Storing list of IsA<gtk::Widget>: trait cannot be made into an object

[kaixoo]

Hi there,

I am writing a generic WidgetPager struct which needs to be able to show different widgets of different kinds. First I just used [gtk::Widget; N], the problem comes when gtk::Button != gtk::Widget. Then I thought of using IsA<gtk::Widget> since that's what the library uses internally.

It seems the trait IsA<> is using Self somewhere, or something inside it like ToGlibPtr, which Rust doesn't accept to use safely.

Log:

error[E0038]: the trait `gtk4::prelude::IsA` cannot be made into an object
   --> src/widgets/widget_pager.rs:7:30
    |
7   |     container_selected_page: dyn IsA<gtk::Widget>,
    |                              ^^^^^^^^^^^^^^^^^^^^ `gtk4::prelude::IsA` cannot be made into an object
    |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> /home/aitor/.cargo/registry/src/index.crates.io-6f17d22bba15001f/glib-0.20.0/src/object.rs:28:7
    |
28  |     + PartialEq
    |       ^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
29  |     + Eq
30  |     + PartialOrd
    |       ^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
...
37  |     + for<'a> ToGlibPtr<'a, *mut <Self as ObjectType>::GlibType>
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
38  |     + IntoGlibPtr<*mut <Self as ObjectType>::GlibType>
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter

error[E0277]: the trait bound `&gtk4::Box: gtk4::prelude::IsA<gtk4::Widget>` is not satisfied
  --> src/main.rs:31:45
   |
31 | ..._pager = WidgetPager::new([&screen_input_select.container], None);
   |             ---------------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `gtk4::prelude::IsA<gtk4::Widget>` is not implemented for `&gtk4::Box`
   |             |
   |             required by a bound introduced by this call
   |
   = help: the following other types implement trait `gtk4::prelude::IsA<T>`:
             <gtk4::Box as gtk4::prelude::IsA<Accessible>>
             <gtk4::Box as gtk4::prelude::IsA<Buildable>>
             <gtk4::Box as gtk4::prelude::IsA<ConstraintTarget>>
             <gtk4::Box as gtk4::prelude::IsA<Orientable>>
             <gtk4::Box as gtk4::prelude::IsA<gtk4::Box>>
             <gtk4::Box as gtk4::prelude::IsA<gtk4::Widget>>
             <gtk4::Box as gtk4::prelude::IsA<gtk4::glib::Object>>
note: required by a bound in `WidgetPager::<'a, N>::new`
  --> src/widgets/widget_pager.rs:14:30
   |
14 | ...b fn new(_pages: [impl IsA<gtk::Widget>; N], active_page: Option<usize>)...
   |                           ^^^^^^^^^^^^^^^^ required by this bound in `WidgetPager::<'a, N>::new`

In this issue you are recommending to use AsRef, but that doesn't work with Gtk::Box().append() or other widget-related methods. gtk-rs/gtk-rs-core#49

[kaixoo]

Solution: https://users.rust-lang.org/t/storing-list-of-isa-gtk-widget-trait-cannot-be-made-into-an-object/115346/3

tl.dr. Vec<gtk::Widget> IS the way to do this, you just need to cast gtk::Button.