"Bootstrap" a toolkit-views Typography spec.

ui/views/style/typography.h

+// Copyright 2017 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 UI_VIEWS_STYLE_TYPOGRAPHY_H_
+#define UI_VIEWS_STYLE_TYPOGRAPHY_H_
+
+#include "ui/views/views_export.h"
+
+namespace views {
+
+// An enum class that can be extended by embedders without losing type safety or
+// the ability to use values in switch statements. Except for the way the values
+// are declared, the syntax for using an ExtendableEnum should be the same as a
+// regular enum class. Uses default copy constructor and assignment operators
+// but note that conversion/assignment between different template types is not
+// permitted.
+template <class ENUM>
+class ExtendableEnum {
+ public:
+  // Used to declare the enum constants.
+  class Value;  // Inherits from this, so can't be defined inline.
+
+  // The enum value, available at compile time when using a Value directly.
+  constexpr int value() const { return value_; }
+
+ protected:
+  explicit constexpr ExtendableEnum(int value) : value_(value) {}
+
+ private:
+  int value_;
+};
+
+template <class ENUM>
+class ExtendableEnum<ENUM>::Value : public ExtendableEnum<ENUM> {
+ public:
+  // Allows use in switch case statements.
+  constexpr operator int() const { return this->value(); }
+
+  // Allow implicit initialization from an integer literal.
+  constexpr Value(int const_value) : ExtendableEnum(const_value) {}
+};
+
+// Where a piece of text appears in the UI. This influences size and weight, but
+// typically not style or color.
+class VIEWS_EXPORT TextContext : public ExtendableEnum<TextContext> {
+ public:
+  // A title for a dialog window. Usually 15pt. Multi-line OK.
+  static constexpr const Value DIALOG_TITLE = 1;

[Peter Kasting, 2017/03/15 10:01:48]

I don't think you need both constexpr and const on a variable declaration, just
on a method (in C++14)?  I think on variables, constexpr still implies const
post-C++11?

[tapted, 2017/03/16 11:13:39]

You are correct! (but, no longer using this)

+
+  // "Body 2". Usually 12pt.
+  static constexpr const Value CONTROL_LABEL = 3;

[Peter Kasting, 2017/03/15 10:01:48]

Skipped 2

[tapted, 2017/03/16 11:13:39]

no longer using this (it is perilous, clearly ;)

+
+  // A text field. Usually matches CONTROL_LABEL.
+  static constexpr const Value FIELD = 4;
+
+  // Text that appears on a button control. Usually 12pt.
+  static constexpr const Value BUTTON_TEXT = 5;
+
+  // Allow implicit conversion from the constants above.
+  constexpr TextContext(const Value& value) : ExtendableEnum(value.value()) {}
+
+  // Allow declaration without assignment.
+  constexpr TextContext() : ExtendableEnum(0) {}
+
+ protected:
+  enum { VIEWS_MAX = 15 };

[Peter Kasting, 2017/03/15 10:01:48]

15?  (2 places)

[tapted, 2017/03/15 11:04:07]

I guess I was going for
 - a buffer
 - not too big
 - 2^N - 1

[Peter Kasting, 2017/03/15 17:52:47]

Do we need a buffer?  That's only important if these values can't be changed. 
But the values aren't persisted on disk/network anywhere, so it seems OK to have
the last value just be the max, and if we add future values, everything gets
renumbered at recompile.

[tapted, 2017/03/16 11:13:39]

buffer thinking was that there would be less chance of someone forgetting to
update VIEWS_MAX when adding something and "15 constants is all that views would
ever need".

We now need a buffer of a different kind (between context and style, I picked
0x1000)

+};
+
+// How a piece of text should be presented. This influences color and style, but
+// typically not size.
+class VIEWS_EXPORT TextStyle : public ExtendableEnum<TextStyle> {
+ public:
+  // Primary text: solid black, normal weight. Converts to DISABLED in some
+  // contexts (e.g. BUTTON_TEXT, FIELD).
+  static constexpr const Value PRIMARY = 1;
+
+  // Disabled "greyed out" text.
+  static constexpr const Value DISABLED = 2;
+
+  // A solid shade of blue.
+  static constexpr const Value LINK = 3;
+
+  // Active tab in a tabbed pane.
+  static constexpr const Value ACTIVE_TAB = 4;
+
+  // Inactive tab in a tabbed pane.
+  static constexpr const Value INACTIVE_TAB = 5;
+
+  // Hovered tab in a tabbed pane.
+  static constexpr const Value HOVERED_TAB = 6;
+
+  constexpr TextStyle(const Value& value) : ExtendableEnum(value.value()) {}
+  constexpr TextStyle() : ExtendableEnum(0) {}
+
+ protected:
+  enum { VIEWS_MAX = 15 };
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_STYLE_TYPOGRAPHY_H_