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# Objective
- Make it possible to know *what* changed your component or resource.
- Common need when debugging, when you want to know the last code
location that mutated a value in the ECS.
- This feature would be very useful for the editor alongside system
stepping.
## Solution
- Adds the caller location to column data.
- Mutations now `track_caller` all the way up to the public API.
- Commands that invoke these functions immediately call
`Location::caller`, and pass this into the functions, instead of the
functions themselves attempting to get the caller. This would not work
for commands which are deferred, as the commands are executed by the
scheduler, not the user's code.
## Testing
- The `component_change_detection` example now shows where the component
was mutated:
```
2024-07-28T06:57:48.946022Z INFO component_change_detection: Entity { index: 1, generation: 1 }: New value: MyComponent(0.0)
2024-07-28T06:57:49.004371Z INFO component_change_detection: Entity { index: 1, generation: 1 }: New value: MyComponent(1.0)
2024-07-28T06:57:49.012738Z WARN component_change_detection: Change detected!
-> value: Ref(MyComponent(1.0))
-> added: false
-> changed: true
-> changed by: examples/ecs/component_change_detection.rs:36:23
```
- It's also possible to inspect change location from a debugger:
<img width="608" alt="image"
src="https://github.com/user-attachments/assets/c90ecc7a-0462-457a-80ae-42e7f5d346b4">
---
## Changelog
- Added source locations to ECS change detection behind the
`track_change_detection` flag.
## Migration Guide
- Added `changed_by` field to many internal ECS functions used with
change detection when the `track_change_detection` feature flag is
enabled. Use Location::caller() to provide the source of the function
call.
---------
Co-authored-by: BD103 <59022059+BD103@users.noreply.github.com>
Co-authored-by: Gino Valente <49806985+MrGVSV@users.noreply.github.com>
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| .. | ||
| cargo_features.md | ||
| debugging.md | ||
| linters.md | ||
| linux_dependencies.md | ||
| profiling.md | ||
| release_checklist.md | ||
| the_bevy_organization.md | ||