class DirectoryWatcher

Synopsis¶ ↑

A class for watching files within a directory and generating events when those files change.

Details¶ ↑

A directory watcher is an Observable object that sends events to registered observers when file changes are detected within the directory being watched.

The directory watcher operates by scanning the directory at some interval and creating a list of the files it finds. File events are detected by comparing the current file list with the file list from the previous scan interval. Three types of events are supported – added, modified, and

removed*.

An added event is generated when the file appears in the current file list but not in the previous scan interval file list. A removed event is generated when the file appears in the previous scan interval file list but not in the current file list. A modified event is generated when the file appears in the current and the previous interval file list, but the file modification time or the file size differs between the two lists.

The file events are collected into an array, and all registered observers receive all file events for each scan interval. It is up to the individual observers to filter the events they are interested in.

File Selection¶ ↑

The directory watcher uses glob patterns to select the files to scan. The default glob pattern will select all regular files in the directory of interest '*'.

Here are a few useful glob examples:

   '*'               => all files in the current directory
   '**  '            => all files in all subdirectories
   '   *.rb'         => all ruby files
   'ext   /*.{h,c}'  => all C source code files

Note*: file events will never be generated for directories. Only regular

files are included in the file scan.

Stable Files¶ ↑

A fourth file event is supported but not enabled by default – the

stable* event. This event is generated after a file has been added or

modified and then remains unchanged for a certain number of scan intervals.

To enable the generation of this event the stable count must be configured. This is the number of scan intervals a file must remain unchanged (based modification time and file size) before it is considered stable.

To disable this event the stable count should be set to nil.

Usage¶ ↑

Learn by Doing – here are a few different ways to configure and use a directory watcher.

Basic¶ ↑

This basic recipe will watch all files in the current directory and generate the three default events. We'll register an observer that simply prints the events to standard out.

require 'directory_watcher'

dw = DirectoryWatcher.new '.'
dw.add_observer {|*args| args.each {|event| puts event}}

dw.start
gets      # when the user hits "enter" the script will terminate
dw.stop

Suppress Initial “added” Events¶ ↑

This little twist will suppress the initial “added” events that are generated the first time the directory is scanned. This is done by pre-loading the watcher with files – i.e. telling the watcher to scan for files before actually starting the scan loop.

require 'directory_watcher'

dw = DirectoryWatcher.new '.', :pre_load => true
dw.glob = '   *.rb'
dw.add_observer {|*args| args.each {|event| puts event}}

dw.start
gets      # when the user hits "enter" the script will terminate
dw.stop

There is one catch with this recipe. The glob pattern must be specified before the pre-load takes place. The glob pattern can be given as an option to the constructor:

dw = DirectoryWatcher.new '.', :glob => '**/*.rb', :pre_load => true

The other option is to use the reset method:

dw = DirectoryWatcher.new '.'
dw.glob = '**/*.rb'
dw.reset true     # the +true+ flag causes the watcher to pre-load
                  # the files

Generate “stable” Events¶ ↑

In order to generate stable events, the stable count must be specified. In this example the interval is set to 5.0 seconds and the stable count is set to 2. Stable events will only be generated for files after they have remain unchanged for 10 seconds (5.0 * 2).

require 'directory_watcher'

dw = DirectoryWatcher.new '.', :glob => '**/*.rb'
dw.interval = 5.0
dw.stable = 2
dw.add_observer {|*args| args.each {|event| puts event}}

dw.start
gets      # when the user hits "enter" the script will terminate
dw.stop

Persisting State¶ ↑

A directory watcher can be configured to persist its current state to a file when it is stopped and to load state from that same file when it starts. Setting the persist value to a filename will enable this feature.

require 'directory_watcher'

dw = DirectoryWatcher.new '.', :glob => '**/*.rb'
dw.interval = 5.0
dw.persist = "dw_state.yml"
dw.add_observer {|*args| args.each {|event| puts event}}

dw.start  # loads state from dw_state.yml
gets      # when the user hits "enter" the script will terminate
dw.stop   # stores state to dw_state.yml

Running Once¶ ↑

Instead of using the built in run loop, the directory watcher can be run one or many times using the run_once method. The state of the directory watcher can be loaded and dumped if so desired.

