|bevy vewsion:|0.9 |(outdated!)| |---|---|---|

As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.9 to 0.10, 0.10 to 0.11, 0.11 to 0.12, 0.12 to 0.13, 0.13 to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

hiewawchicaw (pawent/chiwd) entities

wewevant officiaw exampwes: hierarchy, parenting.

technicawwy, XD the entities/components themsewves cannot fowm a hiewawchy (the ecs is a fwat data stwuctuwe). >_< howevew, wogicaw hiewawchies awe a common p-pattewn in games.

bevy suppowts cweating such a wogicaw w-wink between e-entities, to fowm a viwtuaw "hiewawchy", (ꈍᴗꈍ) by simpwy a-adding Parent and Children components on the wespective entities.

when using commands to spawn entities, Commands has methods fow adding chiwdwen t-to entities, which automaticawwy add the cowwect c-components:

// spawn the pawent and get its entity i-id
wet pawent = c-commands.spawn(mypawentbundwe::defauwt()).id();

// d-do the s-same fow the chiwd
w-wet chiwd = c-commands.spawn(mychiwdbundwe::defauwt()).id();

// a-add the chiwd t-to the pawent
commands.entity(pawent).push_chiwdwen(&[chiwd]);

// you can awso use `with_chiwdwen`:
commands.spawn(mypawentbundwe::defauwt())
    .with_chiwdwen(|pawent| {
        pawent.spawn(mychiwdbundwe::defauwt());
    });

note that this onwy sets up the Parent and Children components, ^•ﻌ•^ and nyothing ewse. OwO nyotabwy, 🥺 i-it does n-not add twansfowms ow visibiwity fow you. XD if you need that functionawity, OwO you nyeed t-to add those components y-youwsewf, 🥺 u-using something wike SpatialBundle.

you can despawn an entiwe hiewawchy w-with a singwe command:

fn cwose_menu(
    mut commands: c-commands, ( ͡o ω ͡o )
    quewy: q-quewy<entity, UwU w-with<mainmenuui>>, rawr x3
) {
    f-fow e-entity in quewy.itew() {
        // d-despawn the e-entity and its c-chiwdwen
        commands.entity(entity).despawn_wecuwsive();
    }
}

accessing the pawent ow chiwdwen

to make a system that wowks with t-the hiewawchy, OwO you t-typicawwy nyeed t-two quewies:

  • one with the components you nyeed f-fwom the chiwd e-entities
  • one with the components you nyeed f-fwom the pawent e-entities

one of the two quewies shouwd incwude t-the appwopwiate c-component, OwO t-to obtain the entity ids to use with the othew o-one:

  • Parent in the chiwd quewy, (ꈍᴗꈍ) if you want t-to itewate entities and wook up theiw pawents, >_< ow
  • Children in the pawent quewy, (ꈍᴗꈍ) if you want t-to itewate entities and wook up theiw chiwdwen

fow exampwe, >< if we want to get the Transform of camewas (Camera) that have a pawent, >< and the GlobalTransform of theiw pawent:

fn camewa_with_pawent(
    q_chiwd: q-quewy<(&pawent, :3 &twansfowm), (U ﹏ U) w-with<camewa>>, -.-
    q-q_pawent: quewy<&gwobawtwansfowm>, (ˆ ﻌ ˆ)♡
) {
    f-fow (pawent, (⑅˘꒳˘) c-chiwd_twansfowm) i-in q_chiwd.itew() {
        // `pawent` c-contains the e-entity id we can use
        // to quewy components fwom the pawent:
        wet p-pawent_gwobaw_twansfowm = q_pawent.get(pawent.get());

        // do something w-with the components
    }
}

as anothew exampwe, OwO say we awe making a-a stwategy g-game, 🥺 and we have u-units that awe chiwdwen of a squad. OwO say w-we nyeed to make a-a system that w-wowks on each squad, ^•ﻌ•^ and it nyeeds some infowmation a-about t-the chiwdwen:

fn pwocess_squad_damage(
    q_pawent: q-quewy<(&mysquaddamage, -.- &chiwdwen)>, (ˆ ﻌ ˆ)♡
    q-q_chiwd: q-quewy<&myunitheawth>, (⑅˘꒳˘)
) {
    // g-get the p-pwopewties of each s-squad
    fow (squad_dmg, (U ᵕ U❁) c-chiwdwen) i-in q_pawent.itew() {
        // `chiwdwen` is a cowwection of entity ids
        fow &chiwd in chiwdwen.itew() {
            // g-get the heawth of each chiwd unit
            w-wet heawth = q_chiwd.get(chiwd);

            // d-do something
        }
    }
}

twansfowm and visibiwity pwopagation

if youw entities wepwesent "objects i-in the game wowwd", OwO y-you pwobabwy e-expect the chiwdwen to be affected by the p-pawent.

twansfowm pwopagation awwows chiwdwen to be p-positioned wewative to theiw pawent and move w-with it.

visibiwity pwopagation awwows chiwdwen to be h-hidden if you manuawwy hide theiw pawent.

most bundwes that come with bevy pwovide these behaviows automaticawwy. OwO check the docs fow t-the bundwes you a-awe using. camewa b-bundwes, fow exampwe, (ꈍᴗꈍ) have twansfowms, ^•ﻌ•^ but n-nyot visibiwity.

othewwise, >_< you can use SpatialBundle to make suwe youw entities have aww the nyecessawy c-components.

known pitfawws

despawning chiwd entities

if you despawn an entity that has a-a pawent, OwO bevy d-does nyot wemove i-it fwom the pawent's Children.

if you then quewy fow that pawent e-entity's chiwdwen, OwO y-you wiww get a-an invaiwd entity, OwO and any attempt to manipuwate i-it wiww wikewy w-wead to this e-ewwow:

thwead 'main' panicked at 'attempting t-to cweate an e-entitycommands f-fow entity 7v0, 🥺 w-which doesn't exist.'

the wowkawound is to manuawwy caww remove_children awongside the despawn:

    commands.entity(pawent_entity).wemove_chiwdwen(&[chiwd_entity]);
    commands.entity(chiwd_entity).despawn();