# Objective
- Fixes#15373
- Fixes
https://github.com/bevyengine/bevy/pull/14920#issuecomment-2370428013
## Solution
- Make `IntoSystem::pipe` and `IntoSystem::map` return two new
(possibly-ZST) types that implement `IntoSystem` and whose `into_system`
method return the systems that were previously being returned by
`IntoSystem::pipe` and `IntoSystem::map`
- Don't eagerly call `IntoSystem::into_system` on the argument given to
`RunSystemCachedWith::new` to avoid losing its ZST-ness
## Testing
- Added a regression test for each issue
## Migration Guide
- `IntoSystem::pipe` and `IntoSystem::map` now return `IntoPipeSystem`
and `IntoAdapterSystem` instead of `PipeSystem` and `AdapterSystem`.
Most notably these types don't implement `System` but rather only
`IntoSystem`.
# Objective
- Fixes#14924
- Closes#9584
## Solution
- We introduce a new trait, `SystemInput`, that serves as a type
function from the `'static` form of the input, to its lifetime'd
version, similarly to `SystemParam` or `WorldQuery`.
- System functions now take the lifetime'd wrapped version,
`SystemInput::Param<'_>`, which prevents the issue presented in #14924
(i.e. `InRef<T>`).
- Functions for running systems now take the lifetime'd unwrapped
version, `SystemInput::Inner<'_>` (i.e. `&T`).
- Due to the above change, system piping had to be re-implemented as a
standalone type, rather than `CombinatorSystem` as it was previously.
- Removes the `Trigger<'static, E, B>` transmute in observer runner
code.
## Testing
- All current tests pass.
- Added additional tests and doc-tests.
---
## Showcase
```rust
let mut world = World::new();
let mut value = 2;
// Currently possible:
fn square(In(input): In<usize>) -> usize {
input * input
}
value = world.run_system_once_with(value, square);
// Now possible:
fn square_mut(InMut(input): InMut<usize>) {
*input *= *input;
}
world.run_system_once_with(&mut value, square_mut);
// Or:
fn square_ref(InRef(input): InRef<usize>) -> usize {
*input * *input
}
value = world.run_system_once_with(&value, square_ref);
```
## Migration Guide
- All current explicit usages of the following types must be changed in
the way specified:
- `SystemId<I, O>` to `SystemId<In<I>, O>`
- `System<In = T>` to `System<In = In<T>>`
- `IntoSystem<I, O, M>` to `IntoSystem<In<I>, O, M>`
- `Condition<M, T>` to `Condition<M, In<T>>`
- `In<Trigger<E, B>>` is no longer a valid input parameter type. Use
`Trigger<E, B>` directly, instead.
---------
Co-authored-by: Giacomo Stevanato <giaco.stevanato@gmail.com>
# Objective
The goal of this PR is to introduce `SystemParam` validation in order to
reduce runtime panics.
Fixes#15265
## Solution
`SystemParam` now has a new method `validate_param(...) -> bool`, which
takes immutable variants of `get_param` arguments. The returned value
indicates whether the parameter can be acquired from the world. If
parameters cannot be acquired for a system, it won't be executed,
similarly to run conditions. This reduces panics when using params like
`Res`, `ResMut`, etc. as well as allows for new, ergonomic params like
#15264 or #15302.
Param validation happens at the level of executors. All validation
happens directly before executing a system, in case of normal systems
they are skipped, in case of conditions they return false.
Warning about system skipping is primitive and subject to change in
subsequent PRs.
## Testing
Two executor tests check that all executors:
- skip systems which have invalid parameters:
- piped systems get skipped together,
- dependent systems still run correctly,
- skip systems with invalid run conditions:
- system conditions have invalid parameters,
- system set conditions have invalid parameters.
# Objective
- I needed to run a system whenever a specific condition became true
after being previously false.
- Other users might also need to run a system when a condition changes,
regardless of if it became true or false.
