ime: Use ScopedObservation for input_method::Accessibility.

ScopedObservation is much safer and cleaner.

Change-Id: I189991b7aafb2cda41fa77a468b1f9a8a213fa10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522811
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826210}

chrome/browser/chromeos/input_method/accessibility.cc

@@ -13,29 +13,24 @@
 namespace chromeos {
 namespace input_method {
 
-Accessibility::Accessibility(InputMethodManager* imm)
-    : imm_(imm) {
-  DCHECK(imm_);
-  imm_->AddObserver(this);
+Accessibility::Accessibility(InputMethodManager* imm) {
+  DCHECK(imm);
+  observed_input_method_manager_.Observe(imm);
 }
 
-Accessibility::~Accessibility() {
-  DCHECK(imm_);
-  imm_->RemoveObserver(this);
-}
+Accessibility::~Accessibility() = default;
 
 void Accessibility::InputMethodChanged(InputMethodManager* imm,
                                        Profile* profile,
                                        bool show_message) {
-  DCHECK_EQ(imm, imm_);
   if (!show_message)
     return;
 
   // Get the medium name of the changed input method (e.g. US, INTL, etc.)
   const InputMethodDescriptor descriptor =
-      imm_->GetActiveIMEState()->GetCurrentInputMethod();
+      imm->GetActiveIMEState()->GetCurrentInputMethod();
   const std::string medium_name = base::UTF16ToUTF8(
-      imm_->GetInputMethodUtil()->GetInputMethodMediumName(descriptor));
+      imm->GetInputMethodUtil()->GetInputMethodMediumName(descriptor));
 
   AutomationManagerAura::GetInstance()->HandleAlert(medium_name);
 }

chrome/browser/chromeos/input_method/accessibility.h

@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_CHROMEOS_INPUT_METHOD_ACCESSIBILITY_H_
 
 #include "base/macros.h"
+#include "base/scoped_observation.h"
 #include "ui/base/ime/chromeos/input_method_manager.h"
 
 namespace chromeos {
@@ -15,17 +16,20 @@ namespace input_method {
 class Accessibility
     : public InputMethodManager::Observer {
  public:
+  // `imm` needs to be alive for the lifetime of this instance.
   explicit Accessibility(InputMethodManager* imm);
   ~Accessibility() override;
+  Accessibility(const Accessibility&) = delete;
+  Accessibility& operator=(const Accessibility&) = delete;
 
  private:
   // InputMethodManager::Observer implementation.
   void InputMethodChanged(InputMethodManager* imm,
                           Profile* profile,
                           bool show_message) override;
-  InputMethodManager* imm_;
 
-  DISALLOW_COPY_AND_ASSIGN(Accessibility);
+  base::ScopedObservation<InputMethodManager, InputMethodManager::Observer>
+      observed_input_method_manager_{this};
 };
 
 }  // namespace input_method