Playground: Show placeholders for unregistered widgets

With this, Playground can be used to interactively edit e.g.
DisplaySettingsWindow.gml.

DevTools/Playground/main.cpp

@@ -40,11 +40,43 @@
 #include <LibGUI/Menu.h>
 #include <LibGUI/MenuBar.h>
 #include <LibGUI/MessageBox.h>
+#include <LibGUI/Painter.h>
 #include <LibGUI/Splitter.h>
 #include <LibGUI/TextEditor.h>
 #include <LibGUI/Window.h>
 #include <string.h>
 
+namespace {
+
+class UnregisteredWidget final : public GUI::Widget {
+    C_OBJECT(UnregisteredWidget);
+
+private:
+    UnregisteredWidget(const String& class_name);
+
+    virtual void paint_event(GUI::PaintEvent& event) override;
+
+    String m_text;
+};
+
+UnregisteredWidget::UnregisteredWidget(const String& class_name)
+{
+    StringBuilder builder;
+    builder.append(class_name);
+    builder.append("\nnot registered");
+    m_text = builder.to_string();
+}
+
+void UnregisteredWidget::paint_event(GUI::PaintEvent& event)
+{
+    GUI::Painter painter(*this);
+    painter.add_clip_rect(event.rect());
+    painter.fill_rect(event.rect(), Gfx::Color::DarkRed);
+    painter.draw_text(rect(), m_text, Gfx::TextAlignment::Center, Color::White);
+}
+
+}
+
 class GMLAutocompleteProvider final : public virtual GUI::AutocompleteProvider {
 public:
     GMLAutocompleteProvider() { }
@@ -292,7 +324,9 @@ int main(int argc, char** argv)
 
     editor.on_change = [&] {
         preview.remove_all_children();
-        preview.load_from_gml(editor.text());
+        preview.load_from_gml(editor.text(), [](const String& class_name) -> RefPtr<GUI::Widget> {
+            return UnregisteredWidget::construct(class_name);
+        });
     };
 
     auto menubar = GUI::MenuBar::construct();

Libraries/LibGUI/Widget.cpp

@@ -953,14 +953,22 @@ void Widget::set_override_cursor(Gfx::StandardCursor cursor)
 }
 
 bool Widget::load_from_gml(const StringView& gml_string)
+{
+    return load_from_gml(gml_string, [](const String& class_name) -> RefPtr<Widget> {
+        dbgln("Class '{}' not registered", class_name);
+        return nullptr;
+    });
+}
+
+bool Widget::load_from_gml(const StringView& gml_string, RefPtr<Widget> (*unregistered_child_handler)(const String&))
 {
     auto value = parse_gml(gml_string);
     if (!value.is_object())
         return false;
-    return load_from_json(value.as_object());
+    return load_from_json(value.as_object(), unregistered_child_handler);
 }
 
-bool Widget::load_from_json(const JsonObject& json)
+bool Widget::load_from_json(const JsonObject& json, RefPtr<Widget> (*unregistered_child_handler)(const String&))
 {
     json.for_each_member([&](auto& key, auto& value) {
         set_property(key, value);
@@ -1004,15 +1012,17 @@ bool Widget::load_from_json(const JsonObject& json)
                 dbgln("No class name in entry");
                 return false;
             }
-            auto* registration = WidgetClassRegistration::find(class_name.as_string());
-            if (!registration) {
-                dbg() << "Class '" << class_name.as_string() << "' not registered";
-                return false;
-            }
 
-            auto child_widget = registration->construct();
+            RefPtr<Widget> child_widget;
+            if (auto* registration = WidgetClassRegistration::find(class_name.as_string())) {
+                child_widget = registration->construct();
+            } else {
+                child_widget = unregistered_child_handler(class_name.as_string());
+                if (!child_widget)
+                    return false;
+            }
             add_child(*child_widget);
-            child_widget->load_from_json(child_json);
+            child_widget->load_from_json(child_json, unregistered_child_handler);
         }
     }
 

Libraries/LibGUI/Widget.h

@@ -298,6 +298,7 @@ public:
     void set_override_cursor(Gfx::StandardCursor);
 
     bool load_from_gml(const StringView&);
+    bool load_from_gml(const StringView&, RefPtr<Widget> (*unregistered_child_handler)(const String&));
 
     void set_shrink_to_fit(bool);
     bool is_shrink_to_fit() const { return m_shrink_to_fit; }
@@ -350,7 +351,7 @@ private:
     void focus_previous_widget(FocusSource, bool siblings_only);
     void focus_next_widget(FocusSource, bool siblings_only);
 
-    bool load_from_json(const JsonObject&);
+    bool load_from_json(const JsonObject&, RefPtr<Widget> (*unregistered_child_handler)(const String&));
 
     // HACK: These are used as property getters for the fixed_* size property aliases.
     int dummy_fixed_width() { return 0; }