bevy_feathers: It is impossible to initialize a checkbox with a variable

[Warhorst]

Bevy version and features

• 0.18
• ["2d", "dynamic_linking", "bevy_dev_tools", "experimental_bevy_ui_widgets", "experimental_bevy_feathers"]

What you did

I tried to create a feathers checkbox and initialize it with a boolean variable. This is currently not possible

What went wrong

I tried to do something like this:

fn create_checkbox(checked: bool) -> impl Bundle {
    checkbox(??? , Spawn(Text::new("Checkbox")))
}

The checkbox state gets set based on the bundle provided to it. So I provide Checked to set it checked or () to set it not checked.

But what do I do if I want to set it checked based on the provided variable? Something like the following does not work:

fn create_checkbox(checked: bool) -> impl Bundle {
    let bundle = if checked {
         Checked
    } else {
        ()
    };

    checkbox(bundle , Spawn(Text::new("Checkbox")))
}

This does not compile, as the branches have different types.

I would expect that I could just initialize the checkbox like the Slider, where I put all the properties into a properties component (like SliderProps). The doc of the checkbox function even mentions a third parameter named props, which does not exist.

[alice-i-cecile]

My understanding from discussions with @viridia is that this is fundamentally a limitation of our spawning setup. They were interested in swapping to a bsn! backed implementation to solve problems like this, which means this is blocked by #23030

[viridia]

This is easy to do in BSN, but I think we should have a solution for non-BSN scenarios.

There's a thread on discord which talks about this: https://discord.com/channels/691052431525675048/749335865876021248/1465422605355061483

The solution I would like is a way to merge bundles. The problem isn't just feathers: any function which returns an impl Bundle has a static type signature which includes all the components in the bundle; attempting to override any of those components generates an error. If we had a way to "merge" two bundles, producing a new bundle which is the union of those two, this would allow us to override any component produced by the template function.

Unfortunately, the discussion stalled out; while I think merging bundles is a cool idea, I couldn't tell you if it's actually possible to implement in Rust.

This is the primary reason why feathers is hidden by an experimental feature flag: because the current API gives a sub-optimal developer experience. If we had bundle merging, I would redo the API in a way that I would feel much more confident about.

Pinging @cart as the expert on Rust magic.

[alice-i-cecile]

Related but subtly distinct: #19715. #19398