Implement Downcast trait for Option<T>

[JMS55]

A lot of the time I do something like this:

widget.get_child().unwrap().downcast::<SomeWidget>().unwrap();

This gets to be really tedious and ugly. I propose gtk-rs should add a function equivalent to the above, something like:

widget.get_child_as::<SomeWidget>();

[sdroege]

Sounds useful, yes

[bilelmoussaoui]

What makes it harder is that get_child is not a WidgetExt method and we will have to add it to a bunch of various types manually. Do you want to send a PR for that? I think having a trait with the provided method implemented for all those types would be an easy way to do it.

[sophie-h]

Is there something special about get_child? Otherwise, I would expect all functions that return a Widget to have a _as variant and I'm not sure if it's worth the effort to hard code that into gir.

[bilelmoussaoui]

Is there something special about get_child?

Nothing special about it, there are definitely other functions that could benefit from a generic alternative. Do you have any others in mind?

Otherwise, I would expect all functions that return a Widget to have a _as variant and I'm not sure if it's worth the effort to hard code that into gir.

Yeah no, there's no way to detect/generate that automatically from gir

[sophie-h]

Stack::get_visible_child for example and I probably a bunch of functions in libadwaita like hdy_action_row_get_activatable_widget (). I am a bit scared that there will be too much manual stuff at some point and I'm not sure about cost/benefit with this issue.

[ricvelozo]

Maybe is possible to specialize, something like:

trait OptionCast { /* ... */ } // Similar to Cast trait, but returns the value or panics

impl OptionCast for Option<glib::Object> {
    fn downcast<T: ObjectType>(self) -> T {
        panic!("failed to cast to `{}`", T::static_type())
    }
}

// Now
list_item.get_child().unwrap().downcast::<gtk::Label>().expect("failed to cast to `GtkLabel`");

// After
list_item.get_child().downcast::<gtk::Label>();

[sophie-h]

@bilelmoussaoui I don't think this needs to be on 0.1.0

[JMS55]

Yeah, my idea was to also do it for get_item() and various other functions, but that's a lot of work manually. I'm thinking we would either want to have gir generate a get_x_as() function for any function that looks like get_x() -> Object, but not sure how feasible that is. The much more realistic option, but perhaps slightly worse to use, is what @ricvelozo posted - Add a new function on the Cast trait or somewhere, that at least lets you get rid of two unwraps.

[bilelmoussaoui]

Maybe is possible to specialize, something like:

trait OptionCast { /* ... */ } // Similar to Cast trait, but returns the value or panics

impl OptionCast for Option<glib::Object> {
    fn downcast<T: ObjectType>(self) -> T {
        panic!("failed to cast to `{}`", T::static_type())
    }
}

// Now
list_item.get_child().unwrap().downcast::<gtk::Label>().expect("failed to cast to `GtkLabel`");

// After
list_item.get_child().downcast::<gtk::Label>();

@sdroege What do you think of something like this? without the panic part. I suppose it's similar to the ListModel::item case where you lose the semantics of the Option in case you're trying to cast to a wrong type.

[sdroege]

You could implement Downcast etc on Option<T> and then you'd end up with fn downcast<U>(Option<T>) -> Result<Option<U>, Option<T>>. That would not hide the errors at least while still allowing to directly work on Options.

[bilelmoussaoui]

I am going to move this to gtk-rs-core as such trait implementation has to be done on glib-rs side

[bilelmoussaoui]

Fixed in #843