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Andy Maleh
Andy Maleh

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Glimmer DSL for LibUI Bidirectional Data-Binding

Glimmer DSL for LibUI v0.4.11 just wrapped up a very important new feature that has been rolling out gradually over the last few 0.4.x releases: Bidirectional Data-Binding

I am including below an explanation of the Observer Pattern and Data-Binding in Glimmer DSL for LibUI.

Afterwards, I conclude by sharing an example that takes full advantage of the data-binding features in Glimmer DSL for LibUI, a contact management example app called Form Table.

Happy Glimmering!

Observer Pattern

The Observer Design Pattern (a.k.a. Observer Pattern) is fundamental to building GUIs (Graphical User Interfaces) following the MVC (Model View Controller) Architectural Pattern or any of its variations like MVP (Model View Presenter). In the original Smalltalk-MVC, the View observes the Model for changes and updates itself accordingly.

MVC - Model View Controller

Glimmer DSL for LibUI supports the Observer Design Pattern via the observe(model, attribute_or_key=nil) keyword, which can observe Object models with attributes, Hashes with keys, and Arrays. It automatically enhances objects as needed to support automatically notifying observers of changes via observable#notify_observers(attribute_or_key = nil) method:

  • Object becomes Glimmer::DataBinding::ObservableModel, which supports observing specified Object model attributes.
  • Hash becomes Glimmer::DataBinding::ObservableHash, which supports observing all Hash keys or a specific Hash key
  • Array becomes Glimmer::DataBinding::ObservableArray, which supports observing Array changes like those done with push, <<, delete, and map! methods (all mutation methods).


  observe(person, :name) do |new_name|
    @name_label.text = new_name
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That observes a person's name attribute for changes and updates the name label text property accordingly.

Learn about Glimmer's Observer Pattern capabilities and options in more detail at the Glimmer project page.

See examples of the observe keyword at Color The Circles, Method-Based Custom Keyword, Snake, and Tetris.


Glimmer DSL for LibUI supports both bidirectional (two-way) data-binding and unidirectional (one-way) data-binding.

Data-binding enables writing very expressive, terse, and declarative code to synchronize View properties with Model attributes without writing many lines or pages of imperative code doing the same thing, increasing productivity immensely.

Data-binding automatically takes advantage of the Observer Pattern behind the scenes and is very well suited to declaring View property data sources piecemeal. On the other hand, explicit use of the Observer Pattern is sometimes more suitable when needing to make multiple View updates upon a single Model attribute change.

Data-binding supports utilizing the MVP (Model View Presenter) flavor of MVC by observing both the View and a Presenter for changes and updating the opposite side upon encountering them. This enables writing more decoupled cleaner code that keeps View code and Model code disentangled and highly maintainable. For example, check out the Snake game presenters for Grid and Cell, which act as proxies for the actual Snake game models Snake and Apple, mediating synchronization of data between them and the Snake View GUI.


Glimmer DSL for LibUI supports bidirectional (two-way) data-binding of the following controls/properties via the <=> operator (indicating data is moving in both directions between View and Model):

  • checkbox: checked
  • check_menu_item: checked
  • color_button: color
  • combobox: selected, selected_item
  • date_picker: time
  • date_time_picker: time
  • editable_combobox: text
  • entry: text
  • font_button: font
  • multiline_entry: text
  • non_wrapping_multiline_entry: text
  • radio_buttons: selected
  • radio_menu_item: checked
  • search_entry: text
  • slider: value
  • spinbox: value
  • table: cell_rows (explicit data-binding by using <=> and implicit data-binding by assigning value directly)
  • time_picker: time

Example of bidirectional data-binding:

entry {
  text <=> [contract, :legal_text]
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That is data-binding a contract's legal text to an entry text property.

Another example of bidirectional data-binding with an option:

entry {
  text <=> [self, :entered_text, after_write: ->(text) {puts text}]
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That is data-binding entered_text attribute on self to entry text property and printing text after write to the model.

Glimmer DSL for LibUI supports unidirectional (one-way) data-binding of any control/shape/attributed-string property via the <= operator (indicating data is moving from the right side, which is the Model, to the left side, which is the GUI View object).

Example of unidirectional data-binding:

square(0, 0, CELL_SIZE) {
  fill <= [@grid.cells[row][column], :color]
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That is data-binding a grid cell color to a square shape's fill property. That means if the color attribute of the grid cell is updated, the fill property of the square shape is automatically updated accordingly.

Another Example of unidirectional data-binding with an option:

window {
  title <= [@game, :score, on_read: -> (score) {"Glimmer Snake (Score: #{@game.score})"}]
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That is data-binding the window title property to the score attribute of a @game, but converting on read from the Model to a String.

To summarize the data-binding API:

  • view_property <=> [model, attribute, *read_or_write_options]: Bidirectional (two-way) data-binding to Model attribute accessor
  • view_property <= [model, attribute, *read_only_options]: Unidirectional (one-way) data-binding to Model attribute reader

This is also known as the Glimmer Shine syntax for data-binding, a Glimmer-only unique innovation that takes advantage of Ruby's highly expressive syntax and malleable DSL support.

