typ_buffer/
watch.rs

1//! Notice when a file changes on disk.
2//!
3//! A rebase, a formatter, or another editor writes the file while it is open.
4//! Without this the editor neither reloads nor warns, and the next save
5//! silently overwrites whatever the other writer did.
6
7use std::path::{Path, PathBuf};
8
9use anyhow::{Context, Result};
10use notify::{RecommendedWatcher, RecursiveMode, Watcher};
11
12/// A live watch. Dropping it stops the watching, which is how opening another
13/// file replaces the old watch rather than accumulating them.
14pub struct FileWatch {
15    _watcher: RecommendedWatcher,
16}
17
18/// Report changes to `path` by calling `on_change` from the watcher's thread.
19///
20/// **Watches the parent directory, not the file.** Editors and formatters write
21/// by rename-over, which destroys the inode a file watch is pinned to and
22/// leaves that watch pointed at nothing — the file keeps changing and the
23/// watcher keeps saying nothing. Watching the directory and filtering by name
24/// survives it, and also sees the file being deleted and recreated.
25///
26/// `on_change` is handed the path as it was given here, not the path the OS
27/// reported, so a caller can compare it against what it has open without
28/// worrying about how each platform spells it.
29pub fn watch_file(path: &Path, on_change: impl Fn(PathBuf) + Send + 'static) -> Result<FileWatch> {
30    let path = path.to_path_buf();
31    let dir = path
32        .parent()
33        .filter(|p| !p.as_os_str().is_empty())
34        .map(Path::to_path_buf)
35        .unwrap_or_else(|| PathBuf::from("."));
36    let name = path
37        .file_name()
38        .context("watching a path with no file name")?
39        .to_os_string();
40
41    let reported = path.clone();
42    let mut watcher = notify::recommended_watcher(move |event: notify::Result<notify::Event>| {
43        let Ok(event) = event else { return };
44        // Access events fire on every read, including our own. Only creation,
45        // modification and removal change what is on disk.
46        if !(event.kind.is_create() || event.kind.is_modify() || event.kind.is_remove()) {
47            return;
48        }
49        if event.paths.iter().any(|p| p.file_name() == Some(&name)) {
50            on_change(reported.clone());
51        }
52    })
53    .context("creating a file watcher")?;
54
55    // ponytail: no debouncing. One save produces several events on every
56    // platform, and the handler on the other end is idempotent — it compares
57    // the file against the buffer and does nothing when they agree. A
58    // debouncer earns its place when an event costs more than that comparison.
59    watcher
60        .watch(&dir, RecursiveMode::NonRecursive)
61        .with_context(|| format!("watching {}", dir.display()))?;
62
63    Ok(FileWatch { _watcher: watcher })
64}
typ_buffer/watch.rs

typ_buffer/
watch.rs
