bilsen 63fee2572b ParamSet for conflicting SystemParam:s (#2765 )

# Objective
Add a system parameter `ParamSet` to be used as container for conflicting parameters.

## Solution
Added two methods to the SystemParamState trait, which gives the access used by the parameter. Did the implementation. Added some convenience methods to FilteredAccessSet. Changed `get_conflicts` to return every conflicting component instead of breaking on the first conflicting `FilteredAccess`.


Co-authored-by: bilsen <40690317+bilsen@users.noreply.github.com>

2022-03-29 23:39:38 +00:00

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B0001

To keep Rust rules on references (either one mutable reference or any number of immutable references) on a component, it is not possible to have two queries on the same component when one request mutable access to it in the same system.

Erroneous code example:

use bevy::prelude::*;

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn move_enemies_to_player(
    mut enemies: Query<&mut Transform, With<Enemy>>,
    player: Query<&Transform, With<Player>>,
) {
    // ...
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(move_enemies_to_player)
        .run();
}

This will panic, as it's not possible to have both a mutable and an immutable query on Transform at the same time.

You have two solutions:

Solution #1: use disjoint queries using Without

As a Player entity won't be an Enemy at the same time, those two queries will acutally never target the same entity. This can be encoded in the query filter with Without:

use bevy::prelude::*;

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn move_enemies_to_player(
    mut enemies: Query<&mut Transform, With<Enemy>>,
    player: Query<&Transform, (With<Player>, Without<Enemy>)>,
) {
    // ...
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(move_enemies_to_player)
        .run();
}

Solution #2: use a ParamSet

A ParamSet will let you have conflicting queries as a parameter, but you will still be responsible of not using them at the same time in your system.

use bevy::prelude::*;

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn move_enemies_to_player(
    mut transforms: ParamSet<(
        Query<&mut Transform, With<Enemy>>,
        Query<&Transform, With<Player>>,
    )>,
) {
    // ...
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(move_enemies_to_player)
        .run();
}

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B0001

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