Axle

Axle is an efficient cross-platform DirectoryWatcher based service for Clojure.

BarbaryWatchService for Mac is considered deprecated and no longers works with newer versions of Java. This means existing watcher implementations that depend on it, like Hawk, no longer perform well. Axle introduces a axle.core/watch! fn that is a thin wrapper of the cross-platform, high performance DirectoryWatcher.

Getting Started

Just add the following dependency in your deps.edn:

io.zalky/axle {:mvn/version "0.2.1"}

You can start a watch task like so:

(require '[axle.core :as axle])

(def watcher
  (axle/watch!
   {:paths   ["src/clojure" "src/assets"]
    :context {:init "context"}
    :handler (fn [context event]
               (update-context ...))}))

Where :handler is a reducing function that accepts the current context and a file event to return an updated context, :paths is a list of paths (to individual files or directories) to watch, and optionally, :context is the initial value of the accumulated context.

File events have the following form:

{:type       :modify,
 :path       "/path/to/file.clj",
 :count      1,
 :root-path  "src",
 :directory? false,

There are four event types, :create, :delete, :modify and :overflow (which is emitted if the file system is generating events faster than the underlying implementation can process them).

Each event can optionally include a file hash. Since this affects performance it is disabled by default, but can be turned on via the :file-hashing true option:

(def watcher
  (axle/watch!
   {:paths        ["src/clojure" "src/assets"]
    :handler      handler
    :file-hashing true}))

Now events will include an additional :hash attribute.

On uncaught exceptions, watch! will print a stacktrace that includes the context at the time of the error. However, you should consider catching errors in your handler, where you have access to both the context and the event, and also have the option to update the context in response to an error.

Finally, a watch task can be stopped via:

(axle/stop! watcher)

Windowing

You can return a windowing handler, which handles a collection of events that have occurred within a certain number of milliseconds, via:

{:handler (axle/window ms (fn handler [context events] ...))}

Where events is just a sequence of event maps, and ms is just an integer number of milliseconds.

Alternatives

Beholder is a similar DirectoryWatcher based wrapper. However, its callback is not a reducing function, so you cannot accumulate a context, and it does not provide windowing functionality.

License

Axle is distributed under the terms of the Apache License 2.0.