StateInspector

This can fully inform on object method calls and parameters as well as instance variables before and after (when called via a setter method). In short this utilizes the decorator patttern and the observer pattern to hook and report when a method is called. In simple terms it will wrap any methods you choose with a hook that sends off all the details of the method call when it is executed to a reporter/observer of your choosing.

This project uses a variation of the observer pattern. There is a hash of Reporters where you can
mark the key as a class instance or the class itself and point it to an Observer object. Three
observers are included for you to use under StateInspector::Observers which are NullObserver (default),
InternalObserver, and SessionLoggerObserver. When you toggle on an “informant” on a class instance or
class then each time a setter method is called it will pass that information on to the relevant observer
which handles the behavior you want to occur with that information.

Installation

Add this line to your application’s Gemfile:

gem 'state_inspector'

And then execute:

$ bundle

Or install it yourself as:

$ gem install state_inspector

Complex Usage

The preferred usage is to pick what classes you want to track state change in and have them logged to
a session logger. To do this you would need to do the following.

require 'state_inspector'
require 'state_inspector/observers/session_logger_observer'
include StateInspector::Observers

class MyClass
  attr_writer :thing
end

StateInspector::Reporter[MyClass] = SessionLoggerObserver

MyClass.toggle_informant

m = MyClass.new
m.thing = :a

StateInspector::Reporter[MyClass].values
# => [["#<MyClass:0x005571fde5e498>", "@thing", nil, :a]]

SessionLoggerObserver will by default log reports to log/state_inspector/.

Now everytime the setter method is used for thing a new line will be entered into a log file
of the object, method, old value, and new value. So you will see what is changed from where in
the order that the changed occurred. This session logger will grab as many objects state changes
as you want and give you a nice ordered history of what has occurred.

If you don’t want to inform on all instances of a class then instead of running toggle_informant
on the class itself then simply execute that method on the instances you want to observe.

If you want to see the expected results of the current observer/reporters then see test/reporter_test.rb.

Simple Usage

If you want to only toggle an informant for a small area you may use a helper method to toggle the
observer on and off for you.

When you include StateInspector::Helper it handles requiring the existing observers and bringing them
into scope. You are free to pass in an observer as a second paramater to the toggle_snoop helper method
which will only be assigned for the scope of the block of code.

require 'state_inspector/helper'
include StateInspector::Helper

# instead of doing MyClass.toggle_informant as above, do this.

class MyClass
  attr_writer :thing
end

m = MyClass.new
observer = InternalObserver.new

# observer parameter optional.  Assign beforehand if not provided here.
toggle_snoop(m, observer) do
  m.thing = 4
end

# look at the results
observer.values
# => [[#<MyClass:0x00562f969dee48 @informant=false, @thing=4>, "@thing", nil, 4]]

When writing tests for code and using StateInspector it’s very important to ensure the informant is
untoggled. Otherwise you will have sporatic behavior in your tests. You may use the helpers provided
here for your tests to ensure you won’t have glitchy tests as a result of using informants.

To include it in Minitest you would do:

# test/test_helper.rb or test/minitest_helper.rb

require 'state_inspector/helper'
class Minitest::Test
  include StateInspector::Helper
end

The default behavior for toggling on an informant for the first time is to inject code to observe all
setter methods. This is true for both the Object method toggle_informant and the helper method
toggle_snoop. If you would like to avoid injecting all setter methods for reporting you may either
use state_inspector.skip_setter_snoops (before any toggling) or the helper toggle_snoop_clean which
will cleanly remove its anti-setter hook once done (meaning the next toggle_informant will inject the
informant code into all setters).

The helper block also pipes in the current observer for the object you’re toggling. So you may access
it within the block by pipe assigning it to a variable.

toggle_snoop(m) do |observer|
  observer # Whatever observer happens to be assigned to this object or its class
end

Observers

To include all Observers into scope you can do:

require 'state_inspector/observers'
include StateInspector::Observers

You may look at the available observers in state_inspector/observers.

