Add access to App within LogPlugin::update_subscriber (#12045)

# Objective

In my library,
[`bevy_dev_console`](https://github.com/doonv/bevy_dev_console) I need
access to `App` within `LogPlugin::update_subscriber` in order to
communicate with the `App` from my custom `Layer`.

## Solution

Give access to `App`.

---

## Changelog

- Added access to `App` within `LogPlugin::update_subscriber`

## Migration Guide

`LogPlugin::update_subscriber` now has a `&mut App` parameter. If you
don't need access to `App`, you can ignore the parameter with an
underscore (`_`).

```diff,rust
- fn update_subscriber(subscriber: BoxedSubscriber) -> BoxedSubscriber {
+ fn update_subscriber(_: &mut App, subscriber: BoxedSubscriber) -> BoxedSubscriber {
      Box::new(subscriber.with(CustomLayer))
  }
```

Cargo.toml

@@ -1105,6 +1105,16 @@ description = "Illustrate how to add custom log layers"
 category = "Application"
 wasm = false
 
+[[example]]
+name = "log_layers_ecs"
+path = "examples/app/log_layers_ecs.rs"
+
+[package.metadata.example.log_layers_ecs]
+name = "Advanced log layers"
+description = "Illustrate how to transfer data between log layers and Bevy's ECS"
+category = "Application"
+wasm = false
+
 [[example]]
 name = "plugin"
 path = "examples/app/plugin.rs"

crates/bevy_log/src/lib.rs

@@ -106,9 +106,12 @@ pub struct LogPlugin {
     /// This can be further filtered using the `filter` setting.
     pub level: Level,
 
-    /// Optionally apply extra transformations to the tracing subscriber.
-    /// For example add [`Layers`](tracing_subscriber::layer::Layer)
-    pub update_subscriber: Option<fn(BoxedSubscriber) -> BoxedSubscriber>,
+    /// Optionally apply extra transformations to the tracing subscriber,
+    /// such as adding [`Layer`](tracing_subscriber::layer::Layer)s.
+    ///
+    /// Access to [`App`] is also provided to allow for communication between the [`Subscriber`]
+    /// and the [`App`].
+    pub update_subscriber: Option<fn(&mut App, BoxedSubscriber) -> BoxedSubscriber>,
 }
 
 /// Alias for a boxed [`Subscriber`].
@@ -193,7 +196,7 @@ impl Plugin for LogPlugin {
             let subscriber = subscriber.with(tracy_layer);
 
             if let Some(update_subscriber) = self.update_subscriber {
-                finished_subscriber = update_subscriber(Box::new(subscriber));
+                finished_subscriber = update_subscriber(app, Box::new(subscriber));
             } else {
                 finished_subscriber = Box::new(subscriber);
             }

examples/README.md

@@ -172,6 +172,7 @@ Example | Description
 
 Example | Description
 --- | ---
+[Advanced log layers](../examples/app/log_layers_ecs.rs) | Illustrate how to transfer data between log layers and Bevy's ECS
 [Custom Loop](../examples/app/custom_loop.rs) | Demonstrates how to create a custom runner (to update an app manually)
 [Drag and Drop](../examples/app/drag_and_drop.rs) | An example that shows how to handle drag and drop in an app
 [Empty](../examples/app/empty.rs) | An empty application (does nothing)

examples/app/log_layers.rs

@@ -22,7 +22,9 @@ impl<S: Subscriber> Layer<S> for CustomLayer {
     }
 }
 
-fn update_subscriber(subscriber: BoxedSubscriber) -> BoxedSubscriber {
+// We don't need App for this example, as we are just printing log information.
+// For an example that uses App, see log_layers_ecs.
+fn update_subscriber(_: &mut App, subscriber: BoxedSubscriber) -> BoxedSubscriber {
     Box::new(subscriber.with(CustomLayer))
 }
 
@@ -30,6 +32,14 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
             update_subscriber: Some(update_subscriber),
+            // You can chain multiple subscriber updates like this:
+            //
+            // update_subscriber: Some(|app, subscriber| {
+            //     let subscriber = update_subscriber_a(app, subscriber);
+            //     let subscriber = update_subscriber_b(app, subscriber);
+            //
+            //     update_subscriber_c(app, subscriber)
+            // }),
             ..default()
         }))
         .add_systems(Update, log_system)
@@ -37,7 +47,7 @@ fn main() {
 }
 
 fn log_system() {
-    // here is how you write new logs at each "log level" (in "most import" to
+    // here is how you write new logs at each "log level" (in "most important" to
     // "least important" order)
     error!("something failed");
     warn!("something bad happened that isn't a failure, but thats worth calling out");

