Introduce AccessibilityFocusRing.

chrome/browser/chromeos/ui/accessibility_focus_ring.h

+// Copyright 2014 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 CHROME_BROWSER_CHROMEOS_UI_ACCESSIBILITY_FOCUS_RING_H_
+#define CHROME_BROWSER_CHROMEOS_UI_ACCESSIBILITY_FOCUS_RING_H_
+
+#include "ui/gfx/point.h"
+#include "ui/gfx/rect.h"
+
+namespace chromeos {
+
+// An AccessibilityFocusRing is a special type of shape designed to
+// outline the focused object on the screen for users with visual
+// impairments. It's specifically designed to outline text ranges that
+// span multiple lines (we'll call this a "paragraph" shape from here on,
+// but it works for any text range), so it can outline a shape defined by a
+// few words from the first line, the complete contents of more lines,
+// followed by a few words from the last line. See the figure below.
+// When highlighting any other object, it outlines a rectangular shape.
+//
+// The outline is outset from the object it's highlighting by a few pixels;
+// this margin distance also determines its border radius for rounded
+// corners.
+//
+// An AccessibilityFocusRing can be initialized with either a rectangle
+// defining the bounds of an object, or a paragraph-shape with three
+// rectangles defining a top line, a body, and a bottom line, which are
+// assumed to be adjacent to one another.
+//
+// Initializing an AccessibilityFocusRing computes the following 36 points
+// that completely define the shape's outline. This shape can be traced
+// using Skia or any other drawing utility just by drawing alternating
+// straight lines and quadratic curves (e.g. a line from 0 to 1, a curve
+// from 1 to 3 with 2 as a control point, then a line from 3 to 4, and so on.
+//
+// The same path should be used even if the focus ring was initialized with
+// a rectangle and not a paragraph shape - this makes it possible to
+// smoothly animate between one object and the next simply by interpolating
+// points.
+//
+// Noncontiguous shapes should be handled by drawing multiple focus rings.
+//
+// The 36 points are defined as follows:
+//
+//          2 3------------------------------4 5
+//           /                                \
+//          1                                  6
+//          |      First line of paragraph     |
+//          0                                  7
+//         /                                    \
+// 32 33-34 35                                 8 9---------------10 11
+//   /                                                             \
+// 31      Middle line of paragraph..........................       12
+// |                                                                |
+// |                                                                |
+// |       Middle line of paragraph..........................       |
+// |                                                                |
+// |                                                                |
+// 30      Middle line of paragraph..........................       13
+//   \                                                             /
+// 29 28---------27 26                             17 16---------15 14
+//                 \                                 /
+//                  25                             18
+//                  |    Last line of paragraph    |
+//                  24                             19
+//                    \                           /
+//                  23 22-----------------------21 20
+
+struct AccessibilityFocusRing {
+  // Construct an AccessibilityFocusRing that outlines a rectangular object.
+  static AccessibilityFocusRing CreateWithRect(
+      const gfx::Rect& bounds, int margin);
+
+  // Construct an AccessibilityFocusRing that outlines a paragraph-shaped
+  // object.
+  static AccessibilityFocusRing CreateWithParagraphShape(
+      const gfx::Rect& top_line,
+      const gfx::Rect& body,
+      const gfx::Rect& bottom_line,
+      int margin);
+
+  gfx::Point points[36];
+  gfx::Rect GetBounds() const;
+};
+
+}  // namespace chromeos
+
+#endif  // CHROME_BROWSER_CHROMEOS_UI_ACCESSIBILITY_FOCUS_RING_H_