Observers will have a few methods they each have in common.

module Observer
  def update *vals
    values() << vals
  end

  def display
    values.join " "
  end

  def values
    @values ||= []
  end

  def purge
    @values = []
  end
end

When you’re writing your own observer you’ll include this Observer onto a module’s instance and
overwrite whatever methods you want there.

module ExampleObserver
  class << self
    include Observer
    def display
      "Custom display code here"
    end
  end
end

And to register this observer to a target class you simply write:

StateInspector::Reporter[MyTargetClass] = ExampleObserver

Manually Create or Remove Informers

To manually create informant methods use state_inspector.snoop_setters :a=, :b= and
state_inspector.snoop_methods :a, :b.

require 'state_inspector'
require 'state_inspector/observers/internal_observer'
include StateInspector::Observers

class MyClass
  attr_writer :a, :b, :c
  def x *args; end
end

# InternalObserver (it can be used with many classes and hold all of the data)
# InternalObserver.new (will hold data for only the specific reporter object pointing to it)
StateInspector::Reporter[MyClass] = InternalObserver

# This will allow us to manually define which setter methods to inform on
MyClass.state_inspector.skip_setter_snoops

m = MyClass.new
m.toggle_informant
m.state_inspector.snoop_setters :b=, :c=
m.state_inspector.snoop_methods :x
m.a= 1
m.b= 2
m.c= 3
m.x 4, 5, 6

StateInspector::Reporter[MyClass].values
# => [
#      [#<MyClass:0x005608eb9aead0 @informant=true, @a=1, @b=2, @c=3>, "@b", nil, 2],
#      [#<MyClass:0x005608eb9aead0 @informant=true, @a=1, @b=2, @c=3>, "@c", nil, 3],
#      [#<MyClass:0x005608eb9aead0 @informant=true, @a=1, @b=2, @c=3>, :x, 4, 5, 6]
#    ]

The nils in the values above represent the previous value the instance variable returned.

Remove Informers

To remove informers simply use state_inspector.restore_methods. This takes out the hook from
each method that waits for the informer? method to be true.

# continuing from the above example

m.state_inspector.restore_methods :b=, :c=, :x

Reporter Legend

The format of a report sent will always start with the object that sent it; aka self. Next you will
have either a string representing and instance variable of a symbol representing a method call. If it’s
an instance variable then the next value will be the old value for that instance variable, and the next
value is the value given to the method to be set as the value for that instance variable. If the second
item is a symbol then every item after that is the parameters sent to that method.

LEGEND FOR SETTER	_	_	_
self	@instance_variable	:old_value	:new_value_given

LEGEND FOR METHOD	_	_
self	:method_name	:arguments

Session Logger

The session logger is an observer that saves all output to a log file. A default log file is saved to log/state_inspector/<timestamp>.log. You can manually set a log file with the file= method on the SessionLoggerObserver which may be helpful for situations where the code runs in a temporary directory and vanishes upon completion (like the rubygems test suite).

Here’s an example that catches all setter method calls from constants within the Gem class. Placed at start of file to observe.

require 'state_inspector'
require 'state_inspector/observers/session_logger_observer'
include StateInspector::Observers

SessionLoggerObserver.file = "/home/myhomedir/dev/rubygems/log/output.log"
StateInspector::Reporter.default SessionLoggerObserver
Gem.constants.each {|c| a = eval("Gem::#{c}"); if a.is_a? Class; a.toggle_informant end}

This example is a very expensive one as we set the default observer/reporter to SessionLoggerObserver which means it catches all reports not previously assigned. The last line simply finds all class objects within the Gem namespace and toggles-on the informants (which by default hooks in listeners to each setter method). The file= method used here overwrites the default and guarantees where the data will be written.

I’ve tried the above code in the rubygems test suite on one file test/rubygems/test_gem_dependency_installer.rb. When running this file it records 7756 lines worth of information. This is clearly too much information to parse manually which is why I highly recommend using the scoped helper methods to toggle on and off the behavior around the code you are specifically interested in. You can still toggle informers on many classes like what was done above, but the more objects you do the more I recommend you narrow down the scope of what you’re capturing (like to one specific test in a test suite).

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/danielpclark/state_inspector.

License

The gem is available as open source under the terms of the MIT License.