examples/app/log_layers_ecs.rs

@@ -0,0 +1,151 @@
+//! This example illustrates how to transfer log events from the [`Layer`] to Bevy's ECS.
+//!
+//! The way we will do this is via a [`mpsc`] channel. [`mpsc`] channels allow 2 unrelated
+//! parts of the program to communicate (in this case, [`Layer`]s and Bevy's ECS).
+//!
+//! Inside the [`update_subscriber`] function we will create a [`mpsc::Sender`] and a [`mpsc::Receiver`] from a
+//! [`mpsc::channel`]. The [`Sender`](mpsc::Sender) will go into the [`AdvancedLayer`] and the [`Receiver`](mpsc::Receiver) will
+//! go into a non-send resource called [`LogEvents`] (It has to be non-send because [`Receiver`](mpsc::Receiver) is [`!Sync`](Sync)).
+//! From there we will use [`transfer_log_events`] to transfer log events from [`LogEvents`] to an ECS event called [`LogEvent`].
+//!
+//! Finally, after all that we can access the [`LogEvent`] event from our systems and use it.
+//! In this example we build a simple log viewer.
+
+use std::sync::mpsc;
+
+use bevy::{
+    log::tracing_subscriber::{self, layer::SubscriberExt, Layer},
+    log::BoxedSubscriber,
+    prelude::*,
+    utils::tracing,
+    utils::tracing::Subscriber,
+};
+
+/// A basic message. This is what we will be sending from the [`CaptureLayer`] to [`CapturedLogEvents`] non-send resource.
+#[derive(Debug, Event)]
+struct LogEvent {
+    message: String,
+}
+
+/// This non-send resource temporarily stores [`LogEvent`]s before they are
+/// written to [`Events<LogEvent>`] by [`transfer_log_events`].
+#[derive(Deref, DerefMut)]
+struct CapturedLogEvents(mpsc::Receiver<LogEvent>);
+
+/// Transfers information from the [`LogEvents`] resource to [`Events<LogEvent>`](LogEvent).
+fn transfer_log_events(
+    reciever: NonSend<CapturedLogEvents>,
+    mut log_events: EventWriter<LogEvent>,
+) {
+    // Make sure to use `try_iter()` and not `iter()` to prevent blocking.
+    log_events.send_batch(reciever.try_iter());
+}
+
+/// This is the [`Layer`] that we will use to capture log events and then send them to Bevy's
+/// ECS via it's [`mpsc::Sender`].
+struct CaptureLayer {
+    sender: mpsc::Sender<LogEvent>,
+}
+impl<S: Subscriber> Layer<S> for CaptureLayer {
+    fn on_event(
+        &self,
+        event: &tracing::Event<'_>,
+        _ctx: tracing_subscriber::layer::Context<'_, S>,
+    ) {
+        // In order to obtain the log message, we have to create a struct that implements
+        // Visit and holds a reference to our string. Then we use the `record` method and
+        // the struct to modify the reference to hold the message string.
+        let mut message = None;
+        event.record(&mut CaptureLayerVisitor(&mut message));
+        if let Some(message) = message {
+            // You can obtain metadata like this, but we wont use it for this example.
+            let _metadata = event.metadata();
+            self.sender
+                .send(LogEvent { message })
+                .expect("LogEvents resource no longer exists!");
+        }
+    }
+}
+
+/// A [`Visit`](tracing::field::Visit)or that records log messages that are transfered to [`CaptureLayer`].
+struct CaptureLayerVisitor<'a>(&'a mut Option<String>);
+impl tracing::field::Visit for CaptureLayerVisitor<'_> {
+    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
+        // This if statement filters out unneeded events sometimes show up
+        if field.name() == "message" {
+            *self.0 = Some(format!("{value:?}"));
+        }
+    }
+}
+fn update_subscriber(app: &mut App, subscriber: BoxedSubscriber) -> BoxedSubscriber {
+    let (sender, reciever) = mpsc::channel();
+
+    let layer = CaptureLayer { sender };
+    let resource = CapturedLogEvents(reciever);
+
+    app.insert_non_send_resource(resource);
+    app.add_event::<LogEvent>();
+    app.add_systems(Update, transfer_log_events);
+
+    Box::new(subscriber.with(layer))
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
+            update_subscriber: Some(update_subscriber),
+            ..default()
+        }))
+        .add_systems(Startup, (log_system, setup))
+        .add_systems(Update, print_logs)
+        .run();
+}
+
+fn log_system() {
+    // here is how you write new logs at each "log level" (in "most important" to
+    // "least important" order)
+    error!("something failed");
+    warn!("something bad happened that isn't a failure, but thats worth calling out");
+    info!("helpful information that is worth printing by default");
+    debug!("helpful for debugging");
+    trace!("very noisy");
+}
+
+#[derive(Component)]
+struct LogViewerRoot;
+
+fn setup(mut commands: Commands) {
+    commands.spawn(Camera2dBundle::default());
+
+    commands.spawn((
+        NodeBundle {
+            style: Style {
+                width: Val::Vw(100.0),
+                height: Val::Vh(100.0),
+                flex_direction: FlexDirection::Column,
+                ..default()
+            },
+            ..default()
+        },
+        LogViewerRoot,
+    ));
+}
+
+// This is how we can read our LogEvents.
+// In this example we are reading the LogEvents and inserting them as text into our log viewer.
+fn print_logs(
+    mut events: EventReader<LogEvent>,
+    mut commands: Commands,
+    log_viewer_root: Query<Entity, With<LogViewerRoot>>,
+) {
+    let root_entity = log_viewer_root.single();
+
+    commands.entity(root_entity).with_children(|child| {
+        for event in events.read() {
+            child.spawn(TextBundle::from_section(
+                &event.message,
+                TextStyle::default(),
+            ));
+        }
+    });
+}