Execution context

Avo enables developers to hook into different points of the application lifecycle using blocks. That functionality can't always be performed in void but requires some pieces of state to set up some context.

Computed fields are one example.

field :full_name, as: :text do
  "#{record.first_name} #{record.last_name}"
end

In that block we need to pass the record so you can compile that value. We send more information than just the record, we pass on the resource, view, view_context, request, current_user and more depending on the block that's being run.

How does the ExecutionContext work?

The ExecutionContext is an object that holds some pieces of state on which we execute a lambda function.

module Avo
  class ExecutionContext

    attr_accessor :target, :context, :params, :view_context, :current_user, :request

    def initialize(**args)
      # If target don't respond to call, handle will return target
      # In that case we don't need to initialize the others attr_accessors
      return unless (@target = args[:target]).respond_to? :call

      args.except(:target).each do |key,value|
        singleton_class.class_eval { attr_accessor "#{key}" }
        instance_variable_set("@#{key}", value)
      end

      # Set defaults on not initialized accessors
      @context      ||= Avo::Current.context
      @params       ||= Avo::Current.params
      @view_context ||= Avo::Current.view_context
      @current_user ||= Avo::Current.current_user
      @request      ||= Avo::Current.request
    end

    delegate :authorize, to: Avo::Services::AuthorizationService

    # Return target if target is not callable, otherwise, execute target on this instance context
    def handle
      target.respond_to?(:call) ? instance_exec(&target) : target
    end
  end
end

# Use it like so.
SOME_BLOCK = -> {
  "#{record.first_name} #{record.last_name}"
}

Avo::ExecutionContext.new(target: &SOME_BLOCK, record: User.first).handle

This means you could throw any type of object at it and it it responds to a call method wil will be called with all those objects.

-> target

The block you'll pass to be evaluated. It may be anything but will only be evaluated if it responds to a call method.

-> context

Aliased to Avo::Current.context.

-> current_user

Aliased to Avo::Current.user.

-> view_context

Aliased to Avo::Current.view_context.

-> request

Aliased to Avo::Current.request.

-> params

Aliased to Avo::Current.params.

-> Custom variables

You can pass any variable to the ExecutionContext and it will be available in that block. This is how we can expose view, record, and resource in the computed field example.

Avo::ExecutionContext.new(target: &SOME_BLOCK, record: User.first, view: :index, resource: resource).handle

-> helpers

Within the ExecutionContext you might want to use some of your already defined helpers. You can do that using the helpers object.

# products_helper.rb
class ProductsHelper
  # Strips the "CODE_" prefix from the name
  def simple_name(name)
    name.gsub "CODE_", ""
  end
end

field :name, as: :text, format_using: -> { helpers.simple_name(value) }