Add a class comment to LayoutText

Source/core/layout/LayoutText.h

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004-2009, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 19 matching lines @@
 #include "platform/LengthFunctions.h"
 #include "platform/text/TextPath.h"
 #include "wtf/Forward.h"
 #include "wtf/PassRefPtr.h"
 
 namespace blink {
 
 class AbstractInlineTextBox;
 class InlineTextBox;
 
+// LayoutText is the root class for anything that represents
+// a text node (see core/dom/Text.h).
+//
+// To limit memory overhead, this class inherits directly
+// from LayoutObject. It also means that it's mostly a
+// placeholder in the tree for the text node's content and
+// the line boxes.
+//
+// The actual layout of text is handled by the containing inline
+// or block flow, that will invoke the bidi algorithm to break the
+// text into lines (m_firstTextBox and m_lastTextBox).
+// See e.g. LayoutBlockFlow::layoutInlineChildren.

[eae, 2015/08/27 22:22:25]

It is probably worth pointing out that this class does contain the logic for
prefs width calculation so it's more than just a placeholder.

+//
+// The previous comment applies also for painting.
 class CORE_EXPORT LayoutText : public LayoutObject {
 public:
     // FIXME: If the node argument is not a Text node or the string argument is
     // not the content of the Text node, updating text-transform property
     // doesn't re-transform the string.
     LayoutText(Node*, PassRefPtr<StringImpl>);
 #if ENABLE(ASSERT)
     ~LayoutText() override;
 #endif
 
@@ skipping 114 matching lines @@
     void computePreferredLogicalWidths(float leadWidth);
     void computePreferredLogicalWidths(float leadWidth, HashSet<const SimpleFontData*>& fallbackFonts, FloatRect& glyphBounds);
 
     bool computeCanUseSimpleFontCodePath() const;
 
     // Make length() private so that callers that have a LayoutText*
     // will use the more efficient textLength() instead, while
     // callers with a LayoutObject* can continue to use length().
     unsigned length() const final { return textLength(); }
 
+    // See the class comment as to why we shouldn't call this function directly.
     void paint(const PaintInfo&, const LayoutPoint&) final { ASSERT_NOT_REACHED(); }
     void layout() final { ASSERT_NOT_REACHED(); }
     bool nodeAtPoint(HitTestResult&, const HitTestLocation&, const LayoutPoint&, HitTestAction) final { ASSERT_NOT_REACHED(); return false; }
 
     void deleteTextBoxes();
     bool containsOnlyWhitespace(unsigned from, unsigned len) const;
     float widthFromFont(const Font&, int start, int len, float leadWidth, float textWidthSoFar, TextDirection, HashSet<const SimpleFontData*>* fallbackFonts, FloatRect* glyphBoundsAccumulation) const;
 
     void secureText(UChar mask);
 
@@ skipping 61 matching lines @@
 inline LayoutText* Text::layoutObject() const
 {
     return toLayoutText(CharacterData::layoutObject());
 }
 
 void applyTextTransform(const ComputedStyle*, String&, UChar);
 
 } // namespace blink
 
 #endif // LayoutText_h