View Models (a.k.a. Cells)

General description

A cell is an object that represent a fragment of your UI. The scope of that fragment can embrace an entire page, a single comment container in a thread or just an avatar image link.

Cells are faster than ActionView. While exposing a better performance, you step-wise encapsulate fragments into cell widgets and enforce interfaces.

View Model

Think of cells, or view models, as small Rails controllers, but without any HTTP coupling. Cells embrace all presentation and rendering logic to present a fragment of the UI.

Decidim Cards

card_for @instance will render the corresponding default card for each component instance. If a card for the given resource is not registered, a basic (default) card is shown.

To render a specified size/variation include the size option as a symbol: card_for @instance, size: :m

Card M

To render a label to identify the type of component renderized add to the context the label option.

The label option accepts this arguments:

  • false or "false" will not render the label from the locales t(model.class.model_name.i18n_key, scope: "activerecord.models", count: 1)

  • true or "true" will render the translation from

  • "whathever string" will render it as String

Introducing a Card Cell to a component

  • Add the module cell paths to cell view paths in decidim-<module>/lib/decidim/<module>/engine.rb

    initializer "decidim_<module>.add_cells_view_paths" do
      Cell::ViewModel.view_paths << File.expand_path("#{Decidim::<Module>::Engine.root}/app/cells")
      Cell::ViewModel.view_paths << File.expand_path("#{Decidim::<Module>::Engine.root}/app/views") # for partials
  • The attribute card of the resource is defined in decidim-core/lib/decidim/resource_manifest.rb:

    # The cell to use to render the card of a resource.
    attribute :card, String

    In your decidim-<component>/lib/decidim/<component>/component.rb register the resource and set the card value. Note that the model class for your resource must include the Decidim::Resourceable concern for this to work:

    component.register_resource(:<my_resource>) do |resource|
      resource.class = "Decidim::<Component>/<MyResource>" # eg. "Decidim::Proposals::ProposalDraft
      resource.card = "decidim/<component>/<my_resource>" # eg. "decidim/proposals/proposal_draft"
      resource.component_manifest = component
    module Decidim
      module <Component>
        class <MyResource> < Decidim::ApplicationRecord
          include Decidim::Resourceable
          # ...
  • The Cell Class, following the convention, will reside in decidim-<component>/app/cells/decidim/<component>/<my_resource>_cell.rb:

    module Decidim
      module <Component>s
        class <MyResource>Cell < Decidim::ViewModel
          def show
            render # renders decidim-<component>/app/cells/decidim/<component>/<my_resource>
  • The Cell Views will be in the decidim-<component>/app/cells/decidim/<component>/<my_resource> and defaults to show.erb