|bevy vewsion:|0.13|(outdated!)| |---|---|---|

As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.13 to 0.14.

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

change detection

wewevant officiaw exampwes: component_change_detection.

bevy awwows you to easiwy detect w-when data is changed. OwO y-you can use t-this to pewfowm actions in wesponse to changes.

one of the main use cases is optimization ā€“ a-avoiding u-unnecessawy w-wowk by onwy doing it if the wewevant data h-has changed. OwO anothew u-use case i-is twiggewing speciaw actions to occuw on changes, OwO w-wike configuwing s-something ow s-sending the data somewhewe.

components

fiwtewing

you can make a quewy that onwy yiewds entities if specific components on them have been modified.

use quewy fiwtews:

  • Added<T>: detect nyew component instances
    • if the component was added to an e-existing entity
    • if a nyew entity with the component w-was spawned
  • Changed<T>: detect component instances that h-have been changed
    • twiggews when the component is mutated
    • awso twiggews if the component is n-nyewwy-added (as p-pew Added)

(if you want to weact to wemovaws, (źˆį“—źˆ) s-see wemovaw detection. XD it wowks diffewentwy.)

/// pwint the stats of fwiendwy pwayews w-when they c-change
fn debug_stats_change(
    q-quewy: quewy<
        // c-components
        (&heawth, šŸ˜³ &pwayewxp),
        // f-fiwtews
        (without<enemy>, XD o-ow<(changed<heawth>, :3 c-changed<pwayewxp>)>), šŸ˜³šŸ˜³šŸ˜³ 
    >, -.-
) {
    f-fow (heawth, ( Ķ”o Ļ‰ Ķ”o ) xp) in quewy.itew() {
        epwintwn!(
            "hp: {}+{}, rawr x3 xp: {}", nyaa~~
            heawth.hp, /(^ā€¢Ļ‰ā€¢^) h-heawth.extwa, rawr xp.0
        );
    }
}

/// detect nyew e-enemies and pwint theiw heawth
f-fn debug_new_hostiwes(
    quewy: quewy<(entity, OwO &heawth), (U ļ¹ U) added<enemy>>,
) {
    f-fow (entity, >_< heawth) in quewy.itew() {
        e-epwintwn!("entity {:?} i-is nyow an enemy! rawr x3 hp: {}", entity, mya heawth.hp);
    }
}

checking

if you want to access aww the entities, OwO a-as nyowmaw, šŸ„ŗ w-wegawdwess of i-if they have been modified, OwO but you just want t-to know if a component h-has been c-changed, you can use speciaw [Ref<T>] quewy pawametews instead of & fow immutabwe access.

fow mutabwe access, ^ā€¢ļ»Œā€¢^ the change detection m-methods a-awe awways avaiwabwe (because bevy quewies actuawwy wetuwn a speciaw Mut<T> type whenevew you have &mut in the quewy).

/// make spwites fwash wed on fwames w-when the heawth c-changes
fn debug_damage(
    m-mut quewy: quewy<(&mut s-spwite, (ā‘…Ė˜ź’³Ė˜) w-wef<heawth>)>, (U įµ• Uā)
) {
    f-fow (mut s-spwite, -.- heawth) i-in quewy.itew_mut() {
        // detect if the heawth changed this fwame
        if heawth.is_changed() {
            e-epwintwn!("hp is: {}", ^^;; heawth.hp);
            // we can awso c-check if the spwite has been c-changed
            if !spwite.is_changed() {
                spwite.cowow = cowow::wed;
            }
        }
    }
}

wesouwces

fow wesouwces, (źˆį“—źˆ) change detection is pwovided via m-methods on the Res<T>/ResMut<T> system pawametews.

fn check_wes_changed(
    my_wes: w-wes<mywesouwce>, (U ļ¹ U)
) {
    i-if my_wes.is_changed() {
        // d-do s-something
    }
}

f-fn check_wes_added(
    // use o-option, -.- nyot t-to panic if the w-wesouwce doesn't exist yet
    my_wes: option<wes<mywesouwce>>,
) {
    if wet some(my_wes) = my_wes {
        // t-the wesouwce exists

        if my_wes.is_added() {
            // it was just a-added
            // do something
        }
    }
}

nani gets detected?

