Provide an API for fetching SystemParam from World #2089
Description
What problem does this solve or what need does it fill?
I want to write tests of a custom SystemParam.
For systems that act on resources and components, I can just use APIs on Worldto fetch the final state of the resources and components.
But a SystemParam doesn't have side effects -- instead, it has an API that I want to exercise.
What solution would you like?
I'd like to be able to fetch SystemParam instances out of World the same way I can fetch components and resources:
let app = new_test_app();
// set up app.world with state
let param: MySystemParam = app.world.get_system_param_mut();
assert_eq!(wanted_value, param.method1(args...));
// Call a method for side effects:
param.method2(args...);
assert_eq!(wanted_state, get_state(&app.world));What alternative(s) have you considered?
I can write a test system, something like this:
struct TestMySystemParam(fn(MySystemParam) -> Result<(), anyhow::Error>);
// The test resource is optional so I can share the app setup between several
// system params in one module.
fn test_my_system_param(
opt_test: Option<Res<TestMySystemParam>>,
mut out: ResMut<Result<anyhow::Error>>,
my_system_param: MySystemParam) {
if let Some(test) = opt_test {
*out = test(my_system_param);
}
}
#[test] my_test() {
let app = new_test_app();
// Set up app.world.
app.world.insert_resource(TestMySystemParam(|param| {
let got = param.method1();
ensure!(want == got, "method1: wanted {:?}, got {:?}", want, got);
}));
app.update();
let result: Result<(), anyhow::Error> = app.world.get_resource();
result.unwrap();
}However, this is kind of fiddly, and I have to do it for every SystemParam I want to test. It would be much nicer if Bevy could handle the details for me.