pub trait MakeWriter<'a> {
type Writer: Write;
// Required method
fn make_writer(&'a self) -> Self::Writer;
// Provided method
fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer { ... }
}Expand description
A type that can create io::Write instances.
MakeWriter is used by fmt::Layer or fmt::Subscriber to print
formatted text representations of Events.
This trait is already implemented for function pointers and
immutably-borrowing closures that return an instance of io::Write, such
as io::stdout and io::stderr. Additionally, it is implemented for
std::sync::Mutex when the type inside the mutex implements
io::Write.
§Examples
The simplest usage is to pass in a named function that returns a writer. For example, to log all events to stderr, we could write:
let subscriber = tracing_subscriber::fmt()
.with_writer(std::io::stderr)
.finish();Any function that returns a writer can be used:
fn make_my_great_writer() -> impl std::io::Write {
// ...
}
let subscriber = tracing_subscriber::fmt()
.with_writer(make_my_great_writer)
.finish();A closure can be used to introduce arbitrary logic into how the writer is created. Consider the (admittedly rather silly) example of sending every 5th event to stderr, and all other events to stdout:
use std::io;
use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
let n = AtomicUsize::new(0);
let subscriber = tracing_subscriber::fmt()
.with_writer(move || -> Box<dyn io::Write> {
if n.fetch_add(1, Relaxed) % 5 == 0 {
Box::new(io::stderr())
} else {
Box::new(io::stdout())
}
})
.finish();A single instance of a type implementing io::Write may be used as a
MakeWriter by wrapping it in a Mutex. For example, we could
write to a file like so:
use std::{fs::File, sync::Mutex};
let log_file = File::create("my_cool_trace.log")?;
let subscriber = tracing_subscriber::fmt()
.with_writer(Mutex::new(log_file))
.finish();Required Associated Types§
Sourcetype Writer: Write
type Writer: Write
The concrete io::Write implementation returned by make_writer.
Required Methods§
Sourcefn make_writer(&'a self) -> Self::Writer
fn make_writer(&'a self) -> Self::Writer
Returns an instance of Writer.
§Implementer notes
fmt::Layer or fmt::Subscriber will call this method each time an event is recorded. Ensure any state
that must be saved across writes is not lost when the Writer instance is dropped. If
creating a io::Write instance is expensive, be sure to cache it when implementing
MakeWriter to improve performance.
Provided Methods§
Sourcefn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer
fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer
Returns a Writer for writing data from the span or event described
by the provided Metadata.
By default, this calls self.make_writer(), ignoring
the provided metadata, but implementations can override this to provide
metadata-specific behaviors.
This method allows MakeWriter implementations to implement different
behaviors based on the span or event being written. The MakeWriter
type might return different writers based on the provided metadata, or
might write some values to the writer before or after providing it to
the caller.
For example, we might want to write data from spans and events at the
ERROR and WARN levels to stderr, and data from spans or events
at lower levels to stdout:
use std::io::{self, Stdout, Stderr, StdoutLock, StderrLock};
use tracing_subscriber::fmt::writer::MakeWriter;
use tracing_core::{Metadata, Level};
pub struct MyMakeWriter {
stdout: Stdout,
stderr: Stderr,
}
/// A lock on either stdout or stderr, depending on the verbosity level
/// of the event being written.
pub enum StdioLock<'a> {
Stdout(StdoutLock<'a>),
Stderr(StderrLock<'a>),
}
impl<'a> io::Write for StdioLock<'a> {
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
match self {
StdioLock::Stdout(lock) => lock.write(buf),
StdioLock::Stderr(lock) => lock.write(buf),
}
}
fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
// ...
}
fn flush(&mut self) -> io::Result<()> {
// ...
}
}
impl<'a> MakeWriter<'a> for MyMakeWriter {
type Writer = StdioLock<'a>;
fn make_writer(&'a self) -> Self::Writer {
// We must have an implementation of `make_writer` that makes
// a "default" writer without any configuring metadata. Let's
// just return stdout in that case.
StdioLock::Stdout(self.stdout.lock())
}
fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
// Here's where we can implement our special behavior. We'll
// check if the metadata's verbosity level is WARN or ERROR,
// and return stderr in that case.
if meta.level() <= &Level::WARN {
return StdioLock::Stderr(self.stderr.lock());
}
// Otherwise, we'll return stdout.
StdioLock::Stdout(self.stdout.lock())
}
}