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Allow piping run conditions (bevyengine#7547)
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# 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 bevyengine#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`.
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JoJoJet authored and myreprise1 committed Feb 15, 2023
1 parent a6ebe22 commit 77d3e36
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Showing 5 changed files with 62 additions and 7 deletions.
9 changes: 4 additions & 5 deletions crates/bevy_ecs/src/schedule/condition.rs
Original file line number Diff line number Diff line change
Expand Up @@ -11,15 +11,14 @@ pub trait Condition<Params>: sealed::Condition<Params> {}
impl<Params, F> Condition<Params> for F where F: sealed::Condition<Params> {}

mod sealed {
use crate::system::{IntoSystem, IsFunctionSystem, ReadOnlySystemParam, SystemParamFunction};
use crate::system::{IntoSystem, ReadOnlySystem};

pub trait Condition<Params>: IntoSystem<(), bool, Params> {}

impl<Params, Marker, F> Condition<(IsFunctionSystem, Params, Marker)> for F
impl<Params, F> Condition<Params> for F
where
F: SystemParamFunction<(), bool, Params, Marker> + Send + Sync + 'static,
Params: ReadOnlySystemParam + 'static,
Marker: 'static,
F: IntoSystem<(), bool, Params>,
F::System: ReadOnlySystem,
{
}
}
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13 changes: 13 additions & 0 deletions crates/bevy_ecs/src/system/function_system.rs
Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,8 @@ use crate::{
use bevy_ecs_macros::all_tuples;
use std::{any::TypeId, borrow::Cow, marker::PhantomData};

use super::ReadOnlySystem;

/// The metadata of a [`System`].
#[derive(Clone)]
pub struct SystemMeta {
Expand Down Expand Up @@ -528,6 +530,17 @@ where
}
}

/// SAFETY: `F`'s param is `ReadOnlySystemParam`, so this system will only read from the world.
unsafe impl<In, Out, Param, Marker, F> ReadOnlySystem for FunctionSystem<In, Out, Param, Marker, F>
where
In: 'static,
Out: 'static,
Param: ReadOnlySystemParam + 'static,
Marker: 'static,
F: SystemParamFunction<In, Out, Param, Marker> + Send + Sync + 'static,
{
}

/// A trait implemented for all functions that can be used as [`System`]s.
///
/// This trait can be useful for making your own systems which accept other systems,
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20 changes: 18 additions & 2 deletions crates/bevy_ecs/src/system/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -117,11 +117,11 @@ pub use system::*;
pub use system_param::*;
pub use system_piping::*;

/// Ensure that a given function is a system
/// Ensure that a given function is a [system](System).
///
/// This should be used when writing doc examples,
/// to confirm that systems used in an example are
/// valid systems
/// valid systems.
pub fn assert_is_system<In, Out, Params, S: IntoSystem<In, Out, Params>>(sys: S) {
if false {
// Check it can be converted into a system
Expand All @@ -130,6 +130,22 @@ pub fn assert_is_system<In, Out, Params, S: IntoSystem<In, Out, Params>>(sys: S)
}
}

/// Ensure that a given function is a [read-only system](ReadOnlySystem).
///
/// This should be used when writing doc examples,
/// to confirm that systems used in an example are
/// valid systems.
pub fn assert_is_read_only_system<In, Out, Params, S: IntoSystem<In, Out, Params>>(sys: S)
where
S::System: ReadOnlySystem,
{
if false {
// Check it can be converted into a system
// TODO: This should ensure that the system has no conflicting system params
IntoSystem::into_system(sys);
}
}

#[cfg(test)]
mod tests {
use std::any::TypeId;
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10 changes: 10 additions & 0 deletions crates/bevy_ecs/src/system/system.rs
Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,16 @@ pub trait System: Send + Sync + 'static {
fn set_last_change_tick(&mut self, last_change_tick: u32);
}

/// [`System`] types that do not modify the [`World`] when run.
/// This is implemented for any systems whose parameters all implement [`ReadOnlySystemParam`].
///
/// [`ReadOnlySystemParam`]: crate::system::ReadOnlySystemParam
///
/// # Safety
///
/// This must only be implemented for system types which do not mutate the `World`.
pub unsafe trait ReadOnlySystem: System {}

/// A convenience type alias for a boxed [`System`] trait object.
pub type BoxedSystem<In = (), Out = ()> = Box<dyn System<In = In, Out = Out>>;

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17 changes: 17 additions & 0 deletions crates/bevy_ecs/src/system/system_piping.rs
Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@ use crate::{
};
use std::{any::TypeId, borrow::Cow};

use super::ReadOnlySystem;

/// A [`System`] created by piping the output of the first system into the input of the second.
///
/// This can be repeated indefinitely, but system pipes cannot branch: the output is consumed by the receiving system.
Expand Down Expand Up @@ -153,6 +155,15 @@ impl<SystemA: System, SystemB: System<In = SystemA::Out>> System for PipeSystem<
}
}

/// SAFETY: Both systems are read-only, so piping them together will only read from the world.
unsafe impl<SystemA: System, SystemB: System<In = SystemA::Out>> ReadOnlySystem
for PipeSystem<SystemA, SystemB>
where
SystemA: ReadOnlySystem,
SystemB: ReadOnlySystem,
{
}

/// An extension trait providing the [`IntoPipeSystem::pipe`] method to pass input from one system into the next.
///
/// The first system must have return type `T`
Expand Down Expand Up @@ -412,10 +423,16 @@ pub mod adapter {
unimplemented!()
}

fn not(In(val): In<bool>) -> bool {
!val
}

assert_is_system(returning::<Result<u32, std::io::Error>>.pipe(unwrap));
assert_is_system(returning::<Option<()>>.pipe(ignore));
assert_is_system(returning::<&str>.pipe(new(u64::from_str)).pipe(unwrap));
assert_is_system(exclusive_in_out::<(), Result<(), std::io::Error>>.pipe(error));
assert_is_system(returning::<bool>.pipe(exclusive_in_out::<bool, ()>));

returning::<()>.run_if(returning::<bool>.pipe(not));
}
}

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