Bug 1896047 part 2: Introduce CssAltContent class to query alternative text specified in the CSS content property. r=eeejay

This allows us to check for alt text and also to output all of it to a string.
For now, this only supports plain strings, but this will be extended in a subsequent patch.

Differential Revision: https://phabricator.services.mozilla.com/D210014

accessible/base/CssAltContent.cpp

@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "CssAltContent.h"
+#include "nsIContent.h"
+#include "nsIFrame.h"
+
+namespace mozilla::a11y {
+
+CssAltContent::CssAltContent(nsIContent* aContent) {
+  nsIFrame* frame = aContent->GetPrimaryFrame();
+  if (!frame) {
+    return;
+  }
+  if (!frame->IsReplaced()) {
+    return;
+  }
+  // Check if this is for a pseudo-element.
+  if (aContent->IsInNativeAnonymousSubtree()) {
+    nsIContent* parent = aContent->GetParent();
+    if (parent && (parent->IsGeneratedContentContainerForBefore() ||
+                   parent->IsGeneratedContentContainerForAfter() ||
+                   parent->IsGeneratedContentContainerForMarker())) {
+      // We need the frame from the pseudo-element to get the content style.
+      frame = parent->GetPrimaryFrame();
+      if (!frame) {
+        return;
+      }
+    }
+  }
+  mItems = frame->StyleContent()->AltContentItems();
+}
+
+void CssAltContent::AppendToString(nsAString& aOut) {
+  // There can be multiple alt text items.
+  for (const auto& item : mItems) {
+    if (item.IsString()) {
+      aOut.Append(NS_ConvertUTF8toUTF16(item.AsString().AsString()));
+    }
+  }
+}
+
+}  // namespace mozilla::a11y

accessible/base/CssAltContent.h

@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef CssAltContent_h_
+#define CssAltContent_h_
+
+#include "nsStyleStruct.h"
+
+namespace mozilla::a11y {
+
+/**
+ * Queries alternative text specified in the CSS content property.
+ */
+class MOZ_STACK_CLASS CssAltContent {
+ public:
+  explicit CssAltContent(nsIContent* aContent);
+
+  /**
+   * Checks whether any CSS alt text has been specified. For example:
+   * if (CssAltContent(someContentNode)) ...
+   */
+  explicit operator bool() const { return !mItems.IsEmpty(); }
+
+  /**
+   * Append all CSS alt text to a string.
+   */
+  void AppendToString(nsAString& aOut);
+
+ private:
+  mozilla::Span<const mozilla::StyleContentItem> mItems;
+};
+
+}  // namespace mozilla::a11y
+
+#endif

accessible/base/moz.build

@@ -48,6 +48,7 @@ UNIFIED_SOURCES += [
     "ARIAStateMap.cpp",
     "Asserts.cpp",
     "CachedTableAccessible.cpp",
+    "CssAltContent.cpp",
     "DocManager.cpp",
     "EmbeddedObjCollector.cpp",
     "EventQueue.cpp",