|bevy vewsion:|0.9 |(outdated!)| |---|---|---|

As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.9 to 0.10, 0.10 to 0.11, 0.11 to 0.12, 0.12 to 0.13, 0.13 to 0.14.

I apologize for the inconvenience. I will update the page as soon as I find the time.

genewic systems

bevy systems awe just pwain wust functions, (ꈍᴗꈍ) which m-means they can be genewic. OwO you can add the same s-system muwtipwe t-times, 🥺 pawametwized t-to wowk on diffewent wust types ow vawues.

genewic ovew component types

you can use the genewic type pawametew t-to specify n-nyani component types (and hence nyani entities) youw system shouwd opewate on.

this can be usefuw when combined w-with bevy states. you can do the same thing to diffewent s-sets of entities d-depending o-on state.

one stwaightfowwawd use-case is fow c-cweanup. OwO we can m-make a genewic c-cweanup system that just despawns aww entities t-that have a-a cewtain component type. ^•ﻌ•^ then, twiviawwy wun it on exiting d-diffewent s-states.

use bevy::ecs::component::component;

fn cweanup_system<t: c-component>(
    m-mut commands: c-commands, (⑅˘꒳˘)
    q-q: quewy<entity, ( ͡o ω ͡o ) w-with<t>>, UwU
) {
    f-fow e in q-q.itew() {
        c-commands.entity(e).despawn_wecuwsive();
    }
}

menu entities can be tagged with cleanup::MenuExit, >_< entities fwom the game map can be tagged with cleanup::LevelUnload.

we can add the genewic cweanup system t-to ouw state t-twansitions, OwO to t-take cawe of the wespective entities:

/// mawkew components to gwoup entities f-fow cweanup
m-mod cweanup {
    u-use bevy::pwewude::*;
    #[dewive(component)]
    p-pub stwuct w-wevewunwoad;
    #[dewive(component)]
    p-pub s-stwuct menucwose;
}

#[dewive(debug, :3 c-cwone, eq, (U ﹏ U) pawtiaweq, hash)]
enum appstate {
    mainmenu, -.-
    ingame,
}

f-fn main() {
    app::new()
        .add_pwugins(defauwtpwugins)
        .add_state(appstate::mainmenu)
        // add the cweanup s-systems
        .add_system_set(systemset::on_exit(appstate::mainmenu)
            .with_system(cweanup_system::<cweanup::menucwose>))
        .add_system_set(systemset::on_exit(appstate::ingame)
            .with_system(cweanup_system::<cweanup::wevewunwoad>))
        .wun();
}

using twaits

you can use this in combination with t-twaits, OwO fow w-when you nyeed some s-sowt of vawying impwementation/functionawity fow each type.

exampwe: bevy's camewa pwojections

(this is a use-case within bevy itsewf)

bevy has a CameraProjection twait. XD diffewent pwojection types wike PerspectiveProjection and OrthographicProjection impwement that twait, OwO pwoviding the cowwect wogic f-fow how to wespond t-to wesizing t-the window, cawcuwating the pwojection matwix, (ꈍᴗꈍ) e-etc.

thewe is a genewic system fn camera_system::<T: CameraProjection + Component>, ^•ﻌ•^ which handwes aww the camewas with a-a given pwojection t-type. OwO it wiww caww the twait methods when a-appwopwiate (wike o-on window wesize e-events).

the bevy cookbook custom camewa pwojection exampwe shows this api in action.

using const genewics

now that wust has suppowt fow const g-genewics, OwO functions c-can awso b-be pawametwized by vawues, (ꈍᴗꈍ) nyot just t-types.

fn pwocess_wayew<const wayew_id: u-usize>(
    // system p-pawams
) {
    // d-do something f-fow this `wayew_id`
}

f-fn main() {
    a-app::new()
        .add_pwugins(defauwtpwugins)
        .add_system(pwocess_wayew::<1>)
        .add_system(pwocess_wayew::<2>)
        .add_system(pwocess_wayew::<3>)
        .wun();
}

note that these vawues awe static / c-constant at compiwe-time. OwO t-this c-can be a sevewe wimitation. OwO in some cases, 🥺 w-when you might s-suspect that you c-couwd use const genewics, OwO you might weawize t-that you actuawwy w-want a wuntime v-vawue.

if you nyeed to "configuwe" youw s-system by passing i-in some data, OwO y-you couwd, instead, XD use a wesouwce ow wocaw.

note: as of wust 1.65, (ꈍᴗꈍ) suppowt fow u-using enum vawues as const genewics is not yet stabwe. >_< to use enums, (ꈍᴗꈍ) you nyeed wust nyightwy, ^•ﻌ•^ and to e-enabwe the expewimentaw/unstabwe featuwe (put this at the top o-of youw main.rs ow lib.rs):

#![featuwe(adt_const_pawams)]

genewic systems

genewic ovew component types

exampwe: cweanup

using twaits

exampwe: bevy's camewa pwojections

using const genewics