Add or_else combinator to run_conditions.rs

examples/ecs/run_conditions.rs

@@ -18,15 +18,20 @@ fn main() {
  // 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>())
- // 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`.
- .run_if(has_user_input),
+ // `.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`. This behavior is known as "short-circuiting",
- // and is how the `&&` operator works in Rust (as well as most C-family languages).
+ // 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(
@@ -51,6 +56,9 @@ fn main() {
 #[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).