Refactor and simplify "arrow drawing" in widgets

"Arrows" in widgets are those GUI elements mostly represented by
triangles pointing in a particular direction as in scrollbars,
choice widgets, some menus, valuators and Fl_Counter widgets.

The code has been simplified and standardized such that all these
GUI elements are drawn identically per FLTK scheme.

Widget authors no longer need to write code to calculate arrow sizes
and draw polygons etc.

Different schemes can and do implement different drawing functions.

Todo: see comments "FIXME_ARROW" in src/Fl_Menu_Button.cxx and
      src/Fl_Menu.cxx

FL/Enumerations.H

@@ -1,7 +1,7 @@
 //
 // Enumerations for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2020 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -1223,4 +1223,57 @@ enum Fl_Damage {
 
 // FLTK 1.0.x compatibility definitions (FLTK_1_0_COMPAT) dropped in 1.4.0
 
+/** Arrow types define the type of arrow drawing function.
+
+  FLTK schemes can draw several graphical elements in their particular way.
+  One of these elements is an arrow type that can be in different GUI
+  elements like scrollbars, choice buttons, and FLTK's Fl_Return_Button.
+
+  \note This enum is not yet stable (as of FLTK 1.4.0) and may be changed
+    without notice as necessary.
+
+  \since 1.4.0
+*/
+
+enum Fl_Arrow_Type {
+  FL_ARROW_SINGLE = 0x01, ///< Single arrow, e.g. in Fl_Scrollbar
+  FL_ARROW_DOUBLE = 0x02, ///< Double arrow, e.g. in Fl_Counter
+  FL_ARROW_CHOICE = 0x03, ///< Dropdown box, e.g. in Fl_Choice
+  FL_ARROW_RETURN = 0x04  ///< Return arrow, e.g. in Fl_Return_Button
+};
+
+/** Fl_Orientation describes the orientation of a GUI element.
+
+  FLTK schemes can draw several graphical elements, for instance arrows,
+  pointing at different directions. This enum defines the direction
+  to use for drawing a particular GUI element.
+
+  The definition of this enum was chosen such that the enum value can
+  be multiplied by 45 to get a rotation angle in degrees starting
+  at the horizontal axis (0 = right, 1 = NE, 2 = up, ...) that can be
+  used with fl_rotate(). Note: angle is counter-clockwise in degrees.
+
+  The 'unspecified' value \b FL_ORIENT_NONE shall be used for elements
+  that would usually not be rotated, like the return arrow of the
+  Fl_Return_Button. It can still be used as an angle though since it is
+  the same value as \p FL_ORIENT_RIGHT (0 degrees).
+
+  \note This enum is not yet stable (as of FLTK 1.4.0) and may be changed
+    without notice as necessary.
+
+  \since 1.4.0
+*/
+
+enum Fl_Orientation {
+  FL_ORIENT_NONE  = 0x00, ///< GUI element direction is unspecified
+  FL_ORIENT_RIGHT = 0x00, ///< GUI element pointing right (  0°)
+  FL_ORIENT_NE    = 0x01, ///< GUI element pointing NE    ( 45°)
+  FL_ORIENT_UP    = 0x02, ///< GUI element pointing up    ( 90°)
+  FL_ORIENT_NW    = 0x03, ///< GUI element pointing NW    (135°)
+  FL_ORIENT_LEFT  = 0x04, ///< GUI element pointing left  (180°)
+  FL_ORIENT_SW    = 0x05, ///< GUI element pointing SW    (225°)
+  FL_ORIENT_DOWN  = 0x06, ///< GUI element pointing down  (270°)
+  FL_ORIENT_SE    = 0x07  ///< GUI element pointing SE    (315°)
+};
+
 #endif

FL/Fl_Counter.H

@@ -49,14 +49,16 @@ class FL_EXPORT Fl_Counter : public Fl_Valuator {
   Fl_Fontsize textsize_;
   Fl_Color textcolor_;
   double lstep_;
-  uchar mouseobj;
+  uchar mouseobj_;
   static void repeat_callback(void *);
   int calc_mouseobj();
   void increment_cb();
 
 protected:
 
   void draw();
+  // compute widths of arrow boxes
+  void arrow_widths(int &w1, int &w2);
 
 public:
 

FL/Fl_Spinner.H

@@ -1,7 +1,7 @@
 //
 // Spinner widget for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2017 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -66,6 +66,8 @@ protected:
                 up_button_,             // Up button
                 down_button_;           // Down button
 
+  virtual void draw();
+
 public:
 