Data-bound model attribute can be:

  • Direct: Symbol representing attribute reader/writer (e.g. [person, :name])
  • Nested: String representing nested attribute path (e.g. [company, 'address.street']). That results in "nested data-binding"
  • Indexed: String containing array attribute index (e.g. [customer, 'addresses[0].street']). That results in "indexed data-binding"

Data-binding options include:

  • before_read {|value| ...}: performs an operation before reading data from Model to update the View.
  • on_read {|value| ...}: converts value read from Model to update the View.
  • after_read {|converted_value| ...}: performs an operation after read from Model and updating the View.
  • before_write {|value| ...}: performs an operation before writing data to Model from View.
  • on_write {|value| ...}: converts value read from View to update the Model.
  • after_write {|converted_value| ...}: performs an operation after writing to Model from View.
  • computed_by attribute or computed_by [attribute1, attribute2, ...]: indicates model attribute is computed from specified attribute(s), thus updated when they are updated (see in Login example version 2). That is known as "computed data-binding".

Note that with both on_read and on_write converters, you could pass a Symbol representing the name of a method on the value object to invoke.


entry {
  text <=> [product, :price, on_read: :to_s, on_write: :to_i]
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Data-binding gotchas:

  • Never data-bind a control property to an attribute on the same view object with the same exact name (e.g. binding entry text property to self text attribute) as it would conflict with it. Instead, data-bind view property to an attribute with a different name on the view object or with the same name, but on a presenter or model object (e.g. data-bind entry text to self legal_text attribute or to contract model text attribute)
  • Data-binding a property utilizes the control's listener associated with the property (e.g. on_changed for entry text), so you cannot hook into the listener directly anymore as that would negate data-binding. Instead, you can add an after_write: ->(val) {} option to perform something on trigger of the control listener instead.

Learn more from data-binding usage in Login (4 data-binding versions), Basic Entry, Form, Form Table (5 data-binding versions), Method-Based Custom Keyword, Snake and Tic Tac Toe examples.

Form Table Example

Run with this command from the root of the project if you cloned the project:

ruby -r './lib/glimmer-dsl-libui' examples/form_table.rb
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Run with this command if you installed the Ruby gem:

ruby -r glimmer-dsl-libui -e "require 'examples/form_table'"
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Mac Windows Linux
glimmer-dsl-libui-mac-form-table.png glimmer-dsl-libui-windows-form-table.png glimmer-dsl-libui-linux-form-table.png

New Glimmer DSL for LibUI Version (with explicit data-binding):

require 'glimmer-dsl-libui'

class FormTable
  Contact =, :email, :phone, :city, :state)

  include Glimmer

  attr_accessor :contacts, :name, :email, :phone, :city, :state, :filter_value

  def initialize
    @contacts = ['Lisa Sky', '', '720-523-4329', 'Denver', 'CO'),'Jordan Biggins', '', '617-528-5399', 'Boston', 'MA'),'Mary Glass', '', '847-589-8788', 'Elk Grove Village', 'IL'),'Darren McGrath', '', '206-539-9283', 'Seattle', 'WA'),'Melody Hanheimer', '', '213-493-8274', 'Los Angeles', 'CA'),

  def launch
    window('Contacts', 600, 600) { |w|
      margined true

      vertical_box {
        form {
          stretchy false

          entry {
            label 'Name'
            text <=> [self, :name] # bidirectional data-binding between entry text and

          entry {
            label 'Email'
            text <=> [self, :email]

          entry {
            label 'Phone'
            text <=> [self, :phone]

          entry {
            label 'City'
            text <=> [self, :city]

          entry {
            label 'State'
            text <=> [self, :state]

        button('Save Contact') {
          stretchy false

          on_clicked do
            new_row = [name, email, phone, city, state]
            if new_row.include?('')
              msg_box_error(w, 'Validation Error!', 'All fields are required! Please make sure to enter a value for all fields.')
              @contacts <<*new_row) # automatically inserts a row into the table due to explicit data-binding
              @unfiltered_contacts = @contacts.dup
     = '' # automatically clears name entry through explicit data-binding
     = ''
     = ''
     = ''
              self.state = ''

        search_entry {
          stretchy false
          # bidirectional data-binding of text to self.filter_value with after_write option
          text <=> [self, :filter_value,
            after_write: ->(filter_value) { # execute after write to self.filter_value
              @unfiltered_contacts ||= @contacts.dup
              # Unfilter first to remove any previous filters
              self.contacts = @unfiltered_contacts.dup # affects table indirectly through explicit data-binding
              # Now, apply filter if entered
              unless filter_value.empty?
                self.contacts = @contacts.filter do |contact| # affects table indirectly through explicit data-binding
                  contact.members.any? do |attribute|

        table {

          editable true
          # explicit data-binding to Model Array (expects model attribute names to be underscored column names by convention [e.g. :state for State], can be customized with :column_attributes option [e.g. {'State/Province' => :state})
          cell_rows <=> [self, :contacts] 

          on_changed do |row, type, row_data|
            puts "Row #{row} #{type}: #{row_data}"
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