Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

lib/integrations/slack/incoming_message_builder.rb

@@ -0,0 +1,78 @@
+class Integrations::Slack::IncomingMessageBuilder
+  attr_reader :params
+
+  SUPPORTED_EVENT_TYPES = %w[event_callback url_verification].freeze
+  SUPPORTED_EVENTS = %w[message].freeze
+  SUPPORTED_MESSAGE_TYPES = %w[rich_text].freeze
+
+  def initialize(params)
+    @params = params
+  end
+
+  def perform
+    return unless valid_event?
+
+    if hook_verification?
+      verify_hook
+    elsif create_message?
+      create_message
+    end
+  end
+
+  private
+
+  def valid_event?
+    supported_event_type? && supported_event?
+  end
+
+  def supported_event_type?
+    SUPPORTED_EVENT_TYPES.include?(params[:type])
+  end
+
+  def supported_event?
+    hook_verification? || SUPPORTED_EVENTS.include?(params[:event][:type])
+  end
+
+  def supported_message?
+    SUPPORTED_MESSAGE_TYPES.include?(message[:type])
+  end
+
+  def hook_verification?
+    params[:type] == 'url_verification'
+  end
+
+  def create_message?
+    supported_message? && integration_hook
+  end
+
+  def message
+    params[:event][:blocks].first
+  end
+
+  def verify_hook
+    {
+      challenge: params[:challenge]
+    }
+  end
+
+  def integration_hook
+    @integration_hook ||= Integrations::Hook.where(reference_id: params[:event][:channel])
+  end
+
+  def conversation
+    @conversation ||= Conversation.where(identifier: params[:event][:thread_ts]).first
+  end
+
+  def create_message
+    return unless conversation
+
+    conversation.messages.create(
+      message_type: 0,
+      account_id: conversation.account_id,
+      inbox_id: conversation.inbox_id,
+      content: message[:elements].first[:elements].first[:text]
+    )
+
+    { status: 'success' }
+  end
+end