public SessionObserver {
   // Observes user profile prefs for touch devices.
   std::unique_ptr<PrefChangeRegistrar> pref_change_registrar_;
 
-  // Used to record pref started UMA.
-  std::unique_ptr<ScopedUmaRecorder> scoped_uma_recorder_;
+  // Used to record pref started UMA, bound on user session added and run on
+  // active user pref service changed. The goal is to record the initial state
+  // of the feature.
+  base::OnceCallback<void(PrefService* prefs)> uma_record_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(TouchDevicesController);
 };