## Solution
- This adds two systems to common_conditions:
- `condition_changed` that changes whenever the inner condition changes
- `condition_became_true` that returns true whenever the inner condition
becomes true after previously being false
## Testing
- I added a doctest for each function
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>
# Objective
Simplify Bevy-provided functions that return a condition-satisfying
closure instead of just being the condition.
## Solution
Become the condition.
## Testing
I did not test. Game jamming. Hopefully CI passes.
---
## Migration Guide
Some run conditions have been simplified.
```rust
// Before:
app.add_systems(Update, (
system_0.run_if(run_once()),
system_1.run_if(resource_changed_or_removed::<T>()),
system_2.run_if(resource_removed::<T>()),
system_3.run_if(on_event::<T>()),
system_4.run_if(any_component_removed::<T>()),
));
// After:
app.add_systems(Update, (
system_0.run_if(run_once),
system_1.run_if(resource_changed_or_removed::<T>),
system_2.run_if(resource_removed::<T>),
system_3.run_if(on_event::<T>),
system_4.run_if(any_component_removed::<T>),
));
```
# Objective
- My attempt at fulfilling #13629.
## Solution
Renames the `and_then` / `or_else` run condition methods to `and` /
`or`, respectively.
Extends the run conditions API to include a suite of binary logical
operators:
- `and`
- `or`
- `nand`
- `nor`
- `xor`
- `xnor`
## Testing
- Did you test these changes? If so, how?
- The test **run_condition_combinators** was extended to include the
added run condition combinators. A **double_counter** system was added
to test for combinators running on even count cycles.
- Are there any parts that need more testing?
- I'm not too sure how I feel about the "counter" style of testing but I
wanted to keep it consistent. If it's just a unit test I would prefer
simply to just assert `true` == _combinator output_ or `false` ==
_combinator output_ .
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
- Nothing too specific. The added methods should be equivalent to the
logical operators they are analogous to (`&&` , `||`, `^`, `!`).
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
- Should not be relevant, I'm using Windows.
## Changelog
- What changed as a result of this PR?
- The run conditions API.
- If applicable, organize changes under "Added", "Changed", or "Fixed"
sub-headings
- Changed:
- `and_then` run condition combinator renamed to simply `and`
- `or_else` run condition combinator renamed to simply `or`
- Added:
- `nand` run condition combinator.
- `nor` run condition combinator.
- `xor` run condition combinator.
- `xnor` run condition combinator.
## Migration Guide
- The `and_then` run condition method has been replaced with the `and`
run condition method.
- The `or_else` run condition method has been replaced with the `or` run
condition method.
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Andres O. Vela <andresovela@users.noreply.github.com>
# Objective
Extracts the state mechanisms into a new crate called "bevy_state".
This comes with a few goals:
- state wasn't really an inherent machinery of the ecs system, and so
keeping it within bevy_ecs felt forced
- by mixing it in with bevy_ecs, the maintainability of our more robust
state system was significantly compromised
moving state into a new crate makes it easier to encapsulate as it's own
feature, and easier to read and understand since it's no longer a
single, massive file.
## Solution
move the state-related elements from bevy_ecs to a new crate
## Testing
- Did you test these changes? If so, how? all the automated tests
migrated and passed, ran the pre-existing examples without changes to
validate.
---
## Migration Guide
Since bevy_state is now gated behind the `bevy_state` feature, projects
that use state but don't use the `default-features` will need to add
that feature flag.
Since it is no longer part of bevy_ecs, projects that use bevy_ecs
directly will need to manually pull in `bevy_state`, trigger the
StateTransition schedule, and handle any of the elements that bevy_app
currently sets up.
---------
Co-authored-by: Kristoffer Søholm <k.soeholm@gmail.com>
# Objective
Improve performance scalability when adding new event types to a Bevy
app. Currently, just using Bevy in the default configuration, all apps
spend upwards of 100+us in the `First` schedule, every app tick,
evaluating if it should update events or not, even if events are not
being used for that particular frame, and this scales with the number of
Events registered in the app.