   // Constructor

FL/fl_draw.H

@@ -1,7 +1,7 @@
 //
 // Portable drawing function header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2021 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -250,15 +250,15 @@ inline void fl_line_style(int style, int width = 0, char *dashes = 0) {
   fl_graphics_driver->line_style(style, width, dashes);
 }
 enum {
-  FL_SOLID = 0,      ///< line style: <tt>___________</tt>
-  FL_DASH = 1,       ///< line style: <tt>_ _ _ _ _ _</tt>
-  FL_DOT = 2,        ///< line style: <tt>. . . . . .</tt>
-  FL_DASHDOT = 3,    ///< line style: <tt>_ . _ . _ .</tt>
-  FL_DASHDOTDOT = 4, ///< line style: <tt>_ . . _ . .</tt>
+  FL_SOLID      = 0,      ///< line style: <tt>___________</tt>
+  FL_DASH       = 1,      ///< line style: <tt>_ _ _ _ _ _</tt>
+  FL_DOT        = 2,      ///< line style: <tt>. . . . . .</tt>
+  FL_DASHDOT    = 3,      ///< line style: <tt>_ . _ . _ .</tt>
+  FL_DASHDOTDOT = 4,      ///< line style: <tt>_ . . _ . .</tt>
 
-  FL_CAP_FLAT = 0x100,   ///< cap style: end is flat
-  FL_CAP_ROUND = 0x200,  ///< cap style: end is round
-  FL_CAP_SQUARE = 0x300, ///< cap style: end wraps end point
+  FL_CAP_FLAT   = 0x100,  ///< cap style: end is flat
+  FL_CAP_ROUND  = 0x200,  ///< cap style: end is round
+  FL_CAP_SQUARE = 0x300,  ///< cap style: end wraps end point
 
   FL_JOIN_MITER = 0x1000, ///< join style: line join extends to a point
   FL_JOIN_ROUND = 0x2000, ///< join style: line join is rounded
@@ -939,6 +939,9 @@ FL_EXPORT void fl_draw_box(Fl_Boxtype, int x, int y, int w, int h, Fl_Color);
 // Draw a check mark in the given color inside the bounding box bb.
 void fl_draw_check(Fl_Rect bb, Fl_Color col);
 
+// Draw one or more "arrows" (triangles)
+FL_EXPORT void fl_draw_arrow(Fl_Rect bb, Fl_Arrow_Type t, Fl_Orientation o, Fl_Color color);
+
 // images:
 
 /**

src/CMakeLists.txt

@@ -136,6 +136,7 @@ set (CPPFILES
   fl_curve.cxx
   fl_diamond_box.cxx
   fl_draw.cxx
+  fl_draw_arrow.cxx
   fl_draw_pixmap.cxx
   fl_encoding_latin1.cxx
   fl_encoding_mac_roman.cxx

src/Fl_Choice.cxx

@@ -1,7 +1,7 @@
 //
 // Choice widget for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2015 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -22,6 +22,11 @@
 // Emulates the Forms choice widget.  This is almost exactly the same
 // as an Fl_Menu_Button.  The only difference is the appearance of the
 // button: it draws the text of the current pick and a down-arrow.
+// The exact layout and the type of arrow can vary by FLTK scheme.
+
+// FIXME: all such variations should be implemented in the "scheme code",
+// hopefully in a future class derived from a base class Fl_Scheme or similar.
+// Albrecht
 
 void Fl_Choice::draw() {
   Fl_Boxtype btype = Fl::scheme() ? FL_UP_BOX           // non-default uses up box
@@ -32,70 +37,74 @@ void Fl_Choice::draw() {
   // Arrow area
   int H = h() - 2 * dy;
   int W = Fl::is_scheme("gtk+")    ? 20 :                       // gtk+  -- fixed size
-          Fl::is_scheme("gleam")   ? 20 :                       // gleam -- fixed size
-          Fl::is_scheme("plastic") ? ((H > 20) ? 20 : H)        // plastic: shrink if H<20
-                                   : ((H > 20) ? 20 : H);       // default: shrink if H<20
+          Fl::is_scheme("gleam")   ? 20                         // gleam -- fixed size
+                                   : ((H > 20) ? 20 : H);       // else: shrink if H<20
   int X = x() + w() - W - dx;
   int Y = y() + dy;
 
