Mac a11y: Improve live region support in UI

- Removes restriction that live region changed events need to be fired from
nodes with the live region role, since we don't actually do this in UI code.
- Announces the node's name as the live region text, falling back to inner text
if name isn't set. Again, this tracks how we actually use the notifications.
- Debounces multiple notifications from the same node to one per 20ms

Bug: 1015002
Change-Id: Id564c85cd0ab0888877d4cf4b12fb872d1306284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871830
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711412}

ui/accessibility/platform/ax_platform_node_mac.mm

@@ -21,21 +21,15 @@
 #import "ui/gfx/mac/coordinate_conversion.h"
 #include "ui/strings/grit/ui_strings.h"
 
-@interface AXPlatformNodeCocoa (Private)
-// Helper function for string attributes that don't require extra processing.
-- (NSString*)getStringAttribute:(ax::mojom::StringAttribute)attribute;
-// Returns AXValue, or nil if AXValue isn't an NSString.
-- (NSString*)getAXValueAsString;
-// Returns the text that should be announced for an event with type |eventType|,
-// or nil if it shouldn't be announced.
-- (NSString*)announcementTextForEvent:(ax::mojom::Event)eventType;
-@end
-
 namespace {
 
+// Same length as web content/WebKit.
+static int kLiveRegionDebounceMillis = 20;
+
 using RoleMap = std::map<ax::mojom::Role, NSString*>;
 using EventMap = std::map<ax::mojom::Event, NSString*>;
 using ActionList = std::vector<std::pair<ax::mojom::Action, NSString*>>;
+using AnnouncementSpec = std::pair<base::scoped_nsobject<NSString>, bool>;
 
 RoleMap BuildRoleMap() {
   const RoleMap::value_type roles[] = {
@@ -317,10 +311,12 @@ const ActionList& GetActionList() {
   return *action_map;
 }
 
-void PostAnnouncementNotification(NSString* announcement) {
+void PostAnnouncementNotification(NSString* announcement, bool is_polite) {
+  NSAccessibilityPriorityLevel priority =
+      is_polite ? NSAccessibilityPriorityMedium : NSAccessibilityPriorityHigh;
   NSDictionary* notification_info = @{
     NSAccessibilityAnnouncementKey : announcement,
-    NSAccessibilityPriorityKey : @(NSAccessibilityPriorityHigh)
+    NSAccessibilityPriorityKey : @(priority)
   };
   NSAccessibilityPostNotificationWithUserInfo(
       [NSApp mainWindow], NSAccessibilityAnnouncementRequestedNotification,
@@ -350,8 +346,27 @@ bool AlsoUseShowMenuActionForDefaultAction(const ui::AXNodeData& data) {
 
 }  // namespace
 
+@interface AXPlatformNodeCocoa (Private)
+// Helper function for string attributes that don't require extra processing.
+- (NSString*)getStringAttribute:(ax::mojom::StringAttribute)attribute;
+// Returns AXValue, or nil if AXValue isn't an NSString.
+- (NSString*)getAXValueAsString;
+// Returns the data necessary to queue an NSAccessibility announcement if
+// |eventType| should be announced, or nullptr otherwise.
+- (std::unique_ptr<AnnouncementSpec>)announcementForEvent:
+    (ax::mojom::Event)eventType;
+// Ask the system to announce |announcementText|. This is debounced to happen
+// at most every |kLiveRegionDebounceMillis| per node, with only the most
+// recent announcement text read, to account for situations with multiple
+// notifications happening one after another (for example, results for
+// find-in-page updating rapidly as they come in from subframes).
+- (void)scheduleLiveRegionAnnouncement:
+    (std::unique_ptr<AnnouncementSpec>)polite;
+@end
+
 @implementation AXPlatformNodeCocoa {
   ui::AXPlatformNodeBase* node_;  // Weak. Retains us.
+  std::unique_ptr<AnnouncementSpec> pendingAnnouncement_;
 }
 
 @synthesize node = node_;
@@ -408,25 +423,49 @@ bool AlsoUseShowMenuActionForDefaultAction(const ui::AXNodeData& data) {
   return [value isKindOfClass:[NSString class]] ? value : nil;
 }
 
