feat(docs): add tracing example (#1192)

Add an example that demonstrates logging to a file for:

<https://forum.ratatui.rs/t/how-do-you-println-debug-your-tui-programs/66>

```shell
cargo run --example tracing
RUST_LOG=trace cargo run --example=tracing
cat tracing.log
```

![Made with VHS](https://vhs.charm.sh/vhs-21jgJCedh2YnFDONw0JW7l.gif)

Cargo.toml

@@ -61,6 +61,9 @@ rand = "0.8.5"
 rand_chacha = "0.3.1"
 rstest = "0.21.0"
 serde_json = "1.0.109"
+tracing = "0.1.40"
+tracing-appender = "0.2.3"
+tracing-subscriber = { version = "0.3.18", features = ["env-filter"] }
 
 [lints.rust]
 unsafe_code = "forbid"
@@ -353,6 +356,11 @@ name = "tabs"
 required-features = ["crossterm"]
 doc-scrape-examples = true
 
+[[example]]
+name = "tracing"
+required-features = ["crossterm"]
+doc-scrape-examples = true
+
 [[example]]
 name = "user_input"
 required-features = ["crossterm"]

examples/tracing.rs

@@ -0,0 +1,154 @@
+//! # [Ratatui] Tracing example
+//!
+//! The latest version of this example is available in the [examples] folder in the repository.
+//!
+//! Please note that the examples are designed to be run against the `main` branch of the Github
+//! repository. This means that you may not be able to compile with the latest release version on
+//! crates.io, or the one that you have installed locally.
+//!
+//! See the [examples readme] for more information on finding examples that match the version of the
+//! library you are using.
+//!
+//! [Ratatui]: https://github.com/ratatui-org/ratatui
+//! [examples]: https://github.com/ratatui-org/ratatui/blob/main/examples
+//! [examples readme]: https://github.com/ratatui-org/ratatui/blob/main/examples/README.md
+
+// A simple example demonstrating how to use the [tracing] with Ratatui to log to a file.
+//
+// This example demonstrates how to use the [tracing] crate with Ratatui to log to a file. The
+// example sets up a simple logger that logs to a file named `tracing.log` in the current directory.
+//
+// Run the example with `cargo run --example tracing` and then view the `tracing.log` file to see
+// the logs. To see more logs, you can run the example with `RUST_LOG=tracing=debug cargo run
+// --example`
+//
+// For a helpful widget that handles logging, see the [tui-logger] crate.
+//
+// [tracing]: https://crates.io/crates/tracing
+// [tui-logger]: https://crates.io/crates/tui-logger
+
+use std::{fs::File, io::stdout, panic, time::Duration};
+
+use color_eyre::{
+    config::HookBuilder,
+    eyre::{self, Context},
+    Result,
+};
+use crossterm::{
+    event::{self, Event, KeyCode},
+    terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
+    ExecutableCommand,
+};
+use ratatui::{
+    backend::{Backend, CrosstermBackend},
+    terminal::Terminal,
+    widgets::{Block, Paragraph},
+};
+use tracing::{debug, info, instrument, trace, Level};
+use tracing_appender::{non_blocking, non_blocking::WorkerGuard};
+use tracing_subscriber::EnvFilter;
+
+fn main() -> Result<()> {
+    init_error_hooks()?;
+    let _guard = init_tracing()?;
+    info!("Starting tracing example");
+
+    let mut terminal = init_terminal()?;
+    let mut events = vec![]; // a buffer to store the recent events to display in the UI
+    while !should_exit(&events) {
+        handle_events(&mut events)?;
+        terminal.draw(|frame| ui(frame, &events))?;
+    }
+    restore_terminal()?;
+    info!("Exiting tracing example");
+    println!("See the tracing.log file for the logs");
+    Ok(())
+}
+
+fn should_exit(events: &[Event]) -> bool {
+    events
+        .iter()