dw = DirectoryWatcher.new '.', :glob => '**/*.rb'
dw.persist = "dw_state.yml"
dw.add_observer {|*args| args.each {|event| puts event}}

dw.load!       # loads state from dw_state.yml
dw.run_once
sleep 5.0
dw.run_once
dw.persist!    # stores state to dw_state.yml

Ordering of Events¶ ↑

In the case, particularly in the initial scan, or in cases where the Scanner may be doing a large pass over the monitored locations, many events may be generated all at once. In the default case, these will be emitted in the order in which they are observed, which tends to be alphabetical, but it not guaranteed. If you wish the events to be order by modified time, or file size this may be done by setting the sort_by and/or the order_by options.

dw = DirectoryWatcher.new '.', :glob => '**/*.rb', :sort_by => :mtime
dw.add_observer {|*args| args.each {|event| puts event}}
dw.start
gets      # when the user hits "enter" the script will terminate
dw.stop

Scanning Strategies¶ ↑

By default DirectoryWatcher uses a thread that scans the directory being watched for files and calls “stat” on each file. The stat information is used to determine which files have been modified, added, removed, etc. This approach is fairly intensive for short intervals and/or directories with many files.

DirectoryWatcher supports using Cool.io, EventMachine, or Rev instead of a busy polling thread. These libraries use system level kernel hooks to receive notifications of file system changes. This makes DirectoryWorker much more efficient.

This example will use Cool.io to generate file notifications.

dw = DirectoryWatcher.new '.', :glob => '**/*.rb', :scanner => :coolio
dw.add_observer {|*args| args.each {|event| puts event}}

dw.start
gets      # when the user hits "enter" the script will terminate
dw.stop

The scanner cannot be changed after the DirectoryWatcher has been created. To use an EventMachine scanner, pass :em as the :scanner option.

If you wish to use the Cool.io scanner, then you must have the Cool.io gem installed. The same goes for EventMachine and Rev. To install any of these gems run the following on the command line:

gem install cool.io
gem install eventmachine
gem install rev

Note: Rev has been replace by Cool.io and support for the Rev scanner will eventually be dropped from DirectoryWatcher.

Contact¶ ↑

A lot of discussion happens about Ruby in general on the ruby-talk mailing list (www.ruby-lang.org/en/ml.html), and you can ask any questions you might have there. I monitor the list, as do many other helpful Rubyists, and you're sure to get a quick answer. Of course, you're also welcome to email me (Tim Pease) directly at the at tim.pease@gmail.com, and I'll do my best to help you out.

(the above paragraph was blatantly stolen from Nathaniel Talbott's Test::Unit documentation)

Author¶ ↑

Tim Pease

Constants

HAVE_COOLIO
HAVE_EM
HAVE_REV

Attributes

config[R]

access the configuration of the DirectoryWatcher

Public Class Methods

new( directory, options ) click to toggle source

Create a new DirectoryWatcher that will generate events when file changes are detected in the given directory. If the directory does not exist, it will be created. The following options can be passed to this method:

 :glob      =>  '*'      file glob pattern to restrict scanning
 :interval  =>  30.0     the directory scan interval (in seconds)
 :stable    =>  nil      the number of intervals a file must remain
                         unchanged for it to be considered "stable"
 :pre_load  =>  false    setting this option to true will pre-load the
                         file list effectively skipping the initial
                         round of file added events that would normally
                         be generated (glob pattern must also be
                         specified otherwise odd things will happen)
 :persist   =>  file     the state will be persisted to and restored
                         from the file when the directory watcher is
                         stopped and started (respectively)
 :scanner   =>  nil      the directory scanning strategy to use with
                         the directory watcher (either :coolio, :em, :rev or nil)
 :sort_by   =>  :path    the sort order of the scans, when there are
                         multiple events ready for deliver. This can be
                         one of:

                            :path  => default, order by file name
                            :mtime => order by last modified time
                            :size  => order by file size
:order_by   => :ascending The direction in which the sorted items are
                          sorted. Either :ascending or :descending
:logger     => nil      An object that responds to the debug, info, warn,
                        error and fatal methods. Using the default will
                        use Logging gem if it is available and then fall
                        back to NullLogger

The default glob pattern will scan all files in the configured directory. Setting the :stable option to nil will prevent stable events from being generated.

Additional information about the available options is documented in the Configuration class.

