A class for watching files within a directory and generating events when those files change.
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.
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.
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.
Learn by Doing – here are a few different ways to configure and use a directory watcher.
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
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
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
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
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
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.
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)
Tim Pease
Returns the library path for the module. If any arguments are given, they will be joined to the end of the libray path using File.join.
# File lib/directory_watcher.rb, line 264 def self.libpath( *args, &block ) rv = args.empty? ? LIBPATH : ::File.join(LIBPATH, args.flatten) if block begin $LOAD_PATH.unshift LIBPATH rv = block.call ensure $LOAD_PATH.shift end end return rv end
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)
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.
# File lib/directory_watcher.rb, line 320 def initialize( directory, opts = {} ) @dir = directory @observer_peers = {} if Kernel.test(ee, @dir) unless Kernel.test(dd, @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
Returns the lpath for the module. If any arguments are given, they will be joined to the end of the path using File.join.
# File lib/directory_watcher.rb, line 280 def self.path( *args, &block ) rv = args.empty? ? PATH : ::File.join(PATH, args.flatten) if block begin $LOAD_PATH.unshift PATH rv = block.call ensure $LOAD_PATH.shift end end return rv end
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 358 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
Return the number of observers associated with this directory watcher..
# File lib/directory_watcher.rb, line 387 def count_observers @observer_peers.size end
Delete observer as an observer of this directory watcher. It will no longer receive notifications.
# File lib/directory_watcher.rb, line 375 def delete_observer( observer ) @observer_peers.delete observer end
Delete all observers associated with the directory watcher.
# File lib/directory_watcher.rb, line 381 def delete_observers @observer_peers.clear end
Returns the array of glob patterns used to monitor files in the directory.
# File lib/directory_watcher.rb, line 412 def glob @scanner.glob end
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 398 def glob=( val ) glob = case val when String; [File.join(@dir, val)] when Array; val.flatten.map! {|g| File.join(@dir, g)} else raise(ArgumentError, 'expecting a glob pattern or an array of glob patterns') end glob.uniq! @scanner.glob = glob end
Returns the directory scan interval in seconds.
# File lib/directory_watcher.rb, line 428 def interval @scanner.interval end
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 420 def interval=( val ) val = Float(val) raise ArgumentError, "interval must be greater than zero" if val <= 0 @scanner.interval = val end
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 560 def join( limit = nil ) @scanner.join limit end
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 493 def load! return if running? @scanner.files = YAML.load_file(@persist) if @persist and test(ff, @persist) self end
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 483 def persist! return if running? File.open(@persist, 'w') {|fd| fd.write YAML.dump(@scanner.files)} if @persist self end
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 474 def persist=( filename ) @persist = filename ? filename.to_s : nil end
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 539 def reset( pre_load = false ) was_running = @scanner.running? stop if was_running File.delete(@persist) if @persist and test(ff, @persist) @scanner.reset pre_load start if was_running self end
Performs exactly one scan of the directory for file changes and notifies the observers.
# File lib/directory_watcher.rb, line 567 def run_once @scanner.run_once self end
Returns true if the directory watcher is currently running. Returns false if this is not the case.
# File lib/directory_watcher.rb, line 502 def running? @scanner.running? end
Returs the number of intervals a file must remain unchanged before it is considered “stable”.
# File lib/directory_watcher.rb, line 466 def stable @scanner.stable end
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 452 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
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