pub trait Condition<Marker, In = ()>: Condition<Marker, In> {
// Provided methods
fn and_then<M, C: Condition<M, In>>(
self,
and_then: C
) -> AndThen<Self::System, C::System> { ... }
fn or_else<M, C: Condition<M, In>>(
self,
or_else: C
) -> OrElse<Self::System, C::System> { ... }
}
Expand description
A system that determines if one or more scheduled systems should run.
Implemented for functions and closures that convert into System<Out=bool>
with read-only parameters.
§Marker type parameter
Condition
trait has Marker
type parameter, which has no special meaning,
but exists to work around the limitation of Rust’s trait system.
Type parameter in return type can be set to <()>
by calling IntoSystem::into_system
,
but usually have to be specified when passing a condition to a function.
fn not_condition<Marker>(a: impl Condition<Marker>) -> impl Condition<()> {
IntoSystem::into_system(a.map(|x| !x))
}
§Examples
A condition that returns true every other time it’s called.
fn every_other_time() -> impl Condition<()> {
IntoSystem::into_system(|mut flag: Local<bool>| {
*flag = !*flag;
*flag
})
}
schedule.add_systems(my_system.run_if(every_other_time()));
A condition that takes a bool as an input and returns it unchanged.
fn identity() -> impl Condition<(), bool> {
IntoSystem::into_system(|In(x)| x)
}
app.add_systems(my_system.run_if(always_true.pipe(identity())));
Provided Methods§
sourcefn and_then<M, C: Condition<M, In>>(
self,
and_then: C
) -> AndThen<Self::System, C::System>
fn and_then<M, C: Condition<M, In>>( self, and_then: C ) -> AndThen<Self::System, C::System>
Returns a new run condition that only returns true
if both this one and the passed and_then
return true
.
The returned run condition is short-circuiting, meaning
and_then
will only be invoked if self
returns true
.
§Examples
use bevy_ecs::prelude::*;
#[derive(Resource, PartialEq)]
struct R(u32);
app.add_systems(
// The `resource_equals` run condition will panic since we don't initialize `R`,
// just like if we used `Res<R>` in a system.
my_system.run_if(resource_equals(R(0))),
);
Use .and_then()
to avoid checking the condition.
app.add_systems(
// `resource_equals` will only get run if the resource `R` exists.
my_system.run_if(resource_exists::<R>.and_then(resource_equals(R(0)))),
);
Note that in this case, it’s better to just use the run condition resource_exists_and_equals
.
sourcefn or_else<M, C: Condition<M, In>>(
self,
or_else: C
) -> OrElse<Self::System, C::System>
fn or_else<M, C: Condition<M, In>>( self, or_else: C ) -> OrElse<Self::System, C::System>
Returns a new run condition that returns true
if either this one or the passed or_else
return true
.
The returned run condition is short-circuiting, meaning
or_else
will only be invoked if self
returns false
.
§Examples
use bevy_ecs::prelude::*;
#[derive(Resource, PartialEq)]
struct A(u32);
#[derive(Resource, PartialEq)]
struct B(u32);
app.add_systems(
// Only run the system if either `A` or `B` exist.
my_system.run_if(resource_exists::<A>.or_else(resource_exists::<B>)),
);