Add more "common run conditions" (#7579)

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

Author: inodentry

crates/bevy_ecs/src/schedule/condition.rs

@@ -133,6 +133,9 @@ mod sealed {
 pub mod common_conditions {
     use super::Condition;
     use crate::{
+        change_detection::DetectChanges,
+        event::{Event, EventReader},
+        prelude::{Component, Query, With},
         schedule::{State, States},
         system::{In, IntoPipeSystem, ReadOnlySystem, Res, Resource},
     };
@@ -187,6 +190,107 @@ pub mod common_conditions {
         }
     }
 
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if the resource of the given type has been added since the condition was last checked.
+    pub fn resource_added<T>() -> impl FnMut(Option<Res<T>>) -> bool
+    where
+        T: Resource,
+    {
+        move |res: Option<Res<T>>| match res {
+            Some(res) => res.is_added(),
+            None => false,
+        }
+    }
+
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if the resource of the given type has had its value changed since the condition
+    /// was last checked.
+    ///
+    /// The value is considered changed when it is added. The first time this condition
+    /// is checked after the resource was added, it will return `true`.
+    /// Change detection behaves like this everywhere in Bevy.
+    ///
+    /// # Panics
+    ///
+    /// The condition will panic if the resource does not exist.
+    pub fn resource_changed<T>() -> impl FnMut(Res<T>) -> bool
+    where
+        T: Resource,
+    {
+        move |res: Res<T>| res.is_changed()
+    }
+
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if the resource of the given type has had its value changed since the condition
+    /// was last checked.
+    ///
+    /// The value is considered changed when it is added. The first time this condition
+    /// is checked after the resource was added, it will return `true`.
+    /// Change detection behaves like this everywhere in Bevy.
+    ///
+    /// This run condition does not detect when the resource is removed.
+    ///
+    /// The condition will return `false` if the resource does not exist.
+    pub fn resource_exists_and_changed<T>() -> impl FnMut(Option<Res<T>>) -> bool
+    where
+        T: Resource,
+    {
+        move |res: Option<Res<T>>| match res {
+            Some(res) => res.is_changed(),
+            None => false,
+        }
+    }
+
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if the resource of the given type has had its value changed since the condition
+    /// was last checked.
+    ///
+    /// The value is considered changed when it is added. The first time this condition
+    /// is checked after the resource was added, it will return `true`.
+    /// Change detection behaves like this everywhere in Bevy.
+    ///
+    /// This run condition also detects removal. It will return `true` if the resource
+    /// has been removed since the run condition was last checked.
+    ///
+    /// The condition will return `false` if the resource does not exist.
+    pub fn resource_changed_or_removed<T>() -> impl FnMut(Option<Res<T>>) -> bool
+    where
+        T: Resource,
+    {
+        let mut existed = false;
+        move |res: Option<Res<T>>| {
+            if let Some(value) = res {
+                existed = true;
+                value.is_changed()
+            } else if existed {
+                existed = false;
+                true
+            } else {
+                false
+            }
+        }
+    }
+
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if the resource of the given type has been removed since the condition was last checked.
+    pub fn resource_removed<T>() -> impl FnMut(Option<Res<T>>) -> bool
+    where
+        T: Resource,
+    {
+        let mut existed = false;
+        move |res: Option<Res<T>>| {
+            if res.is_some() {
+                existed = true;
+                false
+            } else if existed {
+                existed = false;
+                true
+            } else {
+                false
+            }
+        }
+    }
+
     /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
     /// if the state machine exists.
     pub fn state_exists<S: States>() -> impl FnMut(Option<Res<State<S>>>) -> bool {
@@ -216,6 +320,35 @@ pub mod common_conditions {
         }
     }
 
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if the state machine changed state.
+    ///
+    /// To do things on transitions to/from specific states, use their respective OnEnter/OnExit
+    /// schedules. Use this run condition if you want to detect any change, regardless of the value.
+    ///
+    /// # Panics
+    ///
+    /// The condition will panic if the resource does not exist.
+    pub fn state_changed<S: States>() -> impl FnMut(Res<State<S>>) -> bool {
+        move |current_state: Res<State<S>>| current_state.is_changed()
+    }
+
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if there are any new events of the given type since it was last called.
+    pub fn on_event<T: Event>() -> impl FnMut(EventReader<T>) -> bool {
+        // The events need to be consumed, so that there are no false positives on subsequent
+        // calls of the run condition. Simply checking `is_empty` would not be enough.
+        // PERF: note that `count` is efficient (not actually looping/iterating),
+        // due to Bevy having a specialized implementation for events.
+        move |mut reader: EventReader<T>| reader.iter().count() > 0
+    }
+
+    /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
+    /// if there are any entities with the given component type.
+    pub fn any_with_component<T: Component>() -> impl FnMut(Query<(), With<T>>) -> bool {
+        move |query: Query<(), With<T>>| !query.is_empty()
+    }
+
     /// Generates a  [`Condition`](super::Condition) that inverses the result of passed one.
     ///
     /// # Examples