## Solution
As `Events::update` is guaranteed `O(1)` by just checking if a
resource's value, swapping two Vecs, and then clearing one of them, the
actual cost of running `event_update_system` is *very* cheap. The
overhead of doing system dependency injection, task scheduling ,and the
multithreaded executor outweighs the cost of running the system by a
large margin.
Create an `EventRegistry` resource that keeps a number of function
pointers that update each event. Replace the per-event type
`event_update_system` with a singular exclusive system uses the
`EventRegistry` to update all events instead. Update `SubApp::add_event`
to use `EventRegistry` instead.
## Performance
This speeds reduces the cost of the `First` schedule in both many_foxes
and many_cubes by over 80%. Note this is with system spans on. The
majority of this is now context-switching costs from launching
`time_system`, which should be mostly eliminated with #12869.
The actual `event_update_system` is usually *very* short, using only a
few microseconds on average.
---
## Changelog
TODO
## Migration Guide
TODO
---------
Co-authored-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
# Objective
- There are several redundant imports in the tests and examples that are
not caught by CI because additional flags need to be passed.
## Solution
- Run `cargo check --workspace --tests` and `cargo check --workspace
--examples`, then fix all warnings.
- Add `test-check` to CI, which will be run in the check-compiles job.
This should catch future warnings for tests. Examples are already
checked, but I'm not yet sure why they weren't caught.
## Discussion
- Should the `--tests` and `--examples` flags be added to CI, so this is
caught in the future?
- If so, #12818 will need to be merged first. It was also a warning
raised by checking the examples, but I chose to split off into a
separate PR.
---------
Co-authored-by: François Mockers <francois.mockers@vleue.com>
# Objective
We deprecated quite a few APIs in 0.13. 0.13 has shipped already. It
should be OK to remove them in 0.14's release. Fixes#4059. Fixes#9011.
## Solution
Remove them.
# Objective
Adjust bevy internals to utilize `Option<Res<State<S>>>` instead of
`Res<State<S>>`, to allow for adding/removing states at runtime and
avoid unexpected panics.
As requested here:
https://github.com/bevyengine/bevy/pull/10088#issuecomment-1869185413
---
## Changelog
- Changed the use of `world.resource`/`world.resource_mut` to
`world.get_resource`/`world.get_resource_mut` in the
`run_enter_schedule` and `apply_state_transition` systems and handled
the `None` option.
- `in_state` now returns a ` FnMut(Option<Res<State<S>>>) -> bool +
Clone`, returning `false` if the resource doesn't exist.
- `state_exists_and_equals` was marked as deprecated, and now just runs
and returns `in_state`, since their bevhaviour is now identical
- `state_changed` now takes an `Option<Res<State<S>>>` and returns
`false` if it does not exist.
I would like to remove `state_exists_and_equals` fully, but wanted to
ensure that is acceptable before doing so.
---------
Co-authored-by: Mike <mike.hsu@gmail.com>
# Objective
- Shorten paths by removing unnecessary prefixes
## Solution
- Remove the prefixes from many paths which do not need them. Finding
the paths was done automatically using built-in refactoring tools in
Jetbrains RustRover.
# Objective
Resolves#10743.
## Solution
Copied over the documentation written by @stepancheng from PR #10718.
I left out the lines from the doctest where `<()>` is removed, as that
seemed to be the part people couldn't decide on whether to keep or not.
# Objective
The `States::variants` method was once used to construct `OnExit` and
`OnEnter` schedules for every possible value of a given `States` type.
[Since the switch to lazily initialized
schedules](https://github.com/bevyengine/bevy/pull/8028/files#diff-b2fba3a0c86e496085ce7f0e3f1de5960cb754c7d215ed0f087aa556e529f97f),
we no longer need to track every possible value.