# File lib/directory_watcher.rb, line 295
def initialize( directory, opts = {} )
  @observer_peers = {}
  @config = Configuration.new( opts.merge( :dir => directory ) )

  setup_dir(config.dir)

  @notifier = Notifier.new(config, @observer_peers)
  @collector = Collector.new(config)
  @scanner = config.scanner_class.new(config)
end

Public Instance Methods

add_observer( observer, func = :update ) click to toggle source

add_observer {|*events| block}

Adds the given observer as an observer on this directory watcher. The observer will now receive file events when they are generated. The second optional argument specifies a method to notify updates, of which the default value is update.

Optionally, a block can be passed as the observer. The block will be executed with the file events passed as the arguments. A reference to the underlying Proc object will be returned for use with the delete_observer method.

# File lib/directory_watcher.rb, line 335
def add_observer( observer = nil, func = :update, &block )
  unless block.nil?
    observer = block.to_proc
    func = :call
  end

  unless observer.respond_to? func
    raise NoMethodError, "observer does not respond to `#{func.to_s}'"
  end

  logger.debug "Added observer"
  @observer_peers[observer] = func
  observer
end

count_observers() click to toggle source

Return the number of observers associated with this directory watcher..

# File lib/directory_watcher.rb, line 365
def count_observers
  @observer_peers.size
end

delete_observer( observer ) click to toggle source

Delete observer as an observer of this directory watcher. It will no longer receive notifications.

# File lib/directory_watcher.rb, line 353
def delete_observer( observer )
  @observer_peers.delete observer
end

delete_observers() click to toggle source

Delete all observers associated with the directory watcher.

# File lib/directory_watcher.rb, line 359
def delete_observers
  @observer_peers.clear
end

finished_scans?() click to toggle source

Returns true if the maximum number of scans has been reached.

# File lib/directory_watcher.rb, line 556
def finished_scans?
  return true if maximum_iterations and (scans >= maximum_iterations)
  return false
end

glob() click to toggle source

# File lib/directory_watcher.rb, line 380
def glob
  config.glob
end

glob = '*' click to toggle source

glob = ['lib /*.rb', 'test/ *.rb']

Sets the glob pattern that will be used when scanning the directory for files. A single glob pattern can be given or an array of glob patterns.

# File lib/directory_watcher.rb, line 376
def glob=( val )
  config.glob = val
end

interval() click to toggle source

# File lib/directory_watcher.rb, line 392
def interval
  config.interval
end

interval=( val ) click to toggle source

Sets the directory scan interval. The directory will be scanned every interval seconds for changes to files matching the glob pattern. Raises ArgumentError if the interval is zero or negative.

# File lib/directory_watcher.rb, line 388
def interval=( val )
  config.interval = val
end

join( limit = nil ) click to toggle source

If the directory watcher is running, the calling thread will suspend execution and run the directory watcher thread. This method does not return until the directory watcher is stopped or until limit seconds have passed.

If the directory watcher is not running, this method returns immediately with nil.

# File lib/directory_watcher.rb, line 592
def join( limit = nil )
  @scanner.join limit
end

load!() click to toggle source

Loads the state of the directory watcher from the persist file. This method will do nothing if the directory watcher is running or if the persist file is not configured.

# File lib/directory_watcher.rb, line 458
def load!
  return if running?
  File.open(persist, 'r') { |fd| @collector.load_stats(fd) } if persist? and test(?f, persist)
  self
end

maximum_iterations() click to toggle source

Returns the maximum number of scans the directory scanner will perform

# File lib/directory_watcher.rb, line 541
def maximum_iterations
  @scanner.maximum_iterations
end

maximum_iterations=( value ) click to toggle source

Sets the maximum number of scans the scanner is to make on the directory

# File lib/directory_watcher.rb, line 535
def maximum_iterations=( value )
  @scanner.maximum_iterations = value
end

pause() click to toggle source

Pauses the scanner.

# File lib/directory_watcher.rb, line 498
def pause
  @scanner.pause
end

persist() click to toggle source

# File lib/directory_watcher.rb, line 432
def persist
  config.persist
end

persist!() click to toggle source

Write the current state of the directory watcher to the persist file. This method will do nothing if the directory watcher is running or if the persist file is not configured.

# File lib/directory_watcher.rb, line 440
def persist!
  return if running?
  File.open(persist, 'w') { |fd| @collector.dump_stats(fd) } if persist?
  self
rescue => e
  logger.error "Failure to write to persitence file #{persist.inspect} : #{e}"
end

persist=( filename ) click to toggle source

Sets the name of the file to which the directory watcher state will be persisted when it is stopped. Setting the persist filename to nil will disable this feature.

