content/browser/accessibility/browser_accessibility_state_impl.h

// Copyright (c) 2012 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.

#ifndef CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_STATE_IMPL_H_
#define CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_STATE_IMPL_H_

#include <vector>

#include "base/basictypes.h"
#include "base/compiler_specific.h"
#include "base/memory/singleton.h"
#include "content/common/view_message_enums.h"
#include "content/public/browser/browser_accessibility_state.h"

namespace content {

// The BrowserAccessibilityState class is used to determine if Chrome should be
// customized for users with assistive technology, such as screen readers. We
// modify the behavior of certain user interfaces to provide a better experience
// for screen reader users. The way we detect a screen reader program is
// different for each platform.
//
// Screen Reader Detection
// (1) On windows many screen reader detection mechinisms will give false
// positives like relying on the SPI_GETSCREENREADER system parameter. In Chrome
// we attempt to dynamically detect a MSAA client screen reader by calling
// NotifiyWinEvent in NativeWidgetWin with a custom ID and wait to see if the ID
// is requested by a subsequent call to WM_GETOBJECT.
// (2) On mac we detect dynamically if VoiceOver is running.  We rely upon the
// undocumented accessibility attribute @"AXEnhancedUserInterface" which is set
// when VoiceOver is launched and unset when VoiceOver is closed.  This is an
// improvement over reading defaults preference values (which has no callback
// mechanism).
class CONTENT_EXPORT BrowserAccessibilityStateImpl
    : public base::RefCountedThreadSafe<BrowserAccessibilityStateImpl>,
      public BrowserAccessibilityState {
 public:
  BrowserAccessibilityStateImpl();

  static BrowserAccessibilityStateImpl* GetInstance();

  virtual void EnableAccessibility() OVERRIDE;
  virtual void DisableAccessibility() OVERRIDE;
  virtual void OnScreenReaderDetected() OVERRIDE;
  virtual bool IsAccessibleBrowser() OVERRIDE;
  virtual void AddHistogramCallback(base::Closure callback) OVERRIDE;

  virtual void UpdateHistogramsForTesting() OVERRIDE;

  AccessibilityMode accessibility_mode() const { return accessibility_mode_; };
  void SetAccessibilityMode(AccessibilityMode mode);

 private:
  friend class base::RefCountedThreadSafe<BrowserAccessibilityStateImpl>;
  friend struct DefaultSingletonTraits<BrowserAccessibilityStateImpl>;

  // Called a short while after startup to allow time for the accessibility
  // state to be determined. Updates histograms with the current state.
  void UpdateHistograms();

  // Leaky singleton, destructor generally won't be called.
  virtual ~BrowserAccessibilityStateImpl();

  void UpdatePlatformSpecificHistograms();

  AccessibilityMode accessibility_mode_;

  std::vector<base::Closure> histogram_callbacks_;

  DISALLOW_COPY_AND_ASSIGN(BrowserAccessibilityStateImpl);
};

}  // namespace content

#endif  // CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_STATE_IMPL_H_