Listen

The Listen gem listens to file modifications and notifies you about the changes.

Features

• Works everywhere!
• Supports watching multiple directories from a single listener.
• OS-specific adapters for Mac OS X 10.6+, Linux, *BSD and Windows.
• Automatic fallback to polling if OS-specific adapter doesn't work.
• Detects file modification, addition and removal.
• Checksum comparison for modifications made under the same second.
• Allows supplying regexp-patterns to ignore and filter paths for better results.
• Tested on all Ruby environments via travis-ci.

Install

gem install listen

Usage

There are two ways to use Listen:

1. Call Listen.to with either a single directory or multiple directories, then define the change callback in a block.
2. Create a listener object and use it in an (ARel style) chainable way.

Feel free to give your feeback via Listen issues

Block API

# Listen to a single directory.
Listen.to('dir/path/to/listen', :filter => /\.rb$/, :ignore => %r{ignored/path/}) do |modified, added, removed|
  # ...
end

# Listen to multiple directories.
Listen.to('dir/to/awesome_app', 'dir/to/other_app', :filter => /\.rb$/, :latency => 0.1) do |modified, added, removed|
  # ...
end

"Object" API

listener = Listen.to('dir/path/to/listen')
listener = listener.ignore(%r{^ignored/path/})
listener = listener.filter(/\.rb$/)
listener = listener.latency(0.5)
listener = listener.force_polling(true)
listener = listener.polling_fallback_message(false)
listener = listener.change(&callback)
listener.start # blocks execution!

Chainable

Listen.to('dir/path/to/listen')
      .ignore(%r{^ignored/path/})
      .filter(/\.rb$/)
      .latency(0.5)
      .force_polling(true)
      .polling_fallback_message('custom message')
      .change(&callback)
      .start # blocks execution!

Pause/Unpause

Listener can also easily be paused/unpaused:

listener = Listen.to('dir/path/to/listen')
listener.start(false) # non-blocking mode
listener.pause   # stop listening to changes
listener.paused? # => true
listener.unpause
listener.stop

Listening to changes on multiple directories

The Listen gem provides the MultiListener class to watch multiple directories and handle their changes from a single listener:

listener = Listen::MultiListener.new('app/css', 'app/js')
listener.latency(0.5)

# Configure the listener to your needs...

listener.start # blocks execution!

For an easier access, the Listen.to method can also be used to create a multi-listener:

listener = Listen.to('app/css', 'app/js')
                 .ignore(%r{^vendor/}) # both js/vendor and css/vendor will be ignored
                 .change(&assets_callback)

listener.start # blocks execution!

Changes callback

Changes to the listened-to directories gets reported back to the user in a callback. The registered callback gets invoked, when there are changes, with three parameters: modified_paths, added_paths and removed_paths in that particular order.

You can register a callback in two ways. The first way is by passing a block when calling the Listen.to method or when initializing a listener object:

Listen.to('path/to/app') do |modified, added, removed|
  # This block will be called when there are changes.
end

# or ...

listener = Listen::Listener.new('path/to/app') do |modified, added, removed|
  # This block will be called when there are changes.
end

The second way to register a callback is be calling the change method on any listener passing it a block:

# Create a callback
callback = Proc.new do |modified, added, removed|
  # This proc will be called when there are changes.
end

listener = Listen.to('dir')
listener.change(&callback) # convert the callback to a block and register it

listener.start # blocks execution

Paths in callbacks

Listeners invoke callbacks passing them absolute paths by default:

# Assume someone changes the 'style.css' file in '/home/user/app/css' after creating
# the listener.
Listen.to('/home/user/app/css') do |modified, added, removed|
  modified.inspect # => ['/home/user/app/css/style.css']
end

Relative paths in callbacks

When creating a listener for a single path (more specifically a Listen::Listener instance), you can pass :relative_paths => true as an option to get relative paths in your callback:

# Assume someone changes the 'style.css' file in '/home/user/app/css' after creating
# the listener.
Listen.to('/home/user/app/css', :relative_paths => true) do |modified, added, removed|
  modified.inspect # => ['style.css']
end

Passing the :relative_paths => true option won't work when listeneing to multiple directories:

# Assume someone changes the 'style.css' file in '/home/user/app/css' after creating
# the listener.
Listen.to('/home/user/app/css', '/home/user/app/js', :relative_paths => true) do |modified, added, removed|
  modified.inspect # => ['/home/user/app/css/style.css']
end

