A spawn trait for Commands and ChildBuilder

[tguichaoua]

What problem does this solve or what need does it fill?

Both Commands and ChildBuilder share the methods spawn and spawn_bundle.

In my project I create a UiExt that add methods to spawn some ui related structure of entity (e.g. for scrolling list) that look like this :

pub trait UiExt<'w, 's> {
    fn spawn<'a>(&'a mut self) -> EntityCommands<'w, 's, 'a>;

    fn spawn_scroll_list<'a>(
        &'a mut self,
        style: Style,
        spawn_items: impl FnOnce(&mut ChildBuilder),
    ) -> EntityCommands<'w, 's, 'a> {
        let mut entity_commands = self.spawn();
        scroll_list::build_scroll_list(&mut entity_commands, style, spawn_items);
        entity_commands
    }
}

impl<'w, 's> UiExt<'w, 's> for Commands<'w, 's> {
    fn spawn<'a>(&'a mut self) -> EntityCommands<'w, 's, 'a> {
        self.spawn()
    }
}

impl<'w, 's, 'a> UiExt<'w, 's> for ChildBuilder<'w, 's, 'a> {
    fn spawn<'b>(&'b mut self) -> EntityCommands<'w, 's, 'b> {
        self.spawn()
    }
}

The methods are the same whatever if self is Commands or ChildBuilder as long as I can get a EntityCommands.
But because spawn is not implemented via a trait I cannot simply use a blanket implementation.

What solution would you like?

Expose spawn via a trait for Commands and ChildBuilder.

Allowing blanket implementation:

impl<'w, 's, T> UiExt<'w, 's> for T
where
    T : SpawnEntityCommands<'w, 's>
{
    fn spawn_scroll_list<'a>(
        &'a mut self,
        style: Style,
        spawn_items: impl FnOnce(&mut ChildBuilder),
    ) -> EntityCommands<'w, 's, 'a> {
        let mut entity_commands = self.spawn();
        scroll_list::build_scroll_list(&mut entity_commands, style, spawn_items);
        entity_commands
    }
}

What alternative(s) have you considered?

Add a spawn method in my UiExt trait and manually implement it for Commands and ChildBuilder.

[asafigan]

It might also be good to add this trait to World.

[hw762]

This would also be helpful with spawn_batch function, as ChildBuilder doesn't have it and is significantly hitting my performance.