This also opens the door to `States` types that aren't enums.
## Solution
- Remove the unused `States::variants` method and its associated type.
- Remove the enum-only restriction on derived States types.
---
## Changelog
- Removed `States::variants` and its associated type.
- Derived `States` can now be datatypes other than enums.
## Migration Guide
- `States::variants` no longer exists. If you relied on this function,
consider using a library that provides enum iterators.
# Objective
Scheduling low cost systems has significant overhead due to task pool
contention and the extra machinery to schedule and run them. Event
update systems are the prime example of a low cost system, requiring a
guaranteed O(1) operation, and there are a *lot* of them.
## Solution
Add a run condition to every event system so they only run when there is
an event in either of it's two internal Vecs.
---
## Changelog
Changed: Event update systems will not run if there are no events to
process.
## Migration Guide
`Events<T>::update_system` has been split off from the the type and can
be found at `bevy_ecs::event::event_update_system`.
---------
Co-authored-by: IceSentry <IceSentry@users.noreply.github.com>
# Objective
Rename RemovedComponents::iter/iter_with_id to read/read_with_id to make
it clear that it consume the data
Fixes#9755.
(It's my first pull request, if i've made any mistake, please let me
know)
## Solution
Refactor RemovedComponents::iter/iter_with_id to read/read_with_id
## Changelog
Refactor RemovedComponents::iter/iter_with_id to read/read_with_id
Deprecate RemovedComponents::iter/iter_with_id
Remove IntoIterator implementation
Update removal_detection example accordingly
---
## Migration Guide
Rename calls of RemovedComponents::iter/iter_with_id to
read/read_with_id
Replace IntoIterator iteration (&mut <RemovedComponents>) with .read()
---------
Co-authored-by: denshi_ika <mojang2824@gmail.com>
# Objective
- The current `EventReader::iter` has been determined to cause confusion
among new Bevy users. It was suggested by @JoJoJet to rename the method
to better clarify its usage.
- Solves #9624
## Solution
- Rename `EventReader::iter` to `EventReader::read`.
- Rename `EventReader::iter_with_id` to `EventReader::read_with_id`.
- Rename `ManualEventReader::iter` to `ManualEventReader::read`.
- Rename `ManualEventReader::iter_with_id` to
`ManualEventReader::read_with_id`.
---
## Changelog
- `EventReader::iter` has been renamed to `EventReader::read`.
- `EventReader::iter_with_id` has been renamed to
`EventReader::read_with_id`.
- `ManualEventReader::iter` has been renamed to
`ManualEventReader::read`.
- `ManualEventReader::iter_with_id` has been renamed to
`ManualEventReader::read_with_id`.
- Deprecated `EventReader::iter`
- Deprecated `EventReader::iter_with_id`
- Deprecated `ManualEventReader::iter`
- Deprecated `ManualEventReader::iter_with_id`
## Migration Guide
- Existing usages of `EventReader::iter` and `EventReader::iter_with_id`
will have to be changed to `EventReader::read` and
`EventReader::read_with_id` respectively.
- Existing usages of `ManualEventReader::iter` and
`ManualEventReader::iter_with_id` will have to be changed to
`ManualEventReader::read` and `ManualEventReader::read_with_id`
respectively.
# Objective
- Move schedule name into `Schedule` to allow the schedule name to be
used for errors and tracing in Schedule methods
- Fixes#9510
## Solution
- Move label onto `Schedule` and adjust api's on `World` and `Schedule`
to not pass explicit label where it makes sense to.
- add name to errors and tracing.
- `Schedule::new` now takes a label so either add the label or use
`Schedule::default` which uses a default label. `default` is mostly used
in doc examples and tests.
---
## Changelog
- move label onto `Schedule` to improve error message and logging for
schedules.
