mirror of
https://github.com/bevyengine/bevy
synced 2024-11-26 22:50:19 +00:00
4ce37395da
# Objective - Showcase the use of `or_else()` as requested. Fixes https://github.com/bevyengine/bevy/issues/8702 ## Solution - Add an uninitialized resource `Unused` - Use `or_else()` to evaluate a second run condition - Add documentation explaining how `or_else()` works
106 lines
5.2 KiB
Rust
106 lines
5.2 KiB
Rust
//! This example demonstrates how to use run conditions to control when systems run.
|
|
|
|
use bevy::prelude::*;
|
|
|
|
fn main() {
|
|
println!();
|
|
println!("For the first 2 seconds you will not be able to increment the counter");
|
|
println!("Once that time has passed you can press space, enter, left mouse, right mouse or touch the screen to increment the counter");
|
|
println!();
|
|
|
|
App::new()
|
|
.add_plugins(DefaultPlugins)
|
|
.init_resource::<InputCounter>()
|
|
.add_systems(
|
|
Update,
|
|
(
|
|
increment_input_counter
|
|
// The common_conditions module has a few useful run conditions
|
|
// for checking resources and states. These are included in the prelude.
|
|
.run_if(resource_exists::<InputCounter>())
|
|
// `.or_else()` is a run condition combinator that only evaluates the second condition
|
|
// if the first condition returns `false`. This behavior is known as "short-circuiting",
|
|
// and is how the `||` operator works in Rust (as well as most C-family languages).
|
|
// In this case, the `has_user_input` run condition will be evaluated since the `Unused` resource has not been initialized.
|
|
.run_if(resource_exists::<Unused>().or_else(
|
|
// This is a custom run condition, defined using a system that returns
|
|
// a `bool` and which has read-only `SystemParam`s.
|
|
// Both run conditions must return `true` in order for the system to run.
|
|
// Note that this second run condition will be evaluated even if the first returns `false`.
|
|
has_user_input,
|
|
)),
|
|
print_input_counter
|
|
// `.and_then()` is a run condition combinator that only evaluates the second condition
|
|
// if the first condition returns `true`, analogous to the `&&` operator.
|
|
// In this case, the short-circuiting behavior prevents the second run condition from
|
|
// panicking if the `InputCounter` resource has not been initialized.
|
|
.run_if(resource_exists::<InputCounter>().and_then(
|
|
// This is a custom run condition in the form of a closure.
|
|
// This is useful for small, simple run conditions you don't need to reuse.
|
|
// All the normal rules still apply: all parameters must be read only except for local parameters.
|
|
|counter: Res<InputCounter>| counter.is_changed() && !counter.is_added(),
|
|
)),
|
|
print_time_message
|
|
// This function returns a custom run condition, much like the common conditions module.
|
|
// It will only return true once 2 seconds have passed.
|
|
.run_if(time_passed(2.0))
|
|
// You can use the `not` condition from the common_conditions module
|
|
// to inverse a run condition. In this case it will return true if
|
|
// less than 2.5 seconds have elapsed since the app started.
|
|
.run_if(not(time_passed(2.5))),
|
|
),
|
|
)
|
|
.run();
|
|
}
|
|
|
|
#[derive(Resource, Default)]
|
|
struct InputCounter(usize);
|
|
|
|
#[derive(Resource)]
|
|
struct Unused;
|
|
|
|
/// Return true if any of the defined inputs were just pressed.
|
|
/// This is a custom run condition, it can take any normal system parameters as long as
|
|
/// they are read only (except for local parameters which can be mutable).
|
|
/// It returns a bool which determines if the system should run.
|
|
fn has_user_input(
|
|
keyboard_input: Res<Input<KeyCode>>,
|
|
mouse_button_input: Res<Input<MouseButton>>,
|
|
touch_input: Res<Touches>,
|
|
) -> bool {
|
|
keyboard_input.just_pressed(KeyCode::Space)
|
|
|| keyboard_input.just_pressed(KeyCode::Return)
|
|
|| mouse_button_input.just_pressed(MouseButton::Left)
|
|
|| mouse_button_input.just_pressed(MouseButton::Right)
|
|
|| touch_input.any_just_pressed()
|
|
}
|
|
|
|
/// This is a function that returns a closure which can be used as a run condition.
|
|
/// This is useful because you can reuse the same run condition but with different variables.
|
|
/// This is how the common conditions module works.
|
|
fn time_passed(t: f32) -> impl FnMut(Local<f32>, Res<Time>) -> bool {
|
|
move |mut timer: Local<f32>, time: Res<Time>| {
|
|
// Tick the timer
|
|
*timer += time.delta_seconds();
|
|
// Return true if the timer has passed the time
|
|
*timer >= t
|
|
}
|
|
}
|
|
|
|
/// SYSTEM: Increment the input counter
|
|
/// Notice how we can take just the `ResMut` and not have to wrap
|
|
/// it in an option in case it hasn't been initialized, this is because
|
|
/// it has a run condition that checks if the `InputCounter` resource exists
|
|
fn increment_input_counter(mut counter: ResMut<InputCounter>) {
|
|
counter.0 += 1;
|
|
}
|
|
|
|
/// SYSTEM: Print the input counter
|
|
fn print_input_counter(counter: Res<InputCounter>) {
|
|
println!("Input counter: {}", counter.0);
|
|
}
|
|
|
|
/// SYSTEM: Adds the input counter resource
|
|
fn print_time_message() {
|
|
println!("It has been more than 2 seconds since the program started and less than 2.5 seconds");
|
|
}
|