For issue #358 - adds examples/cairo-draw-x.cxx

Since this is the first cairo example in the examples directory,
    it necessarily involved changes to the Makefile and to fltk-config
    to properly handle the absence/existance of the cairo libs.

    TBD: Add docs to the cario widget describing coordinate system
    and how it differs from the default cairo normalized coordinate system.

examples/Makefile

@@ -6,6 +6,7 @@ SHELL = /bin/sh
 
 # Executables
 ALL = browser-simple$(EXEEXT) \
+      cairo-draw-x$(EXEEXT) \
       chart-simple$(EXEEXT) \
       draggable-group$(EXEEXT) \
       howto-add_fd-and-popen$(EXEEXT) \
@@ -44,6 +45,14 @@ ALL = browser-simple$(EXEEXT) \
 # default target -- build everything
 default all: $(ALL)
 
+# Special rules for building cairo app
+cairo-draw-x.o: cairo-draw-x.cxx
+	@echo "*** Compile $<..."
+	$(CXX) -I.. $(CXXFLAGS_CAIRO) -c $< -o $@
+cairo-draw-x$(EXEEXT): cairo-draw-x.o
+	@echo "*** Link $<..."
+	$(CXX) $< $(LINKFLTK) $(LINKFLTK_CAIRO) -o $@
+
 # clean everything
 clean:
 	$(RM) $(ALL)

examples/Makefile.FLTK

@@ -16,13 +16,15 @@ ifeq '$(OS)' "Windows_NT"
 EXEEXT = .exe
 endif
 
-FLTKCONFIG   = ../fltk-config
-CXX          = $(shell $(FLTKCONFIG) --cxx)
-CXXFLAGS     = $(shell $(FLTKCONFIG) --cxxflags) -Wall -I.
-LINKFLTK     = $(shell $(FLTKCONFIG) --ldstaticflags)
-LINKFLTK_GL  = $(shell $(FLTKCONFIG) --use-gl --ldstaticflags)
-LINKFLTK_IMG = $(shell $(FLTKCONFIG) --use-images --ldstaticflags)
-LINKFLTK_ALL = $(shell $(FLTKCONFIG) --use-images --use-gl --ldstaticflags)
+FLTKCONFIG     = ../fltk-config
+CXX            = $(shell $(FLTKCONFIG) --cxx)
+CXXFLAGS       = $(shell $(FLTKCONFIG) --cxxflags) -Wall -I.
+LINKFLTK       = $(shell $(FLTKCONFIG) --ldstaticflags)
+LINKFLTK_GL    = $(shell $(FLTKCONFIG) --use-gl --ldstaticflags)
+LINKFLTK_IMG   = $(shell $(FLTKCONFIG) --use-images --ldstaticflags)
+LINKFLTK_ALL   = $(shell $(FLTKCONFIG) --use-images --use-gl --ldstaticflags)
+CXXFLAGS_CAIRO = $(shell $(FLTKCONFIG) --use-cairo --cxxflags)
+LINKFLTK_CAIRO = $(shell $(FLTKCONFIG) --use-cairo --ldstaticflags)
 .SUFFIXES: .cxx .h .fl .o $(EXEEXT)
 
 # HOW TO COMPILE

examples/cairo-draw-x.cxx

@@ -0,0 +1,58 @@
+//
+// Simple demo of drawing an "X" in Cairo (antialiased lines)
+//
+// 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
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
+
+#include <FL/Fl.H>               // includes <FL/fl_config.h>
+
+#ifdef FLTK_HAVE_CAIRO           // Builds example code only if cairo enabled
+#include <FL/Fl_Cairo_Window.H>
+
+// Cairo rendering cb called during Fl_Cairo_Window::draw()
+static void my_cairo_draw_cb(Fl_Cairo_Window *window, cairo_t *cr) {
+    const double xmax = (window->w() - 1);
+    const double ymax = (window->h() - 1);
+
+    // Draw green "X"
+    //     Draws an X to four corners of resizable window.
+    //     See Fl_Cairo_Window docs for more info.
+    //
+    cairo_set_antialias(cr, CAIRO_ANTIALIAS_BEST);                // use best antialiasing
+    cairo_set_line_width(cr, 1.00);                               // line width for drawing
+    cairo_set_source_rgb(cr, 1.0, 0.5, 0.0);                      // orange
+    cairo_move_to(cr, 0.0, 0.0);  cairo_line_to(cr, xmax, ymax);  // draw diagonal "\"
+    cairo_move_to(cr, 0.0, ymax); cairo_line_to(cr, xmax, 0.0);   // draw diagonal "/"
+    cairo_stroke(cr);                                             // stroke the lines
+}
+
+int main(int argc, char **argv) {
+    Fl_Cairo_Window window(300, 300, "Cairo Draw 'X'");
+    window.color(FL_BLACK);                                       // cairo window's default bg color
+    window.size_range(50,50,-1,-1);                               // allow resize 50,50 and up
+    window.resizable(&window);                                    // allow window to be resized
+    window.set_draw_cb(my_cairo_draw_cb);                         // draw callback for cairo drawing
+    window.show(argc, argv);
+    return Fl::run();
+}
+
+// The code that follows just allows the example to build even if cairo wasn't enabled.
+#else // (!FLTK_HAVE_CAIRO)
+#include <FL/fl_ask.H>
+int main(int argc, char **argv) {
+    fl_message_title("This program needs a Cairo enabled FLTK library");
+    fl_message("Please configure FLTK with Cairo enabled (--enable-cairo or --enable-cairoext)\n"
+               "or one of the CMake options OPTION_CAIRO or OPTION_CAIROEXT, respectively.");
+    return 0;
+}
+#endif // (FLTK_HAVE_CAIRO)

fltk-config.in

@@ -49,6 +49,13 @@ LDLIBS="@LIBS@"
 OPTIM="@OPTIM@"
 CAIROFLAGS="@CAIROFLAGS@"
 
+# Config
+if grep -q '^#define FLTK_HAVE_CAIRO 1' $selfdir/FL/fl_config.h; then
+    FLTK_HAVE_CAIRO=1
+else
+    FLTK_HAVE_CAIRO=0
+fi
+
 # Check for local invocation, and update paths accordingly...
 if test -f "$selfdir/FL/Fl_Window.H"; then
 	includedir="$selfdir"
@@ -75,7 +82,7 @@ if test -d $includedir/FL/images; then
 	CXXFLAGS="-I$includedir/FL/images $CXXFLAGS"
 fi
 
-if test -f "$libdir/libfltk_cairo.a"; then
+if [ $FLTK_HAVE_CAIRO = 1 ]; then
 	CFLAGS="$CAIROFLAGS $CFLAGS"
 	CXXFLAGS="$CAIROFLAGS $CXXFLAGS"
 fi
@@ -249,7 +256,7 @@ if test x$use_images = xyes; then
     LDSTATIC="$libdir/libfltk_images.a $STATICIMAGELIBS $LDSTATIC"
 fi
 
-if test x$use_cairo = xyes; then
+if test x$use_cairo = xyes -a $FLTK_HAVE_CAIRO = 1; then
     LDLIBS="-lfltk_cairo$SHAREDSUFFIX $CAIROLIBS $LDLIBS"
     LDSTATIC="$libdir/libfltk_cairo.a $CAIROLIBS $LDSTATIC"
 fi