[HELP] drag and drop a custom rust struct

[eppixx]

I want to implement some drag and drop.
Sending a String works fine, but sending a struct is difficult for me.
I found BoxedAnyObject which can be compiled without issue like so:

struct Author {
    name: String,
    subscribers: usize,
}
// ...
        // prepare dragging source
        let boxed = glib::BoxedAnyObject::new(Author {
            name: String::from("GLibAuthor"),
            subscribers: 1000,
        });
        let content = gdk::ContentProvider::for_value(&boxed.to_value());
        model.drag_src.set_content(Some(&content));
        model.drag_src.set_actions(gdk::DragAction::MOVE);
// ...

Problem is I can't receive the drop with any Type set on DropTarget.
I expect it something to do with the following logging:
Gdk-CRITICAL **: 02:49:32.415: gdk_content_formats_builder_add_gtype: assertion 'type != G_TYPE_INVALID' failed

Is the way I'm trying to do this wrong or is there a better way to do this?

[eppixx]

I managed to find out where the error comes from. Problem was the types I accepted with set_types on the DropTarget.

I also found out I need to I need to call connect_accept on DropTarget instead of using set_types that implements a custom check if the Drop can be accepted.

This works for a String in the following way in connect_drop

//function connect_drop
    let any = value.get::<String>();
    //work with any

But when using BoxedAnyObject instead of String my application freezes with the following logs:

Glib-GObject-WARNING **: 18:06:30.439: ../../../gobject/gtype.c:4333: type id '0' is invalid
Glib-GObject-WARNING **: 18:06:30.446: can't peek value table for type '<invalid>' which is not currently referenced
Glib-GObject-WARNING **: 18:06:30.446: ../../../gobject/gvalue.c:218: cannot initialize GValue with type '(null)', this type has no GTypeValueTable implementation 
Gdk-CRITICAL **: 18:06:30.447: gdk_content_provider_get_value: assertion 'G_IS_VALUE (value)' failed
Glib-GObject-WARNING **: 18:06:30.447: ../../../gobject/gtype.c:4333: type id '0' is invalid
Glib-GObject-WARNING **: 18:06:30.447: can't peek value table for type '<invalid>' which is not currently referenced
Glib-GObject-WARNING **: 18:06:30.447: ../../../gobject/gvalue.c:218: cannot initialize GValue with type '(null)', this type has no GTypeValueTable implementation 
Gdk-CRITICAL **: 18:06:30.447: gdk_content_provider_get_value: assertion 'G_IS_VALUE (value)' failed
Glib-GIO-CRITICAL **: 18:06:30.447: g_task_return_error 'error != NULL' failed

This is logged before connect _drop is called.
The error suggests that the drop is not a gobject but isn't that what BoxedAnyObject is for?

[bilelmoussaoui]

Please share a link to a repo with an easy to reproduce example. Otherwise no one can help you.

[bilelmoussaoui]

Also please use https://github.com/gtk-rs/gtk4-rs/discussions instead of the issue tracker when asking questions.

[eppixx]

I created a repo here https://github.com/eppixx/gtk4-dnd

The relevant file is in factory/queue_item.rs

[eppixx]

I found out that transfering a struct is possible by deriving glib::Boxed like in https://gtk-rs.org/gtk-rs-core/stable/latest/docs/glib/subclass/index.html#example-for-registering-a-boxed-type-for-a-rust-struct.
Then the DropTarget needs to set_types to $STRUCT::static_type().
An abridged version:

#[derive(Clone, glib::Boxed)]
#[boxed_type(name = "Test")]
struct Test;

// snip

let src = gtk::DragSource::new();
src.set_actions(gdk::DragAction::MOVE);
src.set_content(Some(&gdk::ContentProvider::for_value(&Test.to_value())));

let dest = gtk::DropTarget::new();
dest.set_actions(gdk::DragAction::MOVE):
dest.set_types(&[Test::static_type()]);
dest.connect_drop(move |_widget, value, _x, _y| {
    if let Ok(_test) = value::get::<Test>() {
        // work with test here
    }
    true
});

That solves the part of dropping a custom rust struct, but doesn't explain why the other way doesn't. This can be closed if the behaviour is not deemed a bug.

[bilelmoussaoui]

I found out that transfering a struct is possible by deriving glib::Boxed like in https://gtk-rs.org/gtk-rs-core/stable/latest/docs/glib/subclass/index.html#example-for-registering-a-boxed-type-for-a-rust-struct.
Then the DropTarget needs to set_types to $STRUCT::static_type().

It should work the same with BoxedAnyObject, you would have to call set_types like you did here.

That solves the part of dropping a custom rust struct, but doesn't explain why the other way doesn't. This can be closed if the behaviour is not deemed a bug.

Wdym by the "other way"?