-  // Arrow object
-  int w1 = (W - 4) / 3; if (w1 < 1) w1 = 1;
-  int x1 = X + (W - 2 * w1 - 1) / 2;
-  int y1 = Y + (H - w1 - 1) / 2;
+  Fl_Rect ab(X, Y, W, H); // arrow box
+  int active = active_r();
+  Fl_Color arrow_color = active ? labelcolor() : fl_inactive(labelcolor());
+  Fl_Color box_color = color();
+
+  // From "original" code: modify the box color *only* for the default scheme.
+  // This is weird (why?). I believe we should either make sure that the text
+  // color contrasts well when the text is rendered *or* we should do this for
+  // *all* schemes. Anyway, adapting the old code... (Albrecht)
+
+  if (!Fl::scheme()) {            // default scheme only, see comment above
+    if (fl_contrast(textcolor(), FL_BACKGROUND2_COLOR) == textcolor())
+      box_color = FL_BACKGROUND2_COLOR;
+    else
+      box_color = fl_lighter(color());
+  }
 
-  if (Fl::scheme()) {
-    // NON-DEFAULT SCHEME
+  // Draw the widget box
 
-    // Draw widget box
-    draw_box(btype, color());
+  draw_box(btype, box_color);
 
-    // Draw arrow area
-    fl_color(active_r() ? labelcolor() : fl_inactive(labelcolor()));
-    if (Fl::is_scheme("plastic")) {
-      // Show larger up/down arrows...
-      fl_polygon(x1, y1 + 3, x1 + w1, y1 + w1 + 3, x1 + 2 * w1, y1 + 3);
-      fl_polygon(x1, y1 + 1, x1 + w1, y1 - w1 + 1, x1 + 2 * w1, y1 + 1);
-    } else {
-      // Show smaller up/down arrows with a divider...
-      x1 = x() + w() - 13 - dx;
-      y1 = y() + h() / 2;
-      fl_polygon(x1, y1 - 2, x1 + 3, y1 - 5, x1 + 6, y1 - 2);
-      fl_polygon(x1, y1 + 2, x1 + 3, y1 + 5, x1 + 6, y1 + 2);
+  // Arrow box or horizontal divider line, depending on the current scheme
+
+  // Scheme:          Box or divider line
+  // ----------------------------------------
+  // Default (None):  Arrow box (FL_UP_BOX)
+  // gtk+, gleam:     Divider line
+  // else:            Nothing (!)
+
+  if (Fl::scheme()) {
+    if (Fl::is_scheme("gtk+") ||
+        Fl::is_scheme("gleam")) {
+      // draw the divider
+      int x1 = x() + w() - 20 - dx;
+      int y1 = y() + h() / 2;
 
       fl_color(fl_darker(color()));
-      fl_yxline(x1 - 7, y1 - 8, y1 + 8);
+      fl_yxline(x1, y1 - 8, y1 + 8);
 
       fl_color(fl_lighter(color()));
-      fl_yxline(x1 - 6, y1 - 8, y1 + 8);
-    }
-  } else {
-    // DEFAULT SCHEME
-
-    // Draw widget box
-    if (fl_contrast(textcolor(), FL_BACKGROUND2_COLOR) == textcolor()) {
-      draw_box(btype, FL_BACKGROUND2_COLOR);
-    } else {
-      draw_box(btype, fl_lighter(color()));
+      fl_yxline(x1 + 1, y1 - 8, y1 + 8);
     }
-
-    // Draw arrow area
-    draw_box(FL_UP_BOX,X,Y,W,H,color());
-    fl_color(active_r() ? labelcolor() : fl_inactive(labelcolor()));
-    fl_polygon(x1, y1, x1 + w1, y1 + w1, x1 + 2 * w1, y1);
+  } else { // Default scheme ("None")
+    // Draw arrow box
+    draw_box(FL_UP_BOX, X, Y, W, H, color());
+    ab.inset(FL_UP_BOX);
   }
 
+  // Draw choice arrow(s)
+  fl_draw_arrow(ab, FL_ARROW_CHOICE, FL_ORIENT_NONE, arrow_color);
+
   W += 2 * dx;
 
   // Draw menu item's label
   if (mvalue()) {
     Fl_Menu_Item m = *mvalue();
-    if (active_r()) m.activate(); else m.deactivate();
+    if (active) m.activate(); else m.deactivate();
 
     // Clip
     int xx = x() + dx, yy = y() + dy + 1, ww = w() - W, hh = H - 2;
     fl_push_clip(xx, yy, ww, hh);
 
-    if ( Fl::scheme()) {
+    if (Fl::scheme()) {
       Fl_Label l;
       l.value = m.text;
       l.image = 0;