## Migration Guide
`Schedule::new` and `App::add_schedule`
```rust
// old
let schedule = Schedule::new();
app.add_schedule(MyLabel, schedule);
// new
let schedule = Schedule::new(MyLabel);
app.add_schedule(schedule);
```
if you aren't using a label and are using the schedule struct directly
you can use the default constructor.
```rust
// old
let schedule = Schedule::new();
schedule.run(world);
// new
let schedule = Schedule::default();
schedule.run(world);
```
`Schedules:insert`
```rust
// old
let schedule = Schedule::new();
schedules.insert(MyLabel, schedule);
// new
let schedule = Schedule::new(MyLabel);
schedules.insert(schedule);
```
`World::add_schedule`
```rust
// old
let schedule = Schedule::new();
world.add_schedule(MyLabel, schedule);
// new
let schedule = Schedule::new(MyLabel);
world.add_schedule(schedule);
```
# Objective
Any time we wish to transform the output of a system, we currently use
system piping to do so:
```rust
my_system.pipe(|In(x)| do_something(x))
```
Unfortunately, system piping is not a zero cost abstraction. Each call
to `.pipe` requires allocating two extra access sets: one for the second
system and one for the combined accesses of both systems. This also adds
extra work to each call to `update_archetype_component_access`, which
stacks as one adds multiple layers of system piping.
## Solution
Add the `AdapterSystem` abstraction: similar to `CombinatorSystem`, this
allows you to implement a trait to generically control how a system is
run and how its inputs and outputs are processed. Unlike
`CombinatorSystem`, this does not have any overhead when computing world
accesses which makes it ideal for simple operations such as inverting or
ignoring the output of a system.
Add the extension method `.map(...)`: this is similar to `.pipe(...)`,
only it accepts a closure as an argument instead of an `In<T>` system.
```rust
my_system.map(do_something)
```
This has the added benefit of making system names less messy: a system
that ignores its output will just be called `my_system`, instead of
`Pipe(my_system, ignore)`
---
## Changelog
TODO
## Migration Guide
The `system_adapter` functions have been deprecated: use `.map` instead,
which is a lightweight alternative to `.pipe`.
```rust
// Before:
my_system.pipe(system_adapter::ignore)
my_system.pipe(system_adapter::unwrap)
my_system.pipe(system_adapter::new(T::from))
// After:
my_system.map(std::mem::drop)
my_system.map(Result::unwrap)
my_system.map(T::from)
// Before:
my_system.pipe(system_adapter::info)
my_system.pipe(system_adapter::dbg)
my_system.pipe(system_adapter::warn)
my_system.pipe(system_adapter::error)
// After:
my_system.map(bevy_utils::info)
my_system.map(bevy_utils::dbg)
my_system.map(bevy_utils::warn)
my_system.map(bevy_utils::error)
```
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
# Objective
Fix typos throughout the project.
## Solution
[`typos`](https://github.com/crate-ci/typos) project was used for
scanning, but no automatic corrections were applied. I checked
everything by hand before fixing.
Most of the changes are documentation/comments corrections. Also, there
are few trivial changes to code (variable name, pub(crate) function name
and a few error/panic messages).
## Unsolved
`bevy_reflect_derive` has
[typo](1b51053f19/crates/bevy_reflect/bevy_reflect_derive/src/type_path.rs (L76))
in enum variant name that I didn't fix. Enum is `pub(crate)`, so there
shouldn't be any trouble if fixed. However, code is tightly coupled with
macro usage, so I decided to leave it for more experienced contributor
just in case.
# Objective
Title.
---------
Co-authored-by: François <mockersf@gmail.com>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: James Liu <contact@jamessliu.com>
# Objective
Be consistent with `Resource`s and `Components` and have `Event` types
be more self-documenting.
Although not susceptible to accidentally using a function instead of a
value due to `Event`s only being initialized by their type, much of the
same reasoning for removing the blanket impl on `Resource` also applies
here.
