Add example with custom field

examples/custom-from-event-or-span.rs

@@ -0,0 +1,78 @@
+use std::fmt::Debug;
+
+use tracing::{
+    field::Visit,
+    span::{Attributes, Id, Record},
+    Subscriber,
+};
+use tracing_core::Field;
+use tracing_subscriber::{
+    layer::{Context, SubscriberExt},
+    registry::LookupSpan,
+    util::SubscriberInitExt,
+    Layer,
+    Registry,
+};
+
+fn main() {
+    let mut json_layer = json_subscriber::layer();
+    json_layer
+        .inner_layer_mut()
+        .add_dynamic_field("app_id", |event, ctx| {
+            let mut app_id = None;
+            event.record(&mut AppIdVisitor(&mut app_id));
+            if let Some(app_id) = app_id {
+                return Some(app_id);
+            }
+
+            for span in ctx.event_scope(event)? {
+                if let Some(app_id) = span.extensions().get::<AppId>() {
+                    return Some(app_id.0.clone());
+                }
+            }
+
+            None
+        });
+
+    Registry::default().with(json_layer).with(AppIdLayer).init();
+
+    tracing::info!("Normal log without app_id.");
+    tracing::info!(app_id = 7, "Log with app_id.");
+    let _span = tracing::info_span!("operation", app_id = "from_span").entered();
+    tracing::info!("Log with inherited app_id.");
+    tracing::info!(app_id = "from_event", "Log with overridden app_id.");
+}
+
+struct AppId(String);
+
+struct AppIdLayer;
+
+impl<S: Subscriber + for<'lookup> LookupSpan<'lookup>> Layer<S> for AppIdLayer {
+    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
+        let mut app_id = None;
+        attrs.record(&mut AppIdVisitor(&mut app_id));
+        if let Some(app_id) = app_id {
+            let span = ctx.span(id).expect("This span was just created.");
+            span.extensions_mut().insert(AppId(app_id));
+        }
+    }
+
+    fn on_record(&self, id: &Id, values: &Record<'_>, ctx: Context<'_, S>) {
+        let mut app_id = None;
+        values.record(&mut AppIdVisitor(&mut app_id));
+        if let Some(app_id) = app_id {
+            let span = ctx.span(id).expect("This span was just created.");
+            span.extensions_mut().replace(AppId(app_id));
+        }
+    }
+}
+
+struct AppIdVisitor<'a>(&'a mut Option<String>);
+
+impl<'a> Visit for AppIdVisitor<'a> {
+    fn record_debug(&mut self, field: &Field, value: &dyn Debug) {
+        if field.name() == "app_id" {
+            *self.0 = Some(format!("{value:?}"));
+        }
+    }
+}