Commands::send_event (bevyengine#14933)

# Objective

sending events tends to be low-frequency so ergonomics can be
prioritized over efficiency.
add `Commands::send_event` to send any type of event without needing a
writer in hand.

i don't know how we feel about these kind of ergonomic things, i add
this to all my projects and find it useful. adding `mut
this_particular_event_writer: EventWriter<ThisParticularEvent>` every
time i want to send something is unnecessarily cumbersome.
it also simplifies the "send and receive in the same system" pattern
significantly.

basic example before:
```rs
fn my_func(
    q: Query<(Entity, &State)>,
    mut damage_event_writer: EventWriter<DamageEvent>,
    mut heal_event_writer: EventWriter<HealEvent>,
) {
    for (entity, state) in q.iter() {
        if let Some(damage) = state.get_damage() {
            damage_event_writer.send(DamageEvent { entity, damage });
        }

        if let Some(heal) = state.get_heal() {
            heal_event_writer.send(HealEvent { entity, heal });
        }
    }
}
```

basic example after:
```rs
import bevy::ecs::event::SendEventEx;

fn my_func(
    mut commands: Commands,
    q: Query<(Entity, &State)>,
) {
    for (entity, state) in q.iter() {
        if let Some(damage) = state.get_damage() {
            commands.send_event(DamageEvent { entity, damage });
        }

        if let Some(heal) = state.get_heal() {
            commands.send_event(HealEvent { entity, heal });
        }
    }
}
```

send/receive in the same system before:
```rs
fn send_and_receive_param_set(
    mut param_set: ParamSet<(EventReader<DebugEvent>, EventWriter<DebugEvent>)>,
) {
    // We must collect the events to resend, because we can't access the writer while we're iterating over the reader.
    let mut events_to_resend = Vec::new();

    // This is p0, as the first parameter in the `ParamSet` is the reader.
    for event in param_set.p0().read() {
        if event.resend_from_param_set {
            events_to_resend.push(event.clone());
        }
    }

    // This is p1, as the second parameter in the `ParamSet` is the writer.
    for mut event in events_to_resend {
        event.times_sent += 1;
        param_set.p1().send(event);
    }
}
```

after:
```rs
use bevy::ecs::event::SendEventEx;

fn send_via_commands_and_receive(
    mut reader: EventReader<DebugEvent>,
    mut commands: Commands,
) {
    for event in reader.read() {
        if event.resend_via_commands {
            commands.send_event(DebugEvent {
                times_sent: event.times_sent + 1,
                ..event.clone()
            });
        }
    }
}
```

---------

Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>

crates/bevy_ecs/src/event/mod.rs

@@ -7,6 +7,7 @@ mod mut_iterators;
 mod mutator;
 mod reader;
 mod registry;
+mod send_event;
 mod update;
 mod writer;
 
@@ -24,6 +25,7 @@ pub use mut_iterators::{EventMutIterator, EventMutIteratorWithId};
 pub use mutator::EventMutator;
 pub use reader::EventReader;
 pub use registry::{EventRegistry, ShouldUpdateEvents};
+pub use send_event::SendEvent;
 pub use update::{
     event_update_condition, event_update_system, signal_event_update_system, EventUpdates,
 };

crates/bevy_ecs/src/event/send_event.rs

@@ -0,0 +1,15 @@
+use super::{Event, Events};
+use crate::world::{Command, World};
+
+/// A command to send an arbitrary [`Event`], used by [`Commands::send_event`](crate::system::Commands::send_event).
+pub struct SendEvent<E: Event> {
+    /// The event to send.
+    pub event: E,
+}
+
+impl<E: Event> Command for SendEvent<E> {
+    fn apply(self, world: &mut World) {
+        let mut events = world.resource_mut::<Events<E>>();
+        events.send(self.event);
+    }
+}

crates/bevy_ecs/src/system/commands/mod.rs

@@ -8,7 +8,7 @@ use crate::{
     bundle::{Bundle, InsertMode},
     component::{ComponentId, ComponentInfo},
     entity::{Entities, Entity},
-    event::Event,
+    event::{Event, SendEvent},
     observer::{Observer, TriggerEvent, TriggerTargets},
     system::{RunSystemWithInput, SystemId},
     world::{
@@ -788,6 +788,21 @@ impl<'w, 's> Commands<'w, 's> {
     ) -> EntityCommands {
         self.spawn(Observer::new(observer))
     }
+
+    /// Sends an arbitrary [`Event`].
+    ///
+    /// This is a convenience method for sending events without requiring an [`EventWriter`].
+    /// ## Performance
+    /// Since this is a command, exclusive world access is used, which means that it will not profit from
+    /// system-level parallelism on supported platforms.
+    /// If these events are performance-critical or very frequently
+    /// sent, consider using a typed [`EventWriter`] instead.
+    ///
+    /// [`EventWriter`]: crate::event::EventWriter
+    pub fn send_event<E: Event>(&mut self, event: E) -> &mut Self {
+        self.add(SendEvent { event });
+        self
+    }
 }
 
 /// A [`Command`] which gets executed for a given [`Entity`].