Trait tracing_subscriber::field::Visit
source · pub trait Visit {
// Required method
fn record_debug(&mut self, field: &Field, value: &dyn Debug);
// Provided methods
fn record_f64(&mut self, field: &Field, value: f64) { ... }
fn record_i64(&mut self, field: &Field, value: i64) { ... }
fn record_u64(&mut self, field: &Field, value: u64) { ... }
fn record_i128(&mut self, field: &Field, value: i128) { ... }
fn record_u128(&mut self, field: &Field, value: u128) { ... }
fn record_bool(&mut self, field: &Field, value: bool) { ... }
fn record_str(&mut self, field: &Field, value: &str) { ... }
fn record_error(&mut self, field: &Field, value: &(dyn Error + 'static)) { ... }
}Expand description
Visits typed values.
An instance of Visit (“a visitor”) represents the logic necessary to
record field values of various types. When an implementor of Value is
recorded, it calls the appropriate method on the provided visitor to
indicate the type that value should be recorded as.
When a Subscriber implementation records an Event or a
set of Values added to a Span, it can pass an &mut Visit to the
record method on the provided ValueSet or Event. This visitor
will then be used to record all the field-value pairs present on that
Event or ValueSet.
§Examples
A simple visitor that writes to a string might be implemented like so:
use std::fmt::{self, Write};
use tracing::field::{Value, Visit, Field};
pub struct StringVisitor<'a> {
string: &'a mut String,
}
impl<'a> Visit for StringVisitor<'a> {
fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
write!(self.string, "{} = {:?}; ", field.name(), value).unwrap();
}
}This visitor will format each recorded value using fmt::Debug, and
append the field name and formatted value to the provided string,
regardless of the type of the recorded value. When all the values have
been recorded, the StringVisitor may be dropped, allowing the string
to be printed or stored in some other data structure.
The Visit trait provides default implementations for record_i64,
record_u64, record_bool, record_str, and record_error, which simply
forward the recorded value to record_debug. Thus, record_debug is the
only method which a Visit implementation must implement. However,
visitors may override the default implementations of these functions in
order to implement type-specific behavior.
Additionally, when a visitor receives a value of a type it does not care about, it is free to ignore those values completely. For example, a visitor which only records numeric data might look like this:
pub struct SumVisitor {
sum: i64,
}
impl Visit for SumVisitor {
fn record_i64(&mut self, _field: &Field, value: i64) {
self.sum += value;
}
fn record_u64(&mut self, _field: &Field, value: u64) {
self.sum += value as i64;
}
fn record_debug(&mut self, _field: &Field, _value: &fmt::Debug) {
// Do nothing
}
}This visitor (which is probably not particularly useful) keeps a running
sum of all the numeric values it records, and ignores all other values. A
more practical example of recording typed values is presented in
examples/counters.rs, which demonstrates a very simple metrics system
implemented using tracing.
Note: Therecord_errortrait method is only available when the Rust standard library is present, as it requires thestd::error::Errortrait.
Required Methods§
sourcefn record_debug(&mut self, field: &Field, value: &dyn Debug)
fn record_debug(&mut self, field: &Field, value: &dyn Debug)
Visit a value implementing fmt::Debug.
Provided Methods§
sourcefn record_f64(&mut self, field: &Field, value: f64)
fn record_f64(&mut self, field: &Field, value: f64)
Visit a double-precision floating point value.
sourcefn record_i64(&mut self, field: &Field, value: i64)
fn record_i64(&mut self, field: &Field, value: i64)
Visit a signed 64-bit integer value.
sourcefn record_u64(&mut self, field: &Field, value: u64)
fn record_u64(&mut self, field: &Field, value: u64)
Visit an unsigned 64-bit integer value.
sourcefn record_i128(&mut self, field: &Field, value: i128)
fn record_i128(&mut self, field: &Field, value: i128)
Visit a signed 128-bit integer value.
sourcefn record_u128(&mut self, field: &Field, value: u128)
fn record_u128(&mut self, field: &Field, value: u128)
Visit an unsigned 128-bit integer value.
sourcefn record_bool(&mut self, field: &Field, value: bool)
fn record_bool(&mut self, field: &Field, value: bool)
Visit a boolean value.
sourcefn record_str(&mut self, field: &Field, value: &str)
fn record_str(&mut self, field: &Field, value: &str)
Visit a string value.
sourcefn record_error(&mut self, field: &Field, value: &(dyn Error + 'static))
fn record_error(&mut self, field: &Field, value: &(dyn Error + 'static))
Records a type implementing Error.
Note: This is only enabled when the Rust standard library is present.