In rust and gtk4, why does not gtk::Label satisfy gtk::Widget? - rust

Question

I'm trying to learn rust with its gtk4 bindings and decided to try ListStore as a model for ListBox. In order to bind the model, function which returns Widget is needed, but i need a Label. Why doesn't Label, which inherits Widget (implements IsA<Widget>), satisfy also Widget type?
Error:
error[E0308]: mismatched types
--> src/main.rs:22:5
|
21 | fn create_widget<'r>(label: &'r Object) -> Widget {
| ------ expected `gtk4::Widget` because of return type
22 | Label::new(Some(label.property_value("name").get().unwrap()))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `gtk4::Widget`, found struct `gtk4::Label`
Full code:
use gtk::prelude::*;
use gtk::{Application, ApplicationWindow, Paned, Label, Orientation, ListBox, ListBoxRow, Widget};
use gtk::gio::ListStore;
use gtk::glib::Type;
use gtk::glib::Object;
fn main() {
println!("Hello, world!");// Create a new application
let app = Application::builder()
.application_id("org.gtk-rs.example")
.build();
// Connect to "activate" signal of `app`
app.connect_activate(build_ui);
// Run the application
app.run();
}
fn create_widget<'r>(label: &'r Object) -> Widget {
Label::new(Some(label.property_value("name").get().unwrap()))
}
fn build_ui(app: &Application) {
let listbox_left = ListBox::new();
let listbox_right = ListBox::new();
let list_model = ListStore::new(Type::STRING);
listbox_right.bind_model(Some(&list_model), create_widget);
let paned = Paned::builder()
.start_child(&listbox_left)
.end_child(&listbox_right)
.orientation(Orientation::Horizontal)
.build();
// Create a window
let window = ApplicationWindow::builder()
.application(app)
.title("My GTK App")
.child(&paned)
.build();
for n in 1..11 {
let n_as_string =(n as u32).to_string();
let mut my = String::from("Left:");
my.push_str(&n_as_string);
let m = Object::with_values(Type::STRING, &[("name", my.to_value())]);
list_model.append(&m.unwrap());
}
// Present window
window.present();
}

Answer 1

Inheritance is not a language feature in Rust. Instead, the gtk4 crate has Label impl IsA<Widget> (here) which allows you to call Cast::upcast like this:
Label::new(Some(label.property_value("name").get().unwrap())).upcast()
You might need to add
use gtk4::glib::object::Cast;
to the top of your file.
Instead you can also make your method return impl IsA<Widget> instead.