globwatcher

watch a set of files for changes (including create/delete) by glob patterns

npm install globwatcher
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globwatcher

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Globwatcher is a node library that monitors a filesystem for changes to files specified by glob patterns, and triggers events when a file is added, changed, or deleted. It's built on top of node's fs.watch mechanism, and uses the glob pattern syntax of the glob module.

Sample usage:

var globwatcher = require("globwatcher").globwatcher;
var watcher = globwatcher("/albums/**/*.mp3");
watcher.on("added", function (filename) {
  console.log("New MP3 detected: " + filename);
});
watcher.ready.then(function () {
  console.log("Globwatcher is now actively scanning!");
});

API

globwatcher(patterns, options)

Create a new GlobWatcher object, which will monitor for files that match the given patterns (or a single pattern), and trigger events. The options object can contain:

  • cwd - folder to use for relative patterns. If not given, the process's current working directory is used. Absolute patterns (patterns that start with "/") don't use this option.

  • interval - frequency (in milliseconds) to monitor existing files for changes. Default is 250, or 1/4 second.

  • debounceInterval - delay (in milliseconds) to wait after receiving a "folder has changed" notification before scanning the folder. This helps alleviate thundering-herd problems where many files may be created or deleted in a short period. Default is 10 milliseconds.

  • snapshot - previous state to resume from, as captured with snapshot() (see "Snapshots" below).

  • persistent - if false, unref the watches so they don't keep node running. Default is false.

  • debug - a function to call to log debug info while running. This function will be called with a string to log whenever certain changes or events occur, so it can be noisy, but may be useful for general debugging.

Useful fields:

  • ready - a promise that will be fulfilled once all the watches have been created and the initial scan is complete. This can be used to run code that depends on globwatcher being active.

  • originalPatterns - the original (non-normalized) patterns used to create this globwatcher, including any added with add().

Useful methods:

  • add(patterns...) - Add new glob patterns to be scanned. This will reset the ready promise (described above) so that it's fulfilled only when this new set of patterns are active.

  • close() - Stop monitoring and free all resources. No new events will be sent after this call returns, and all "watch" resources will be freed.

  • stopWatches() - Stop monitoring temporarily. Pending events may still arrive after this call returns, but no new events will be triggered, and all "watch" resources will be freed. The watched patterns will be remembered, so you can call startWatches() again to resume monitoring.

  • startWatches() - Resume monitoring after a stopWatches() call. You don't need to call this function when first monitoring -- it's called by the constructor automatically.

  • check() - Immediately scan interesting folders and files and trigger events on changes. You don't need to call this function normally, but if you believe files have changed, and want to bypass the normal scan interval, this call may speed up the latency between a filesystem event and the event signalled by globwatcher. Returns a promise that is fulfilled when the check is finished.

  • currentSet() - Return the set of filenames that currently exist and match the glob pattern being scanned. The filenames are all absolute.

  • snapshot() - Return an object representing the current state of watched folders and files. (See "Snapshots" below.)

Events signalled:

  • added(filename) - a new file that matches one of the watched patterns was created

  • deleted(filename) - a file that matches one of the watched patterns was deleted

  • changed(filename) - a file that matches one of the watched patterns was changed, either by modification time or size

The filename argument to events is always an absolute filename.

new FileWatcher(options)

The FileWatcher class is a pure-js replacement for the fs.watchFile interface, with an extra check method. It's primarily an implementation detail of globwatcher, but feel free to use it directly.

A FileWatcher calls fs.stat on a set of files, on a recurring timer. If any of the files has a changed modification time or size, a "changed" event is signalled. File creation/deletion is ignored.

The options object can contain:

  • period - the period of the timer. Default is 250, or 1/4 second.

  • persistent - if false, unref the timer so that it doesn't keep node running. Default is true.

Useful methods:

  • close() - Stop the timer and clear the list of filenames being watched.

  • watch(filename) - Add this filename to the list being watched, if it isn't already there. Returns the watch object for this filename.

  • unwatch(filename) - Remove this filename from the list being watched, if it's there.

  • watchFor(filename) - Return the watch object for a filename, if it's in the list of filenames benig watched. If not, return undefined.

  • check() - Immediately scan every filename currently being watched, and trigger events on changes. This has the same purpose as the "check" function on globwatcher.

The "watch" object returned by watch and watchFor has one function and one event:

  • check() - Immediately scan this filename for changes.

  • changed - the event signalled when a file has changed

How it works

node (as of v0.10) provides two ways to "watch" files/folders for changes:

  • fs.watchFile, which calls fs.stat on the file at regular intervals
  • fs.watch, which tries to take advantage of OS-level file watching system calls

Of the two, watch seems most obviously the best, but is the least portable. Linux has the best support with inotify, but OS X still uses kqueue, and can only notify when the contents of a folder have changed. Nested sub-folders aren't monitored either.

So globwatch uses the minimatch library to parse the globs, and sets OS-level folder watches on any interesting folders that exist. If a named folder doesn't exist, it walks up the tree until it finds one that does exist, and watches that, looking for the subtree to be created.

OS-level watches are only placed on folders, since they're the only ones guaranteed to work, and they only trigger a re-scan of the folder contents. Any matching filenames are watched with file-level watches, which are implemented in FileWatcher.

Snapshots

The current state of a globwatcher can be captured with snapshot(), which returns an object containing metadata about files that currently match. This lets you "resume" a session later, potentially in a new process or at a much later date.

To resume watching, pass the snapshot object as a snapshot option into a new globwatcher:

var watcher = globwatcher("/albums/**/*.mp3");
var snapshot = watcher.snapshot();
// ... time passes ...
val newWatcher = globwatcher("/albums/**/*.mp3", { "snapshot": snapshot });

When restoring from a snapshot, globwatcher will wait for the debounce interval to pass, then scan the filesystem and trigger events for any files that have changed since the snapshot. (Naturally, this may have odd results if your match patterns have changed.)

Caveats

On OS X, the modification time ("mtime") of files is only stored to the nearest second, so if a file is modified twice in one second, and stays the same size, globwatch won't notice the second change. I can't think of a way to work around this -- it's a limitation of the filesystem -- but I'm open to suggestions.

The timer for file-level scanning determines how rapidly the "change" event will trigger. At the default timer interval of 1/4 second, at the worst case, it may be 1/4 second between a file modification and the "change" event triggering. If you've performed operations that might have caused files to change, you can alleviate this somewhat by calling check() to cause an immediate rescan.

License

Apache 2 (open-source) license, included in 'LICENSE.txt'.

Authors

@robey - Robey Pointer robeypointer@gmail.com

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