# Objective
Fixes#16850
## Solution
Add a new function `SubApp::take_extract()`, similar to
`Option::take()`, which allows stealing the currently installed extract
function of a sub-app, with the intent to replace it with a custom one
calling the original one via `set_extract()`.
This pattern enables registering a custom "world sync" function similar
to the existing one `entity_sync_system()`, to run custom world sync
logic with mutable access to both the main and render worlds.
## Testing
`cargo r -p ci` currently doesn't build locally, event after upgrading
rustc to latest and doing a `cargo update`.
f9c8f511fd