How to obtain a reference to widgets in SimpleComponent / Component?

[gtamas]

Hi

I'm building an app using the SimpleComponent / Component macro. It works great. But I don't understand how can I refer to the widgets generated by view! macro later?

My understanding is that the update_view() method is automatically handled by the Component, so I can't really use it.
I know I have access to the generated widgets in init() fn using:

let widgets = view_output!();

But what if I need those widgets later, for example in the event handlers in update() fn or in the post_view() fn or elsewhere?

I'm new to Relm4 and Rust, so probably there is some simple solution, but I can't see how this is supposed to work.

[AaronErhardt]

If you name widgets with #[name(my_widget)] you should be able to access them in post_view() or pre_view(). Accessing widgets in update() is not supported because this is usually a bad pattern, but you can overwrite update_with_view if you really need widgets and model at the same time.

[gtamas]

Thanks for the reply.

So the problem is I cannot use the #[name] or stuff like that, because my UI is dynamically generated. I just have a Frame widget in the view! macro call and its child is generated by a method, which returns a Box containing the form. Something like this:

view! { #[root] gtk::Box { append: form = &gtk::Frame { set_hexpand: true, #[track = "model.changed(SomModel::json())"] set_child: Some(&model.build_form(&model.json).unwrap()) } } }

That's why I think I need access to the widgets later. I need to extract the user input from those widgets on form submit.

So in such a case the update_with_view() is the right way to go? I also think that I cannot use SimpleComponent in this case, but I have to implement Component. Is that correct?

Again, thanks in advance for any help. I'm totally new to Rust and GTK4 and Relm4 so probably there is a much better way to implement all this. Please let me know!

[AaronErhardt]

You could do the #[local] attribute to use a local variable from the init method. This will work similarly to #[name(...)] and "my_box" in the following example should be available as field of the generated Widgets type.

view! {
    gtk::Box {
        ... gtk::Frame {
            #[local]
            my_box -> gtk::Box {},
        }
    }
}

fn init(...) {
    let my_box = model.build_form(...);
    let widgets = view_output!();
    // ...
}

This means that you can also use pre_view() and post_view() to access the widgets. update_with_view() in the Component trait is also an option, but it is usually not necessary to mix state and widgets.

You can also add your own custom fields to the generated Widgets type btw.: https://relm4.org/book/stable/component_macro/reference.html#add-more-fields-to-your-widgets

I need to extract the user input from those widgets on form submit.

Since 0.6, Relm4 has bindings which might be even better for your use-case: https://docs.rs/relm4/latest/relm4/binding/index.html

Basically, the idea is, that if UI state and application state are identical (which is usually the case in forms) you can just have a binding in your model that is synchronized with a property of the UI (such as the state of a toggle button). Something similar is already possible for gtk::Entry by storing an EntryBuffer in your model and setting it as the buffer of an gtk::Entry.

[gtamas]

Thanks for all the info and the detailed explanation! Cool, everything works now.

[chrsoo]

Since 0.6, Relm4 has bindings which might be even better for your use-case: https://docs.rs/relm4/latest/relm4/binding/index.html

Would you have an example on how to use this feature to bind text from a model to a label or the name of an icon to a button?

Looking at the ConnectBinding documentation, support for this would appear to be available per default but I am unable to figure out on how to use it or find the relevant documentation...

[chrsoo]

... and of course I managed to find an example just after posting. Sorry for the noise!

For posterity the binding is done like so in the view macro:

gtk::Label::with_binding(&model.text) {
    set_margin_all: 5,
},

... with the model's text property required to be of the right binding type:

struct App {
    ...
    text: StringBinding,
}