[gtk-rs] Passing Widget as GValue through Drag and drop with composite template

[Jokari]

Hello, I am looking for a bit of help regarding Drag and Drop with gtk-rs please.
I use the composite template feature.
I have a widget FooFrame, subclass of gtkFrame. It contains another pok_frame gtk::Frame and a target gtk::Box to receive the drop.
The goal is to be able to get a reference for the Frame widget that was dragged and dropped onto the target box.

/// FooFrame widget Subclass of Gtk::Frame widget
 #[derive(Debug, Default, CompositeTemplate)]
 #[template(resource = "/ui/layout.ui")]
 pub struct FooFrame {
    #[template_child]
    pub target_box: TemplateChild<gtk::Box>,
    #[template_child]
    pub pok_frame: TemplateChild<gtk::Frame>,
 }

I call a function to setup a DragSource in

impl ObjectImpl for FooFrame {
        fn constructed(&self) {
            self.parent_constructed();
            
            self.obj().set_drag();
        }
}

Details of the setup function and the “on_drop” callback for drop signal:

#[gtk::template_callbacks]
impl FooFrame {
    /// Setup DragSource.
    fn set_drag(&self) {
        let frame: gtk::Frame = self.imp().pok_frame.get();
        
        let drag = gtk::DragSource::new();
        drag.set_actions(gtk::gdk::DragAction::COPY);

        drag.connect_prepare(
        clone!(@weak frame => @default-return None, move |_, _, _| Some(ContentProvider::for_value(&frame.to_value()))));

        frame.add_controller(drag);
    }
        
    /// Callback used for drop signal.
    #[template_callback]
    pub fn on_drop(&self, value: &glib::Value, x: f64, y: f64) -> bool {
        // Program panic here
        let res_frame: Frame = value.get().expect("Error getting GtkFrame from Drop");
	
	// Do something after.
        info!("In connect_drop. Drop at {x} {y}");
        true
    }
}

The DropTarget is define in the XML file of FooFrame.

XML definition of the widget:

<?xml version='1.0' encoding='UTF-8'?>
<interface>
  <template class="FooFrame" parent="GtkFrame">
    <child>
      <object class="GtkBox" id="target_box">
        <child>
          <object class="GtkDropTarget" id="drop_target">
            <property name="actions">copy</property>
            <property name="formats">GtkFrame</property>
            <signal name="drop" handler="on_drop" swapped="true"/>
          </object>
        </child>
        <child>
          <object class="GtkBox">
            <child>
              <object class="GtkFrame" id="pok_frame"/>
            </child>
          </object>
        </child>
    </child>
  </template>
</interface>

I run the app, drag the frame widget and drop it on the frame target. The app panic and output:

thread ‘main’ panicked at ‘Error getting GtkFrame from Drop: WrongValueType(ValueTypeMismatchError { actual: GValue, requested: GtkFrame })’

I would like to get the GtkFrame but I manage only to get a GValue which can’t, obviously, be “converted” into GtkFrame.
I don’t understand what is wrong with my setup and how GtkFrame value is passed with the DropTarget.
Is there something obviously wrong in my setup please?

Thanks for your time.

[jfrancis (Jason Francis)]

Hello, this is a quirk of the way the Rust bindings work and should probably be documented better. Using a glib::Value will give you the raw value passed to the closure but will not work right when the argument itself is a Value. To get a Value that is “inside” another Value you have to use BoxedValue:

    pub fn on_drop(&self, value: glib::BoxedValue, x: f64, y: f64) -> bool {
        // ...
   }

[Jokari]

Thanks a lot!

It works like a charm now. I agree that the documentation should reflect this because this is impossible/very hard to guess otherwise.