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Opal: Shimmer

Shimmer is an application state and configuration management library built with Opal, a Ruby-to-JS compiler.


Add this line to your application's Gemfile:

gem 'opal-shimmer'

And then execute:

$ bundle

Or install it yourself as:

$ gem install opal-shimmer


Shimmer is very easy to use right out of the box. I'm assuming you'll be using it within the context of a Rails application for this tutorial. Make sure you add //= require shimmer to your application.js manifest file.

# Set up the config object:
config =

# Set some values:
config.somevalue = "Wow"
config.othervalue = ['This', 'is', 'great!']

# Get some values:
puts config.somevalue  # > "Wow"

# Use namespaces to define very specific values:
config.several.levels.deep.stringvalue = "Your string here"
puts config.several.levels.deep.stringvalue  # > "Your string here"

# Check whether a value exists:

# Set stuff in a namespace using a block:
config.really.deep.namespace do |c|
  c.value1 = 1
  c.value2 = 2

Persist values across sessions using the browser's localStorage

Normally when you set and retrieve values in a config object, those values are only stored in-memory, which means when you reload the browser the values are no longer available.

Shimmer provides a persist method that allows you to specify that a value key should be stored in localStorage so that it will be automatically available in a later browser session (or even in the same session if you have a different config instance.

# Persist values across sessions using localStorage:
config.cease_and_persist = "abc123"

config2 =  # this loads up a brand new object

puts config2.cease_and_persist  # > "abc123"  Aha! it works!

If you want to make sure you don't overwrite previous persisted values by setting default values, there is a persist_defaults method available. It creates a special block context where you can set numerous values, and those values will only be set if the values aren't already saved in localStorage. Example:

# An easier way to persist values by setting initial defaults
# and not overwriting values that get set differently later:
config.persist_defaults do |c|
  c.somevalue = "abc"
  c.othervalue = 123

# ...user triggers some action...
config.somevalue = "xyz"

# ...days later on a subsquent browser session...
puts config.somevalue  # > "xyz"  not "abc" - Yay!

Use Observers to execute code when config values change

Shimmer lets you attach an observer (essentially an event handler) to a config object key. The observer will be notified when the value for that key changes, and will receive both the old and the new values as method arguments. Example:

config =

oldval = nil
newval = nil do
  on :somevalue do |new, old|
    oldval = old
    newval = new

config.somevalue = "Totally cool"

puts newval  # > "Totally cool"

config.somevalue = "Another cool thing"

puts newval  # > "Another cool thing"
puts oldval  # > "Totally cool"

For more examples, look at the *_spec.rb files in the spec folder.


  1. Fork it ( )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


Simply run rackup at your command line when you're in the project folder. It will load a webserver at port 9292. Then just go to your browser and access http://localhost:9292. You should get the full rspec suite runner output. (And hopefully, everything's green!)

If you have trouble using Safari, try using Chrome instead. I'm not sure why this is sometimes an issue...


Shimmer is an application state and configuration management library built with Opal, a Ruby-to-JS compiler.







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