Implement accessibility enhancements (issue #1217)

- Add new CefBrowserHost::SetAccessibilityState method for toggling
  accessibility state when readers are detected by the client.
- Add new CefAccessibilityHandler interface for the delivery of
  accessibility notifications to windowless (OSR) clients.
- Fix delivery of CefFocusHandler callbacks to windowless clients.
- cefclient: Add example windowless accessibility implementation on Windows and macOS.
- cefclient: Automatically detect screen readers on Windows and macOS.

include/capi/cef_accessibility_handler_capi.h

@@ -0,0 +1,79 @@
+// Copyright (c) 2017 Marshall A. Greenblatt. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the name Chromium Embedded
+// Framework nor the names of its contributors may be used to endorse
+// or promote products derived from this software without specific prior
+// written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// ---------------------------------------------------------------------------
+//
+// This file was generated by the CEF translator tool and should not edited
+// by hand. See the translator.README.txt file in the tools directory for
+// more information.
+//
+
+#ifndef CEF_INCLUDE_CAPI_CEF_ACCESSIBILITY_HANDLER_CAPI_H_
+#define CEF_INCLUDE_CAPI_CEF_ACCESSIBILITY_HANDLER_CAPI_H_
+#pragma once
+
+#include "include/capi/cef_values_capi.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+///
+// Implement this structure to receive accessibility notification when
+// accessibility events have been registered. The functions of this structure
+// will be called on the UI thread.
+///
+typedef struct _cef_accessibility_handler_t {
+  ///
+  // Base structure.
+  ///
+  cef_base_ref_counted_t base;
+
+  ///
+  // Called after renderer process sends accessibility tree changes to the
+  // browser process.
+  ///
+  void (CEF_CALLBACK *on_accessibility_tree_change)(
+      struct _cef_accessibility_handler_t* self, struct _cef_value_t* value);
+
+  ///
+  // Called after renderer process sends accessibility location changes to the
+  // browser process.
+  ///
+  void (CEF_CALLBACK *on_accessibility_location_change)(
+      struct _cef_accessibility_handler_t* self, struct _cef_value_t* value);
+} cef_accessibility_handler_t;
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // CEF_INCLUDE_CAPI_CEF_ACCESSIBILITY_HANDLER_CAPI_H_

include/capi/cef_browser_capi.h

@@ -748,6 +748,34 @@ typedef struct _cef_browser_host_t {
   ///
   struct _cef_navigation_entry_t* (CEF_CALLBACK *get_visible_navigation_entry)(
       struct _cef_browser_host_t* self);
+
+  ///
+  // Set accessibility state for all frames. |accessibility_state| may be
+  // default, enabled or disabled. If |accessibility_state| is STATE_DEFAULT
+  // then accessibility will be disabled by default and the state may be further
+  // controlled with the "force-renderer-accessibility" and "disable-renderer-
+  // accessibility" command-line switches. If |accessibility_state| is
+  // STATE_ENABLED then accessibility will be enabled. If |accessibility_state|
+  // is STATE_DISABLED then accessibility will be completely disabled.
+  //
+  // For windowed browsers accessibility will be enabled in Complete mode (which
+  // corresponds to kAccessibilityModeComplete in Chromium). In this mode all
+  // platform accessibility objects will be created and managed by Chromium's
+  // internal implementation. The client needs only to detect the screen reader
+  // and call this function appropriately. For example, on macOS the client can
+  // handle the @"AXEnhancedUserStructure" accessibility attribute to detect
+  // VoiceOver state changes and on Windows the client can handle WM_GETOBJECT
+  // with OBJID_CLIENT to detect accessibility readers.
+  //
+  // For windowless browsers accessibility will be enabled in TreeOnly mode
+  // (which corresponds to kAccessibilityModeWebContentsOnly in Chromium). In
+  // this mode renderer accessibility is enabled, the full tree is computed, and
+  // events are passed to CefAccessibiltyHandler, but platform accessibility
+  // objects are not created. The client may implement platform accessibility
+  // objects using CefAccessibiltyHandler callbacks if desired.
+  ///
+  void (CEF_CALLBACK *set_accessibility_state)(struct _cef_browser_host_t* self,
+      cef_state_t accessibility_state);
 } cef_browser_host_t;
 
 

include/capi/cef_render_handler_capi.h

@@ -38,6 +38,7 @@
 #define CEF_INCLUDE_CAPI_CEF_RENDER_HANDLER_CAPI_H_
 #pragma once
 
+#include "include/capi/cef_accessibility_handler_capi.h"
 #include "include/capi/cef_base_capi.h"
 #include "include/capi/cef_browser_capi.h"
 #include "include/capi/cef_drag_data_capi.h"
@@ -57,6 +58,14 @@ typedef struct _cef_render_handler_t {
   ///
   cef_base_ref_counted_t base;
 
+  ///
+  // Return the handler for accessibility notifications. If no handler is
+  // provided the default implementation will be used.
+  ///
+  struct _cef_accessibility_handler_t* (
+      CEF_CALLBACK *get_accessibility_handler)(
+      struct _cef_render_handler_t* self);
+
   ///
   // Called to retrieve the root window rectangle in screen coordinates. Return
   // true (1) if the rectangle was provided.