Events that store a Component trait object

[alice-i-cecile]

I wanted to make an event which could store a boxed trait object, where the trait required the Component subtrait.

/// Trait for implementable actions
trait Ability:  Component  {}

/// Event type that stores the action that is currently being executed
struct QueuedAbility {
    ability: Box<dyn Ability>,
}

Looks good, but the compiler is complaining to me that

the value of the associated type Storage (from trait bevy::prelude::Component) must be specified

What the heck should I set that associated type to? Let's look at how to solve it.

(Already solved thanks to help on Discord, but written up here for Google and confused users).

[alice-i-cecile]

The trick is that you need to set the associated type to a type that implements ComponentStorage: currently SparseStorage or TableStorage.

/// Trait for implementable actions
trait Ability:  Component<Storage = SparseStorage>  {}

/// Event type that stores the action that is currently being executed
struct QueuedAbility {
    // Storage is the associated type of the Component trait
    // Here, we're setting it to be the standard enum type
    ability: Box<dyn Ability<Storage = SparseStorage>>,
}

impl QueuedAbility {
    fn new(ability: impl Ability) -> Self {
        QueuedAbility {
            ability: Box::new(ability),
        }
    }
}

#[derive(Component)]
// TableStorage is the default
#[component(storage = "SparseSet")]
struct Dash;

impl Ability for Dash {}

There's one minor limitation: all of the components must have the same storage type. You could probably get around this with an enum if you cared.

P.S. Specifying dyn ComponentStorage as the object type doesn't work: it's not object safe.