Class: DirectoryWatcher

Inherits:
Object
  • Object
show all
Defined in:
lib/directory_watcher.rb

Overview

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

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 Rev () or EventMachine () 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 Rev to generate file notifications.

dw = DirectoryWatcher.new '.', :glob => '**/*.rb', :scanner => :rev
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.

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 [email protected], 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

Defined Under Namespace

Classes: EmScanner, Event, FileStat, RevScanner, Scanner

Constant Summary collapse

VERSION =

:nodoc:

'1.3.1'

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(directory, opts = {}) ⇒ DirectoryWatcher

call-seq:

DirectoryWatcher.new( directory, options )

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 :em :rev or nil)

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.



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# File 'lib/directory_watcher.rb', line 269

def initialize( directory, opts = {} )
  @dir = directory
  @observer_peers = {}

  if Kernel.test(?e, @dir)
    unless Kernel.test(?d, @dir)
      raise ArgumentError, "'#{@dir}' is not a directory"
    end
  else
    Dir.mkdir @dir
  end

  klass = opts[:scanner].to_s.capitalize + 'Scanner'
  klass = DirectoryWatcher.const_get klass rescue Scanner
  @scanner = klass.new {|events| notify_observers(events)}

  self.glob = opts[:glob] || '*'
  self.interval = opts[:interval] || 30
  self.stable = opts[:stable] || nil
  self.persist = opts[:persist]

  @scanner.reset opts[:pre_load]
end

Instance Attribute Details

#glob ⇒ Object

Returns the value of attribute glob.



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# File 'lib/directory_watcher.rb', line 358

def glob
  @glob
end

#persist ⇒ Object

Returns the value of attribute persist.



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# File 'lib/directory_watcher.rb', line 421

def persist
  @persist
end

Instance Method Details

#add_observer(observer = nil, func = :update, &block) ⇒ Object

call-seq:

add_observer( observer, func = :update )
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.



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# File 'lib/directory_watcher.rb', line 307

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

  @observer_peers[observer] = func
  observer
end

#count_observers ⇒ Object

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



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# File 'lib/directory_watcher.rb', line 336

def count_observers
  @observer_peers.size
end

#delete_observer(observer) ⇒ Object

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



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# File 'lib/directory_watcher.rb', line 324

def delete_observer( observer )
  @observer_peers.delete observer
end

#delete_observers ⇒ Object

Delete all observers associated with the directory watcher.



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# File 'lib/directory_watcher.rb', line 330

def delete_observers
  @observer_peers.clear
end

#interval ⇒ Object

Returns the directory scan interval in seconds.



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# File 'lib/directory_watcher.rb', line 372

def interval
  @scanner.interval
end

#interval=(val) ⇒ Object

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.

Raises:

  • (ArgumentError) 


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# File 'lib/directory_watcher.rb', line 364

def interval=( val )
  val = Float(val)
  raise ArgumentError, "interval must be greater than zero" if val <= 0
  @scanner.interval = Float(val)
end

#join(limit = nil) ⇒ Object

call-seq:

join( limit = nil )

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.



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# File 'lib/directory_watcher.rb', line 504

def join( limit = nil )
  @scanner.join limit
end

#load! ⇒ Object

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.



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# File 'lib/directory_watcher.rb', line 437

def load!
  return if running?
  @scanner.files = YAML.load_file(@persist) if @persist and test(?f, @persist)
  self
end

#persist! ⇒ Object

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.



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# File 'lib/directory_watcher.rb', line 427

def persist!
  return if running?
  File.open(@persist, 'w') {|fd| fd.write YAML.dump(@scanner.files)} if @persist
  self
end

#reset(pre_load = false) ⇒ Object

call-seq:

reset( pre_load = false )

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.



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# File 'lib/directory_watcher.rb', line 483

def reset( pre_load = false )
  was_running = @scanner.running?

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

#run_once ⇒ Object

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



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# File 'lib/directory_watcher.rb', line 511

def run_once
  @scanner.run_once
  self
end

#running? ⇒ Boolean

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

Returns:

  • (Boolean) 


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# File 'lib/directory_watcher.rb', line 446

def running?
  @scanner.running?
end

#stable ⇒ Object

Returs the number of intervals a file must remain unchanged before it is considered “stable”.



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# File 'lib/directory_watcher.rb', line 410

def stable
  @scanner.stable
end

#stable=(val) ⇒ Object

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).

Raises:

  • (ArgumentError) 


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# File 'lib/directory_watcher.rb', line 396

def stable=( val )
  if val.nil?
    @scanner.stable = nil
    return
  end

  val = Integer(val)
  raise ArgumentError, "stable must be greater than zero" if val <= 0
  @scanner.stable = val
end

#start ⇒ Object

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



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# File 'lib/directory_watcher.rb', line 453

def start
  return self if running?

  load!
  @scanner.start
  self
end

#stop ⇒ Object

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



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# File 'lib/directory_watcher.rb', line 464

def stop
  return self unless running?

  @scanner.stop
  self
ensure
  persist!
end