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.

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;
@@ -520,11 +520,20 @@ void Menu_Item_Node::write_item(fld::io::Code_Writer& f) {
     f.write_c(", 0, ");
   }
   if (callback()) {
-    const char* k = is_name(callback()) ? nullptr : class_name(1);
-    if (k) {
-      f.write_c(" (Fl_Callback*)%s::%s,", k, callback_name(f));
+    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 {
-      f.write_c(" (Fl_Callback*)%s,", callback_name(f));
+      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,");
@@ -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()) {
-    f.write_c("%s%s->callback((Fl_Callback*)%s", f.indent(), var, callback_name(f));
+    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