Options

These options can be set through Listen.to params or via methods (see the "Object" API)

:filter => /\.rb$/, /\.coffee$/                # Filter files to listen to via a regexps list.
                                               # default: none

:ignore => %r{app/CMake/}, /\.pid$/            # Ignore a list of paths (root directory or sub-dir)
                                               # default: See DEFAULT_IGNORED_DIRECTORIES and DEFAULT_IGNORED_EXTENSIONS in Listen::DirectoryRecord

:latency => 0.5                                # Set the delay (**in seconds**) between checking for changes
                                               # default: 0.25 sec (1.0 sec for polling)

:relative_paths => true                        # Enable the use of relative paths in the callback.
                                               # default: false

:force_polling => true                         # Force the use of the polling adapter
                                               # default: none

:polling_fallback_message => 'custom message'  # Set a custom polling fallback message (or disable it with `false`)
                                               # default: "WARNING: Listen fallen back to polling, learn more at https://github.com/guard/listen#fallback."

The patterns for filtering and ignoring paths

Just like the unix convention of beginning absolute paths with the directory-separator (forward slash / in unix) and with no prefix for relative paths, Listen doesn't prefix relative paths (to the watched directory) with a directory-separator.

Therefore make sure NOT to prefix your regexp-patterns for filtering or ignoring paths with a directory-separator, otherwise they won't work as expected.

As an example: to ignore the build directory in a C-project, use %r{build/} and not %r{/build/}.

Use #filter! and #ignore! methods to overwrites default patterns.

Non-blocking listening to changes

Starting a listener blocks the current thread by default. That means any code after the start call won't be run until the listener is stopped (which needs to be done from another thread).

For advanced usage there is an option to disable this behavior and have the listener start working in the background without blocking. To enable non-blocking listening the start method of the listener (be it Listener or MultiListener) needs to be called with false as a parameter.

Here is an example of using a listener in the non-blocking mode:

listener = Listen.to('dir/path/to/listen')
listener.start(false) # doesn't block execution

# Code here will run immediately after starting the listener

note: Using the Listen.to helper-method with a callback-block will always block execution. See the "Block API" section for an example.

Listen adapters

The Listen gem has a set of adapters to notify it when there are changes. There are 3 OS-specific adapters to support Mac, Linux, *BSD and Windows. These adapters are fast as they use some system-calls to implement the notifying function.

There is also a polling adapter which is a cross-platform adapter and it will work on any system. This adapter is unfortunately slower than the rest of the adapters.

The Listen gem will choose the best and working adapter for your machine automatically. If you want to force the use of the polling adapter, either use the :force_polling option while initializing the listener or call the force_polling method on your listener before starting it.

Polling fallback

When a OS-specific adapter doesn't work the Listen gem automatically falls back to the polling adapter. Here are some things you could try to avoid the polling fallback:

• Update your Dropbox client (if used).
• Increase latency. (Please open an issue if you think that default is too low.)
• Move or rename the listened folder.
• Update/reboot your OS.

If your application keeps using the polling-adapter and you can't figure out why, feel free to open an issue (and be sure to give all the details).

Development

• Documentation hosted at RubyDoc.
• Source hosted at GitHub.

Pull requests are very welcome! Please try to follow these simple rules if applicable:

• Please create a topic branch for every separate change you make.
• Make sure your patches are well tested. All specs run with rake spec:portability must pass.
• Update the Yard documentation.
• Update the README.
• Update the CHANGELOG for noteworthy changes.
• Please do not change the version number.

For questions please join us in our Google group or on #guard (irc.freenode.net).

Acknowledgment

• Michael Kessler (netzpirat) for having written the initial specs.
• Travis Tilley (ttilley) for this awesome work on fssm & rb-fsevent.
• Nathan Weizenbaum (nex3) for rb-inotify, a thorough inotify wrapper.
• Mathieu Arnold (mat813) for [rb-kqueue][], a simple kqueue wrapper.
• stereobooster for rb-fchange, windows support wouldn't exist without him.
• Yehuda Katz (wycats) for vigilo, that has been a great source of inspiration.

Authors

• Thibaud Guillaume-Gentil (@thibaudgg)
• Maher Sallam (@mahersalam)

Contributors

https://github.com/guard/listen/contributors