# File lib/directory_watcher.rb, line 428
def persist=( filename )
  config.persist = filename
end

persist?() click to toggle source

Is persistence done on this DirectoryWatcher

# File lib/directory_watcher.rb, line 450
def persist?
  config.persist
end

reset( pre_load = false ) click to toggle source

Reset the directory watcher state by clearing the stored file list. If the directory watcher is running, it will be stopped, the file list cleared, and then restarted. Passing true to this method will cause the file list to be pre-loaded after it has been cleared effectively skipping the initial round of file added events that would normally be generated.

# File lib/directory_watcher.rb, line 571
def reset( pre_load = false )
  was_running = @scanner.running?

  stop if was_running
  File.delete(config.persist) if persist? and test(?f, config.persist)
  @scanner.reset pre_load
  start if was_running
  self
end

resume() click to toggle source

Resume the emitting of events

# File lib/directory_watcher.rb, line 504
def resume
  @scanner.resume
end

run_once() click to toggle source

Performs exactly one scan of the directory for file changes and notifies the observers.

# File lib/directory_watcher.rb, line 599
def run_once
  @scanner.run
  @collector.start unless running?
  @notifier.start unless running?
  self
end

running?() click to toggle source

Returns true if the directory watcher is currently running. Returns false if this is not the case.

# File lib/directory_watcher.rb, line 467
def running?
  @scanner.running?
end

scans() click to toggle source

Returns the number of scans of the directory scanner it has completed thus far.

This will always report 0 unless a maximum number of scans has been set

# File lib/directory_watcher.rb, line 550
def scans
  @scanner.iterations
end

setup_dir( dir ) click to toggle source

Setup the directory existence.

Raise an error if the item passed in does exist but is not a directory

Returns nothing

# File lib/directory_watcher.rb, line 311
def setup_dir( dir )
  if Kernel.test(?e, dir)
    unless Kernel.test(?d, dir)
      raise ArgumentError, "'#{dir}' is not a directory"
    end
  else
    Dir.mkdir dir
  end
end

stable() click to toggle source

# File lib/directory_watcher.rb, line 420
def stable
  config.stable
end

stable=( val ) click to toggle source

Sets the number of intervals a file must remain unchanged before it is considered “stable”. When this condition is met, a stable event is generated for the file. If stable is set to nil then stable events will not be generated.

A stable event will be generated once for a file. Another stable event will only be generated after the file has been modified and then remains unchanged for stable intervals.

Example:

dw = DirectoryWatcher.new( '/tmp', :glob => 'swap.*' )
dw.interval = 15.0
dw.stable = 4

In this example, a directory watcher is configured to look for swap files in the /tmp directory. Stable events will be generated every 4 scan intervals iff a swap remains unchanged for that time. In this case the time is 60 seconds (15.0 * 4).

# File lib/directory_watcher.rb, line 416
def stable=( val )
  config.stable = val
end

start() click to toggle source

Start the directory watcher scanning thread. If the directory watcher is already running, this method will return without taking any action.

Start returns one the scanner and the notifier say they are running

# File lib/directory_watcher.rb, line 476
def start
  logger.debug "start (running -> #{running?})"
  return self if running?

  load!
  logger.debug "starting notifier #{@notifier.object_id}"
  @notifier.start
  Thread.pass until @notifier.running?

  logger.debug "starting collector"
  @collector.start
  Thread.pass until @collector.running?

  logger.debug "starting scanner"
  @scanner.start
  Thread.pass until @scanner.running?

  self
end

stop() click to toggle source

Stop the directory watcher scanning thread. If the directory watcher is already stopped, this method will return without taking any action.

Stop returns once the scanner and notifier say they are no longer running

# File lib/directory_watcher.rb, line 512
def stop
  logger.debug "stop (running -> #{running?})"
  return self unless running?

  logger.debug"stopping scanner"
  @scanner.stop
  Thread.pass while @scanner.running?

  logger.debug"stopping collector"
  @collector.stop
  Thread.pass while @collector.running?

  logger.debug"stopping notifier"
  @notifier.stop
  Thread.pass while @notifier.running?

  self
ensure
  persist!
end