Cannot easily spawn an entity with one component

If I try spawning an entity with just one component, I get the following error:

error[E0277]: the trait bound `Pawn: bevy::bevy_ecs::DynamicBundle` is not satisfied
  --> examples/balls.rs:69:20
   |
69 |     commands.spawn(Pawn { controller: 1 }).with(Ball {
   |                    ^^^^^^^^^^^^^^^^^^^^^^ the trait `bevy::bevy_ecs::DynamicBundle` is not implemented for `Pawn`

I need to spawn a one element tuple:

commands.spawn((Pawn { controller: 1 },)).with(Ball {

There is no way of coming up to that solution from the error message.

Bevy's selling point is ECS ergonomics - i think this needs to be fixed.

Since there are multiple components in your example, could do .spawn((Pawn, Ball)), but I understand there can be a preference to use the methods since it allows quickly commenting out withs. spawn(()).with(component) works but also isn't entirely ergonomic.

We could add a method like Commands::spawn_with(&mut self, component: impl Component), and then spawn could be renamed to spawn_with_bundle for clarity.

To add spawn_with to your app today, an option is to use a function or add this to an extension trait.

trait CommandsExtension {
    fn spawn_with(&mut self, components: impl Component) -> &mut Self;
}

impl CommandsExtension for Commands {
    fn spawn_with(&mut self, component: impl Component) -> &mut Self {
        self.spawn((component,));
        self
    }
}

fn system(mut commands: Commands) {
    commands.spawn_with(Pawn);
}

Note: make sure during bevy upgrades that any new inherent methods on Commands don't shadow your extension methods' names since Rust will prefer the inherent method first.

@memoryruins

Yeah there is unfortunately no way to make spawn() work with both tuples and individual components (because a component could be, itself a tuple). So they are ambiguous to rustc.

I like @memoryruins idea to make "bundles" a clear part of the api and to add a new api for spawning single components.

@memoryruins

Thank you for the explanation.
I also like the idea of making the bundle spawning more clear. "Explicit is better than implicit."

BTW, I didn't know you can extend an imported type like that. Thank you @memoryruins for this example. :-)

This sounds like a fun change to make, so I figured I'd give it a shot. Please take a look at #611 and let me know if I'm going in the right direction (naming is hard).

This is fixed now that #611 is merged in and can be closed.

#611 was closed, not merged. The discussion in it contains good points to consider when this issue is picked back up again.

This should be greatly improved or closed by #1562.

#1703 (seems improved version of #1562 ) has been closed.
So was this issue solved?

Yes, you can now do:

commands.spawn().insert(MyComponent)

No weird tuple shenanigans needed.

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Cannot easily spawn an entity with one component #607

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Cannot easily spawn an entity with one component #607

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memoryruins commented on Sep 30, 2020

cart commented on Oct 1, 2020

smokku commented on Oct 1, 2020

CleanCut commented on Oct 1, 2020

alice-i-cecile commented on Jan 16, 2021

memoryruins commented on Jan 16, 2021

alice-i-cecile commented on Mar 7, 2021

ledyba commented on Apr 1, 2021

alice-i-cecile commented on Apr 1, 2021

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