* Not immediately obvious if a type is intended to be an event
* Prevent invisible conflicts if the same third-party or primitive types
are used as events
* Allows for further extensions (e.g. opt-in warning for missed events)
## Solution
Remove the blanket impl for the `Event` trait. Add a derive macro for
it.
---
## Changelog
- `Event` is no longer implemented for all applicable types. Add the
`#[derive(Event)]` macro for events.
## Migration Guide
* Add the `#[derive(Event)]` macro for events. Third-party types used as
events should be wrapped in a newtype.
# Objective
- `apply_system_buffers` is an unhelpful name: it introduces a new
internal-only concept
- this is particularly rough for beginners as reasoning about how
commands work is a critical stumbling block
## Solution
- rename `apply_system_buffers` to the more descriptive `apply_deferred`
- rename related fields, arguments and methods in the internals fo
bevy_ecs for consistency
- update the docs
## Changelog
`apply_system_buffers` has been renamed to `apply_deferred`, to more
clearly communicate its intent and relation to `Deferred` system
parameters like `Commands`.
## Migration Guide
- `apply_system_buffers` has been renamed to `apply_deferred`
- the `apply_system_buffers` method on the `System` trait has been
renamed to `apply_deferred`
- the `is_apply_system_buffers` function has been replaced by
`is_apply_deferred`
- `Executor::set_apply_final_buffers` is now
`Executor::set_apply_final_deferred`
- `Schedule::apply_system_buffers` is now `Schedule::apply_deferred`
---------
Co-authored-by: JoJoJet <21144246+JoJoJet@users.noreply.github.com>
# Objective
The `Condition` trait is only implemented for systems and system
functions that take no input. This can make it awkward to write
conditions that are intended to be used with system piping.
## Solution
Add an `In` generic to the trait. It defaults to `()`.
---
## Changelog
- Made the `Condition` trait generic over system inputs.
# Objective
Fix#7833.
Safety comments in the multi-threaded executor don't really talk about
system world accesses, which makes it unclear if the code is actually
valid.
## Solution
Update the `System` trait to use `UnsafeWorldCell`. This type's API is
written in a way that makes it much easier to cleanly maintain safety
invariants. Use this type throughout the multi-threaded executor, with a
liberal use of safety comments.
---
## Migration Guide
The `System` trait now uses `UnsafeWorldCell` instead of `&World`. This
type provides a robust API for interior mutable world access.
- The method `run_unsafe` uses this type to manage world mutations
across multiple threads.
- The method `update_archetype_component_access` uses this type to
ensure that only world metadata can be used.
```rust
let mut system = IntoSystem::into_system(my_system);
system.initialize(&mut world);
// Before:
system.update_archetype_component_access(&world);
unsafe { system.run_unsafe(&world) }
// After:
system.update_archetype_component_access(world.as_unsafe_world_cell_readonly());
unsafe { system.run_unsafe(world.as_unsafe_world_cell()) }
```
---------
Co-authored-by: James Liu <contact@jamessliu.com>
Added helper extracted from #7711. that PR contains some controversy
conditions, but this one should be good to go.
---
## Changelog
### Added
- `any_component_removed` condition.
---------
Co-authored-by: François <mockersf@gmail.com>
# Objective
State requires a kind of awkward `state.0` to get the current state and
exposes the field directly to manipulation.
## Solution
Make it accessible through a getter method as well as privatize the
field to make sure false assumptions about setting the state aren't
made.
## Migration Guide
- Use `State::get` instead of accessing the tuple field directly.
# Objective
While we use `#[doc(hidden)]` to try and hide marker generics from the user, these types reveal themselves in compiler errors, adding visual noise and confusion.
## Solution
Replace the `AlreadyWasSystem` marker generic with `()`, to reduce visual noise in error messages. This also makes it possible to return `impl Condition<()>` from combinators.
For function systems, use their function signature as the marker type. This should drastically improve the legibility of some error messages.
The `InputMarker` type has been removed, since it is unnecessary.
