What is @parcel/watcher?
The @parcel/watcher package is a file system watcher that provides an efficient way to subscribe to changes in the file system. It is designed to be fast and works across multiple platforms. It can be used to watch for changes in files and directories and perform actions in response to those changes.
What are @parcel/watcher's main functionalities?
Subscribing to file changes
This feature allows you to subscribe to changes in a specific directory. The callback function is called with an array of events whenever a change occurs. Each event contains the type of change (e.g., 'create', 'update', 'delete') and the path to the affected file or directory.
const {subscribe} = require('@parcel/watcher');
async function run() {
await subscribe('/path/to/directory', (err, events) => {
if (err) {
// Handle the error
return;
}
// Handle file system change events
events.forEach(event => {
console.log(`Type: ${event.type}, Path: ${event.path}`);
});
});
}
run();
Unsubscribing from file changes
This feature allows you to unsubscribe from a previously established subscription. This is useful when you no longer need to watch for changes or when your application is shutting down.
const {subscribe, unsubscribe} = require('@parcel/watcher');
let subscription;
async function run() {
subscription = await subscribe('/path/to/directory', (err, events) => {
// Handle file system change events
});
}
async function stop() {
await unsubscribe(subscription);
}
run();
// Later on, when you want to stop watching
stop();
Other packages similar to @parcel/watcher
chokidar
Chokidar is a popular file watching package that provides a high-level API to watch file system changes. It is known for its stability and compatibility with multiple platforms. Chokidar is often used because of its extensive feature set and ease of use, but it may be slower than @parcel/watcher for large directories or projects.
watchpack
Watchpack is a wrapper around the file-watching functionality of Node.js and other file-watching libraries. It is used by webpack to detect changes in files. Watchpack offers a good balance of features and performance, but @parcel/watcher might provide better performance in certain scenarios due to its focus on speed and efficiency.
node-watch
Node-watch is a simple and lightweight file watching library. It is easy to use and works across different platforms. While it may not have as many features as @parcel/watcher, it is suitable for simpler use cases where a straightforward file watching solution is needed.
@parcel/watcher
A native C++ Node module for querying and subscribing to filesystem events. Used by Parcel 2.
Features
- Watch - subscribe to realtime recursive directory change notifications when files or directories are created, updated, or deleted.
- Query - performantly query for historical change events in a directory, even when your program is not running.
- Native - implemented in C++ for performance and low-level integration with the operating system.
- Cross platform - includes backends for macOS, Linux, Windows, and Watchman.
- Performant - events are throttled in C++ so the JavaScript thread is not overwhelmed during large filesystem changes (e.g.
git checkout
or npm install
). - Scalable - tens of thousands of files can be watched or queried at once with good performance.
Example
const watcher = require('@parcel/watcher');
const path = require('path');
let subscription = await watcher.subscribe(process.cwd(), (err, events) => {
console.log(events);
});
await subscription.unsubscribe();
let snapshotPath = path.join(process.cwd(), 'snapshot.txt');
let events = await watcher.getEventsSince(process.cwd(), snapshotPath);
await watcher.writeSnapshot(process.cwd(), snapshotPath);
Watching
@parcel/watcher
supports subscribing to realtime notifications of changes in a directory. It works recursively, so changes in sub-directories will also be emitted.
Events are throttled and coalesced for performance during large changes like git checkout
or npm install
, and a single notification will be emitted with all of the events at the end.
Only one notification will be emitted per file. For example, if a file was both created and updated since the last event, you'll get only a create
event. If a file is both created and deleted, you will not be notifed of that file. Renames cause two events: a delete
for the old name, and a create
for the new name.
let subscription = await watcher.subscribe(process.cwd(), (err, events) => {
console.log(events);
});
Events have two properties:
type
- the event type: create
, update
, or delete
.path
- the absolute realpath to the file.
To unsubscribe from change notifications, call the unsubscribe
method on the returned subscription object.
await subscription.unsubscribe();
@parcel/watcher
has the following watcher backends, listed in priority order:
You can specify the exact backend you wish to use by passing the backend
option. If that backend is not available on the current platform, the default backend will be used instead. See below for the list of backend names that can be passed to the options.
Querying
@parcel/watcher
also supports querying for historical changes made in a directory, even when your program is not running. This makes it easy to invalidate a cache and re-build only the files that have changed, for example. It can be significantly faster than traversing the entire filesystem to determine what files changed, depending on the platform.
In order to query for historical changes, you first need a previous snapshot to compare to. This can be saved to a file with the writeSnapshot
function, e.g. just before your program exits.
await watcher.writeSnapshot(dirPath, snapshotPath);
When your program starts up, you can query for changes that have occurred since that snapshot using the getEventsSince
function.
let events = await watcher.getEventsSince(dirPath, snapshotPath);
The events returned are exactly the same as the events that would be passed to the subscribe
callback (see above).
@parcel/watcher
has the following watcher backends, listed in priority order:
The FSEvents (macOS) and Watchman backends are significantly more performant than the brute force backends used by default on Linux and Windows, for example returning results in miliseconds instead of seconds for large directory trees. This is because a background daemon monitoring filesystem changes on those platforms allows us to query cached data rather than traversing the filesystem manually (brute force).
macOS has good performance with FSEvents by default. For the best performance on other platforms, install Watchman and it will be used by @parcel/watcher
automatically.
You can specify the exact backend you wish to use by passing the backend
option. If that backend is not available on the current platform, the default backend will be used instead. See below for the list of backend names that can be passed to the options.
Options
All of the APIs in @parcel/watcher
support the following options, which are passed as an object as the last function argument.
ignore
- an array of paths to ignore. They can be either files or directories. No events will be emitted about these files or directories or their children.backend
- the name of an explicitly chosen backend to use. Allowed options are "fs-events"
, "watchman"
, "inotify"
, "windows"
, or "brute-force"
(only for querying). If the specified backend is not available on the current platform, the default backend will be used instead.
License
MIT