-- (NSString*)announcementTextForEvent:(ax::mojom::Event)eventType {
-  if (eventType == ax::mojom::Event::kAlert &&
-      node_->GetData().role == ax::mojom::Role::kAlert) {
-    // If there's no explicitly set accessible name, fall back to
-    // the inner text.
-    NSString* name =
-        [self getStringAttribute:ax::mojom::StringAttribute::kName];
-    return [name length] > 0 ? name
-                             : base::SysUTF16ToNSString(node_->GetInnerText());
-  } else if (eventType == ax::mojom::Event::kLiveRegionChanged &&
-             node_->GetData().HasStringAttribute(
-                 ax::mojom::StringAttribute::kContainerLiveStatus)) {
-    // Live regions announce their inner text.
-    return base::SysUTF16ToNSString(node_->GetInnerText());
+- (std::unique_ptr<AnnouncementSpec>)announcementForEvent:
+    (ax::mojom::Event)eventType {
+  // Only alerts and live region changes should be announced.
+  DCHECK(eventType == ax::mojom::Event::kAlert ||
+         eventType == ax::mojom::Event::kLiveRegionChanged);
+  std::string liveStatus =
+      node_->GetStringAttribute(ax::mojom::StringAttribute::kLiveStatus);
+  // If live status is explicitly set to off, don't announce.
+  if (liveStatus == "off")
+    return nullptr;
+
+  NSString* name = [self getStringAttribute:ax::mojom::StringAttribute::kName];
+  NSString* announcementText =
+      [name length] > 0 ? name
+                        : base::SysUTF16ToNSString(node_->GetInnerText());
+  if ([announcementText length] == 0)
+    return nullptr;
+
+  return std::make_unique<AnnouncementSpec>(
+      base::scoped_nsobject<NSString>([announcementText retain]),
+      liveStatus != "assertive");
+}
+
+- (void)scheduleLiveRegionAnnouncement:
+    (std::unique_ptr<AnnouncementSpec>)announcement {
+  if (pendingAnnouncement_) {
+    // An announcement is already in flight, so just reset the contents. This is
+    // threadsafe because the dispatch is on the main queue.
+    pendingAnnouncement_ = std::move(announcement);
+    return;
   }
-  // Only alerts and live regions have something to announce.
-  return nil;
-}
 
+  pendingAnnouncement_ = std::move(announcement);
+  dispatch_after(kLiveRegionDebounceMillis * NSEC_PER_MSEC,
+                 dispatch_get_main_queue(), ^{
+                   if (!pendingAnnouncement_) {
+                     return;
+                   }
+                   PostAnnouncementNotification(pendingAnnouncement_->first,
+                                                pendingAnnouncement_->second);
+                   pendingAnnouncement_.reset();
+                 });
+}
 // NSAccessibility informal protocol implementation.
 
 - (BOOL)accessibilityIsIgnored {
@@ -1163,10 +1202,14 @@ gfx::NativeViewAccessible AXPlatformNodeMac::GetNativeViewAccessible() {
 void AXPlatformNodeMac::NotifyAccessibilityEvent(ax::mojom::Event event_type) {
   GetNativeViewAccessible();
   // Handle special cases.
-  NSString* announcement_text =
-      [native_node_ announcementTextForEvent:event_type];
-  if (announcement_text) {
-    PostAnnouncementNotification(announcement_text);
+
+  // Alerts and live regions go through the announcement API instead of the
+  // regular NSAccessibility notification system.
+  if (event_type == ax::mojom::Event::kAlert ||
+      event_type == ax::mojom::Event::kLiveRegionChanged) {
+    if (auto announcement = [native_node_ announcementForEvent:event_type]) {
+      [native_node_ scheduleLiveRegionAnnouncement:std::move(announcement)];
+    }
     return;
   }
   if (event_type == ax::mojom::Event::kSelection) {
@@ -1193,7 +1236,7 @@ void AXPlatformNodeMac::NotifyAccessibilityEvent(ax::mojom::Event event_type) {
 }
 
 void AXPlatformNodeMac::AnnounceText(const base::string16& text) {
-  PostAnnouncementNotification(base::SysUTF16ToNSString(text));
+  PostAnnouncementNotification(base::SysUTF16ToNSString(text), false);
 }
 
 int AXPlatformNodeMac::GetIndexInParent() {