how to get field value #3215

kongbai1996 · 2025-02-10T13:48:13Z

I can get the field name by Field: 8000 :name method, but how to get field value?

tglane · 2025-02-10T18:38:45Z

I had the same issue today and found a workaround by using a custom visitor that implements tracing::field::Visit and passed that to tracing::Event::record(&self, visitor: &mut dyn Visit). Maybe that's useful for you too.

But I wonder if there is a good reason why there is no function like Field::value? If this would be something desirable I could take a look at this issue?

kongbai1996 · 2025-02-11T02:40:09Z

I had the same issue today and found a workaround by using a custom visitor that implements tracing::field::Visit and passed that to tracing::Event::record(&self, visitor: &mut dyn Visit). Maybe that's useful for you too.

How to use it?

tglane · 2025-02-11T19:56:22Z

Suppose you want to read the field value from an tracing::Event as an example. Then you could do the following to read the field value from a field with name EXAMPLE into a string:

#[derive(Default)]
struct ExampleVisitor(pub Option<String>);

impl tracing::field::Visit for ExampleVisitor {
    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
        // Mandatory to implement
    }
    
    fn record_str(&mut self, field: &tracing::field::Field, value: &str) {
        if field.name() == "EXAMPLE" {
            // Extract the field value if the name matches our searched field name
            self.0 = Some(value.to_string());
        }
    }
}

struct MyLoggingLayer;

impl<S: tracing::Subscriber> tracing_subscriber::layer::Layer<S> for MyLoggingLayer {
    fn on_event(&self, event: &tracing_core::event::Event<'_>, _ctx: tracing_subscriber::layer::Context<'_, S>) {
        // Listen for logging events in this logging layer
        // Visit the records/fields of the event with out custom visitor
        let my_visitor = ExampleVisitor::default();
        event.record(&mut my_visitor);
        
        if let Some(field_val) = my_visitor.0 {
            println!("Found {} at the field value EXAMPLE", field_val);
        }
    }
}

The idea here is that the event allows you to pass a custom struct that implements tracing::field::Visit to all the internal fields. Here you can use the record_<type you are interested in> function of the trait to visit the types you are interested in and do what ever you want with it after visiting them within Event::record. The same procedure can be applied in the other event handler function provided by tracing_subscriber::layer::Layer. I have to admit that is feels a bit clumsy but it works for me and is the best I could come up with.

I hope this helps you.

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