FLUID: Add support for lambda callbacks.

Starting the callback text with a '[' assumes that
the rest of the callback is a lambda and generates
inlined code for it.

documentation/src/common.dox

@@ -552,11 +552,10 @@ int xyz_data;
 button->callback(xyz_callback, &xyz_data);
 \endcode
 
-\note You cannot delete a widget inside a callback, as the
-widget may still be accessed by FLTK after your callback
-is completed. Instead, use the Fl::delete_widget()
-method to mark your widget for deletion when it is safe
-to do so.
+It's also possible to use a non-capturing lambda function directly, for example:
+```
+button->callback( [](Fl_Widget*, void*) { puts("Hello"); } , nullptr );
+```
 
 Many programmers new to FLTK or C++ try to use a
 non-static class method instead of a static class method

fluid/documentation/src/page_widget_panel.dox

@@ -369,13 +369,18 @@
  \image html wp_cpp_callback.png
  \image latex wp_cpp_callback.png "" width=7cm
 
- The callback field can be interpreted in two ways. If the callback text is only
- a single word, FLUID assumes that this is the name of an external callback
- function and declares it in the header as
+ The callback field can be interpreted in three ways. If the callback text is
+ only a single word, FLUID assumes that this is the name of an external
+ callback function and declares it in the header as
  `extern void my_button_action(Fl_Button*, void*);`.
 
- Otherwise, FLUID assumes that the text is the body of a C++ callback function
- and instead creates a local static callback function. The name of the callback
+ If the first letter of the callback text is a '[', FLUID expects a lambda
+ function which will be inlined into the widget creation code. The lambda
+ signature must be `[](Fl_Widget*, void*)->void { ... }`. The widget pointer
+ can be casted to another type inside the lambda.
+
+ Otherwise, FLUID assumes that the text is the body of a C++ function,
+ and a local static callback function is created. The name of the callback
  function is generated by FLUID and guaranteed to be unique within the file.
  ```
  static void cb_input(Fl_Input *o, void *v) {

fluid/nodes/Menu_Node.cxx

@@ -329,7 +329,7 @@ void Menu_Item_Node::write_static(fld::io::Code_Writer& f) {
     if (extra_code(n) && isdeclare(extra_code(n)))
       f.write_h_once("%s", extra_code(n));
   }
-  if (callback() && !is_name(callback())) {
+  if (callback() && !is_name(callback()) && (callback()[0] != '[')) {
     // see if 'o' or 'v' used, to prevent unused argument warnings:
     int use_o = 0;
     int use_v = 0;
@@ -520,12 +520,21 @@ void Menu_Item_Node::write_item(fld::io::Code_Writer& f) {
     f.write_c(", 0, ");
   }
   if (callback()) {
+    if (callback()[0] == '[') {
+      f.write_c("\n");
+      f.tag(Mergeback::Tag::GENERIC, Mergeback::Tag::WIDGET_CALLBACK, 0);
+      f.write_c_indented(callback(), 1, 0);
+      f.write_c("\n");
+      f.tag(Mergeback::Tag::WIDGET_CALLBACK, Mergeback::Tag::GENERIC, get_uid());
+      f.write_c("%s, ", f.indent_plus(1));
+    } else {
       const char* k = is_name(callback()) ? nullptr : class_name(1);
       if (k) {
         f.write_c(" (Fl_Callback*)%s::%s,", k, callback_name(f));
       } else {
         f.write_c(" (Fl_Callback*)%s,", callback_name(f));
       }
+    }
   } else
     f.write_c(" 0,");
   if (user_data())
@@ -571,7 +580,7 @@ void Menu_Item_Node::write_code1(fld::io::Code_Writer& f) {
   }
 
   if (callback()) {
-    if (!is_name(callback()) && class_name(1)) {
+    if (!is_name(callback()) && (callback()[0] != '[') && class_name(1)) {
       const char* cn = callback_name(f);
       const char* ut = user_data_type() ? user_data_type() : "void*";
       f.write_public(0);

fluid/nodes/Widget_Node.cxx

@@ -1499,7 +1499,7 @@ void Widget_Node::write_static(fld::io::Code_Writer& f) {
     if (strchr(c, '[') == nullptr) f.write_c("%s *%s=(%s *)0;\n", t, c, t);
     else f.write_c("%s *%s={(%s *)0};\n", t, c, t);
   }
-  if (callback() && !is_name(callback())) {
+  if (callback() && !is_name(callback()) && (callback()[0] != '[')) {
     // see if 'o' or 'v' used, to prevent unused argument warnings:
     int use_o = 0;
     int use_v = 0;
@@ -1883,7 +1883,16 @@ void Widget_Node::write_widget_code(fld::io::Code_Writer& f) {
   const char* ud = user_data();
   if (class_name(1) && !parent->is_widget()) ud = "this";
   if (callback()) {
+    if (callback()[0] == '[') {
+      f.write_c("%s%s->callback(\n", f.indent(), var);
+      f.tag(Mergeback::Tag::GENERIC, Mergeback::Tag::WIDGET_CALLBACK, 0);
+      f.write_c_indented(callback(), 1, 0);
+      f.write_c("\n");
+      f.tag(Mergeback::Tag::WIDGET_CALLBACK, Mergeback::Tag::GENERIC, get_uid());
+      f.write_c("%s", f.indent_plus(1));
+    } else {
       f.write_c("%s%s->callback((Fl_Callback*)%s", f.indent(), var, callback_name(f));
+    }
     if (ud)
       f.write_c(", (void*)(%s));\n", ud);
     else

test/preferences.fl

@@ -7,6 +7,7 @@ i18n_gnu_function gettext
 i18n_gnu_static_function gettext_noop
 header_name {.h}
 code_name {.cxx}
+include_guard {}
 comment {About test/preferences:
 
 The preferences app shows two features of FLTK and FLUID.
@@ -184,6 +185,10 @@ Fl_Input::paste_menu_text = gettext("Paste");} {}
         }
         MenuItem {} {
           label sandals
+          callback {[](Fl_Widget*, void*)
+{
+  puts("The shoe is the sign!");
+}} selected
           xywh {0 0 100 20}
         }
         MenuItem {} {
@@ -203,7 +208,7 @@ Fl_Input::paste_menu_text = gettext("Paste");} {}
           xywh {35 112 100 24} type Radio down_box ROUND_DOWN_BOX
         }
         Fl_Round_Button wRight {
-          label {right side} selected
+          label {right side}
           xywh {35 136 100 24} type Radio down_box ROUND_DOWN_BOX
         }
         Fl_Box {} {
@@ -217,6 +222,15 @@ Fl_Input::paste_menu_text = gettext("Paste");} {}
       }
       Fl_Check_Button wShave {
         label shave
+        callback {[](Fl_Widget* w, void*)->void {
+  auto* btn = static_cast<Fl_Check_Button*>(w);
+  if (btn->value()) {
+    puts("Shave.");
+  } else {
+    puts("Don't shave.");
+  }
+}}
+        comment {// Testing lambdas for callbacks}
         xywh {25 212 105 24} down_box DOWN_BOX
       }
       Fl_Check_Button wBrush {