+        .any(|event| matches!(event, Event::Key(key) if key.code == KeyCode::Char('q')))
+}
+
+/// Handle events and insert them into the events vector keeping only the last 10 events
+#[instrument(skip(events))]
+fn handle_events(events: &mut Vec<Event>) -> Result<()> {
+    // Render the UI at least once every 100ms
+    if event::poll(Duration::from_millis(100))? {
+        let event = event::read()?;
+        debug!(?event);
+        events.insert(0, event);
+    }
+    events.truncate(10);
+    Ok(())
+}
+
+#[instrument(skip_all)]
+fn ui(frame: &mut ratatui::Frame, events: &[Event]) {
+    // To view this event, run the example with `RUST_LOG=tracing=debug cargo run --example tracing`
+    trace!(frame_count = frame.count(), event_count = events.len());
+    let area = frame.size();
+    let events = events.iter().map(|e| format!("{e:?}")).collect::<Vec<_>>();
+    let paragraph = Paragraph::new(events.join("\n"))
+        .block(Block::bordered().title("Tracing example. Press 'q' to quit."));
+    frame.render_widget(paragraph, area);
+}
+
+/// Initialize the tracing subscriber to log to a file
+///
+/// This function initializes the tracing subscriber to log to a file named `tracing.log` in the
+/// current directory. The function returns a [`WorkerGuard`] that must be kept alive for the
+/// duration of the program to ensure that logs are flushed to the file on shutdown. The logs are
+/// written in a non-blocking fashion to ensure that the logs do not block the main thread.
+fn init_tracing() -> Result<WorkerGuard> {
+    let file = File::create("tracing.log").wrap_err("failed to create tracing.log")?;
+    let (non_blocking, guard) = non_blocking(file);
+
+    // By default, the subscriber is configured to log all events with a level of `DEBUG` or higher,
+    // but this can be changed by setting the `RUST_LOG` environment variable.
+    let env_filter = EnvFilter::builder()
+        .with_default_directive(Level::DEBUG.into())
+        .from_env_lossy();
+
+    tracing_subscriber::fmt()
+        .with_writer(non_blocking)
+        .with_env_filter(env_filter)
+        .init();
+    Ok(guard)
+}
+
+/// Initialize the error hooks to ensure that the terminal is restored to a sane state before
+/// exiting
+fn init_error_hooks() -> Result<()> {
+    let (panic, error) = HookBuilder::default().into_hooks();
+    let panic = panic.into_panic_hook();
+    let error = error.into_eyre_hook();
+    eyre::set_hook(Box::new(move |e| {
+        let _ = restore_terminal();
+        error(e)
+    }))?;
+    panic::set_hook(Box::new(move |info| {
+        let _ = restore_terminal();
+        panic(info);
+    }));
+    Ok(())
+}
+
+#[instrument]
+fn init_terminal() -> Result<Terminal<impl Backend>> {
+    enable_raw_mode()?;
+    stdout().execute(EnterAlternateScreen)?;
+    let backend = CrosstermBackend::new(stdout());
+    let terminal = Terminal::new(backend)?;
+    debug!("terminal initialized");
+    Ok(terminal)
+}
+
+#[instrument]
+fn restore_terminal() -> Result<()> {
+    disable_raw_mode()?;
+    stdout().execute(LeaveAlternateScreen)?;
+    debug!("terminal restored");
+    Ok(())
+}

examples/vhs/tracing.tape

@@ -0,0 +1,12 @@
+# This is a vhs script. See https://github.com/charmbracelet/vhs for more info.
+# To run this script, install vhs and run `vhs ./examples/barchart.tape`
+Output "target/tracing.gif"
+Set Theme "Aardvark Blue"
+Set Width 1200
+Set Height 800
+Type "RUST_LOG=trace cargo run --example=tracing" Enter
+Sleep 1s
+Type @100ms "jjjjq"
+Sleep 1s
+Type "cat tracing.log" Enter
+Sleep 10s