Controlling the sensitive value for list items

[pieterdd]

I'd like to apply a data binding to all RelmListItems in a TypedListView. Specifically, I'd like to control the underlying ListItem's sensitive value via an expression that relies on the state of the model item. In gtk4-rs, the idiomatic way to disable certain items within a list appears to be to call a_list_item.set_sensitive(false) and make sure your connect_activate filters out events from the disabled items.

I know RelmListItem::bind has a reference to both my data model and the ListItem where the sensitive value should be applied. What is the recommended way to bind an expression to sensitive? I know BoolBinding exists, but in my case the expression is of the form condition_1 && condition_2.

[AaronErhardt]

I'd say you have to evaluate the expression and then update the bool binding. Or is there something that prevents you from doing that?

[pieterdd]

Wouldn't that require being notified whenever one of the two variables that make up the expression changes?

One of the variables is an enum wrapped in SharedState, the other is a boolean attribute from the model instance that's represented by the ListItem. So the expression is something along the lines of GLOBAL_ENUM_STATE.read() == GlobalEnum::Foo && model.is_a_foo == true. Usually a #[watch] would take care of the heavy lifting but it appears this is not an option here.

[AaronErhardt]

Wouldn't that require being notified whenever one of the two variables that make up the expression changes?

Yes, that is certainly the case. I don't know if there's a better solution that updating each list item because every item has to be notified. Bindings are not supposed to handle expressions and that would not change the fact that you have to run some code for each list item, it would just be hidden. So in the end an hypothetical "expression binding" would just make the code a little bit cleaner.

[pieterdd]

Noted. I suppose I could instrument my data model by replacing the bool with BoolBinding and subscribing to the binding via BoolBinding::connect_value_notify, and use SharedState::subscribe for the global variable. Thanks for the guidance Aaron!

[pieterdd]

Ah dangit, it appears that RelmListItem doesn't have a sender I can pass to SharedState::subscribe 🤔

[AaronErhardt]

You can use the Sender of your component for that probably (I think you just need one message and then update all items from your component). Or you create new channels and handle the messages yourself (maybe using relm4::spawn_local).

[pieterdd]

Sounds interesting. Can the parent component pass on its sender to the RelmListItem? Can I simply pass a fresh instance of relm4::channel:channel() to the subscribe call within the RelmListItem instance? I'm not too familiar with the inner workings of Relm4 yet.