Ref<T> for change detection, but mutable | Bevy | Page 1

dreamy epoch May 31, 2023, 3:11 PM

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Hi 👋

I have a query like

Query<&mut MyComponent>

and I want to detect if the component has changed by using the new my_component.is_changed() function of Ref

But if I switch my query to Query<Ref<MyComponent>> then I lose the mutability, and I need it

I read in the documentation about Ref (https://docs.rs/bevy/latest/bevy/prelude/struct.Ref.html) I quote: Similar to Mut but is immutable and so doesn’t require unique access., but it's misleading because I can't just replace it by Query<Mut<MyComponent>>

If I do that, I get an error like this:

error: the trait bound `bevy::prelude::Mut<'_, my_crate::MyComponent>: WorldQuery` is not satisfied
label: the trait `WorldQuery` is not implemented for `bevy::prelude::Mut<'_, my_crate::MyComponent>`

Am I doing something wrong, or I did missunderstand the docs? 🤔

Ref in bevy::prelude - Rust

Shared borrow of an entity’s component with access to change detection. Similar to Mut but is immutable and so doesn’t require unique access.

river karma May 31, 2023, 3:36 PM

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[edited, see MrGVSV's answer]
Also, you can (maybe, depending on your usecase) use Changed<MyComponent> : https://discord.com/channels/691052431525675048/1113464686566965308

fresh salmon May 31, 2023, 3:38 PM

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&mut MyComponent will give you a Mut<MyComponent>

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And you’ll be able to use all the normal methods from the DetectChanges trait

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But yeah you could also consider using Changed like Nocta suggested (which I believe should be more efficient if you just need to filter down values)

dreamy epoch May 31, 2023, 3:52 PM

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oh, you're right! I didn't notice 🤦‍♂️ thx!

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and yep, in fact I changed to Changed<MyComponent as a workaround, but still wanted to understand how is_changed works

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btw, Changed<MyComponent> detects both changes and new additions, right? I don't need to do Or<(Changed<MyComponent>, Added<MyComponent>)>

#Ref<T> for change detection, but mutable