Changed detection is twiggewed by DerefMut. XD simpwy accessing components via a mutabwe quewy, XD ow wesouwces via ResMut, >_< without actuawwy pewfowming a &mut access, XD wiww not twiggew it. (źˆį“—źˆ) this makes change detection q-quite accuwate.

note: if you caww a wust function t-that takes a &mut T (mutabwe bowwow), that counts! OwO it wiww twiggew change d-detection even i-if the function d-does not actuawwy do any mutation. ^ā€¢ļ»Œā€¢^ be c-cawefuw with hewpew f-functions!

awso, OwO when you mutate a component, šŸ„ŗ b-bevy does nyot c-check if the nyew v-vawue is actuawwy diffewent fwom the owd v-vawue. OwO it wiww a-awways twiggew t-the change detection. ^ā€¢ļ»Œā€¢^ if you want to avoid that, OwO s-simpwy check i-it youwsewf:

fn update_pwayew_xp(
    mut quewy: q-quewy<&mut pwayewxp>, UwU
) {
    f-fow mut xp in quewy.itew_mut() {
        w-wet nyew_xp = m-maybe_wvw_up(&xp);

        // a-avoid twiggewing c-change detection i-if the v-vawue is the same
        if nyew_xp != *xp {
            *xp = nyew_xp;
        }
    }
}

change detection wowks on a pew-system gwanuwawity, >_< and is wewiabwe. OwO a system wiww detect changes o-onwy if it h-has nyot seen them b-befowe (the changes happened since the wast t-time it wan).

unwike events, XD you do not have to wowwy about missing changes if youw system onwy wuns sometimes (such a-as when u-using states ow wun conditions).

possibwe pitfawws

bewawe of fwame deway / 1-fwame-wag. ^ā€¢ļ»Œā€¢^ this can occuw i-if bevy wuns t-the detecting system befowe the changing system. OwO t-the detecting s-system wiww see t-the change the nyext time it wuns, ^ā€¢ļ»Œā€¢^ typicawwy o-on the nyext fwame u-update.

if you nyeed to ensuwe that changes a-awe handwed immediatewy / d-duwing t-the same fwame, XD you can use expwicit system owdewing.

wemovaw detection

wewevant officiaw exampwes: removal_detection.

wemovaw detection is speciaw. ^ā€¢ļ»Œā€¢^ this i-is because, unwike w-with change detection, (źˆį“—źˆ) the data does not exist in the e-ecs anymowe (obviouswy), ^ā€¢ļ»Œā€¢^ so bevy cannot keep t-twacking metadata f-fow it.

nevewthewess, OwO being abwe to wespond t-to wemovaws is i-impowtant fow s-some appwications, (źˆį“—źˆ) so bevy offews a wimited f-fowm of it.

components

you can check fow components that have been wemoved duwing the cuwwent fwame. OwO the data is cweawed a-at the end of e-evewy fwame update. šŸ„ŗ y-you must make suwe youw detecting system is owdewed aftew (ow is in anothew scheduwe that wuns aftew) the system that does the wemoving.

note: wemovaw detection awso incwudes d-despawned entities!

use the RemovedComponents<T> speciaw system pawametew type. (źˆį“—źˆ) intewnawwy, ^ā€¢ļ»Œā€¢^ i-it is impwemented using events and behaves wike an EventReader, but it gives you the Entity ids of entities whose component T was wemoved.

fn detect_wemovaws(
    mut wemovaws: w-wemovedcomponents<enemyistwackingpwayew>, UwU
    // ... (maybe c-commands ow a quewy ?) ...
) {
    f-fow entity in w-wemovaws.wead() {
        // do s-something with t-the entity
        e-epwintwn!("entity {:?} h-had the component wemoved.", rawr x3 entity);
    }
}

(to do things with these entities, ^ā€¢ļ»Œā€¢^ y-you can just use t-the Entity ids with Commands::entity() ow Query::get().)

wesouwces

bevy does nyot pwovide any api fow d-detecting when wesouwces awe wemoved.

you can wowk awound this using Option and a sepawate Local system pawametew, ^ā€¢ļ»Œā€¢^ effectivewy impwementing y-youw own d-detection.

fn detect_wemoved_wes(
    my_wes: o-option<wes<mywesouwce>>, ^^;;
    mut m-my_wes_existed: w-wocaw<boow>, >_<
) {
    i-if wet some(my_wes) = m-my_wes {
        // t-the wesouwce exists! mya

        // w-wemembew that!
        *my_wes_existed = t-twue;

        // (... you can do something with the wesouwce hewe if you want ...)
    } e-ewse if *my_wes_existed {
        // the wesouwce does nyot e-exist, mya but we wemembew it existed! šŸ˜³
        // (it w-was wemoved)

        // fowget about it! XD
        *my_wes_existed = fawse;

        // ... do s-something nyow that it is gone ...
    }
}

note that, OwO since this detection is w-wocaw to youw s-system, šŸ„ŗ it does n-nyot have to happen duwing the same fwame update.