# Objective
Several places in the ECS use marker generics to avoid overlapping trait implementations, but different places alternately refer to it as `Params` and `Marker`. This is potentially confusing, since it might not be clear that the same pattern is being used. Additionally, users might be misled into thinking that the `Params` type corresponds to the `SystemParam`s of a system.
## Solution
Rename `Params` to `Marker`.
# Objective
The `BoxedCondition` type alias does not require the underlying system to be read-only.
## Solution
Define the type alias using `ReadOnlySystem` instead of `System`.
Add some more useful common run conditions.
Some of these existed in `iyes_loopless`. I know people used them, and it would be a regression for those users, when they try to migrate to new Bevy stageless, if they are missing.
I also took the opportunity to add a few more new ones.
---
## Changelog
### Added
- More "common run conditions": on_event, resource change detection, state_changed, any_with_component
# Objective
Fix#7584.
## Solution
Add an abstraction for creating custom system combinators with minimal boilerplate. Use this to implement AND/OR combinators. Use this to simplify the implementation of `PipeSystem`.
## Example
Feel free to bikeshed on the syntax.
I chose the names `and_then`/`or_else` to emphasize the fact that these short-circuit, while I chose method syntax to empasize that the arguments are *not* treated equally.
```rust
app.add_systems((
my_system.run_if(resource_exists::<R>().and_then(resource_equals(R(0)))),
our_system.run_if(resource_exists::<R>().or_else(resource_exists::<S>())),
));
```
---
## Todo
- [ ] Decide on a syntax
- [x] Write docs
- [x] Write tests
## Changelog
+ Added the extension methods `.and_then(...)` and `.or_else(...)` to run conditions, which allows combining run conditions with short-circuiting behavior.
+ Added the trait `Combine`, which can be used with the new `CombinatorSystem` to create system combinators with custom behavior.
# Objective
The trait `Condition<>` is implemented for any type that can be converted into a `ReadOnlySystem` which takes no inputs and returns a bool. However, due to the current implementation, consumers of the trait cannot rely on the fact that `<T as Condition>::System` implements `ReadOnlySystem`. In cases such as the `not` combinator (added in #7559), we are required to add redundant `T::System: ReadOnlySystem` trait bounds, even though this should be implied by the `Condition<>` trait.
## Solution
Add a hidden associated type which allows the compiler to figure out that the `System` associated type implements `ReadOnlySystem`.
# Objective
- Improve readability of the run condition for systems only running in a certain state
## Solution
- Rename `state_equals` to `in_state` (see [comment by cart](https://github.com/bevyengine/bevy/pull/7634#issuecomment-1428740311) in #7634 )
- `.run_if(state_equals(variant))` now is `.run_if(in_state(variant))`
This breaks the naming pattern a bit with the related conditions `state_exists` and `state_exists_and_equals` but I could not think of better names for those and think the improved readability of `in_state` is worth it.
# Objective
Closes#7202
## Solution
~~Introduce a `not` helper to pipe conditions. Opened mostly for discussion. Maybe create an extension trait with `not` method? Please, advice.~~
Introduce `not(condition)` condition that inverses the result of the passed.
---
## Changelog
### Added
- `not` condition.
# Objective
Run conditions are a special type of system that do not modify the world, and which return a bool. Due to the way they are currently implemented, you can *only* use bare function systems as a run condition. Among other things, this prevents the use of system piping with run conditions. This make very basic constructs impossible, such as `my_system.run_if(my_condition.pipe(not))`.
Unblocks a basic solution for #7202.
## Solution
Add the trait `ReadOnlySystem`, which is implemented for any system whose parameters all implement `ReadOnlySystemParam`. Allow any `-> bool` system implementing this trait to be used as a run condition.
---
## Changelog
+ Added the trait `ReadOnlySystem`, which is implemented for any `System` type whose parameters all implement `ReadOnlySystemParam`.
+ Added the function `bevy::ecs::system::assert_is_read_only_system`.
