feat: extract and move implementation to the gem

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Ruby version
+2. Rails version
+3. Run command
+4. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Context (please complete the following information):**
+ - OS: [e.g. Ubuntu 18.04.3]
+ - Version [e.g. binary 4.3.2]
+
+**Additional context**
+Add any other context about the problem here.

.github/PULL_REQUEST_TEMPLATE.md

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+# Context
+
+<!--
+Short description about the feature and the motivation/issue behind it
+-->
+
+## Related tickets
+
+
+
+# What's inside
+
+<!--
+List of features and changes (or highlights) (from the code perspective)
+The purpose of this list is to track the progress if it's WIP (use checkboxes)
+and to emphasize the critical parts (which you'd like to pay reviewers attention to)
+-->
+- [x] A
+- [ ] B
+
+# Checklist:
+
+- [ ] I have added tests
+- [ ] I have made corresponding changes to the documentation

.github/workflows/ruby.yml

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+name: Ruby
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ '**' ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [ '2.6', '2.7', '3.0' ]
+    env:
+      RUBY_IMAGE: ${{ matrix.ruby }}
+    name: Ruby ${{ matrix.ruby }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+      - name: Install dependencies
+        run: |
+          gem install dip
+          dip provision
+      - name: Run tests
+        run: dip appraisal rspec

.gitignore

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+/.bundle/
+/.yardoc
+/_yardoc/
+/coverage/
+/doc/
+/gemfiles/
+/pkg/
+/spec/reports/
+/tmp/
+.pry_history
+.rspec_status
+Gemfile.lock
+lefthook-local.yml

.rspec

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+--format progress
+--color
+--require spec_helper

.standard.yml

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+ruby_version: 2.6

Appraisals

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+# frozen_string_literal: true
+
+if RUBY_VERSION < "3"
+  appraise "rails-5" do
+    gem "rails", "~> 5.0"
+  end
+end
+
+appraise "rails-6" do
+  gem "rails", "~> 6.0"
+end
+
+if RUBY_VERSION >= "2.7"
+  appraise "rails-7" do
+    gem "rails", "~> 7.0.0.alpha2"
+  end
+end

Gemfile

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+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gemspec

LICENSE.txt

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+The MIT License (MIT)
+
+Copyright (c) 2018 Michael Merkushin
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

README.md

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+[![Gem Version](https://badge.fury.io/rb/downstream.svg)](https://badge.fury.io/rb/downstream)
+[![Build Status](https://github.com/bibendi/downstream/workflows/Ruby/badge.svg?branch=master)](https://github.com/bibendi/downstream/actions?query=branch%3Amaster)
+
+# Downstream
+
+This gem provides a straightforward way to implement communication between Rails Engines using the Publish-Subscribe pattern. The gem allows decreasing decoupling engines with events. An event is a recorded object in the system that reflects an action that the engine performs, and the params that lead to its creation.
+
+The gem inspired by [`active_event_store`](https://github.com/palkan/active_event_store), and initially based on its codebase. Having said that, it does not store in a database all happened events which ensures simplicity and performance.
+
+<a href="https://evilmartians.com/?utm_source=bibendi-downstream">
+<img src="https://evilmartians.com/badges/sponsored-by-evil-martians.svg" alt="Sponsored by Evil Martians" width="236" height="54"></a>
+
+## Installation
+
+Add this line to your application's Gemfile:
+
+```ruby
+gem "downstream", "~> 1.0"
+```
+
+## Usage
+
+Under the hood it's a wrapper for `ActiveSupport::Notifications`. But it provides a way more handy interface to build reactive apps. Each event has a strict schema described by a separate class. Also, the gem has convenient tooling to write tests.
+
+### Describe events
+
+Events are represented by _event classes_, which describe events payloads and identifiers:
+
+```ruby
+class ProfileCreated < Downstream::Event
+  # (optional)
+  # Event identifier is used for streaming events to subscribers.
+  # By default, identifier is equal to underscored class name.
+  # You don't need to specify identifier manually, only for backward compatibility when
+  # class name is changed.
+  self.identifier = "profile_created"
+
+  # Add attributes accessors
+  attributes :user
+end
+```
+
+Each event has predefined (_reserved_) fields:
+- `event_id` – unique event id
+- `type` – event type (=identifier)
+
+**NOTE:** events should be in the past tense and describe what happened (e.g. "ProfileCreated", "EventPublished", etc.).
+
+Events are stored in `app/events` folder.
+
+### Publish events
+
+To publish an event you must first create an instance of the event class and call `Downstream.publish` method:
+
+```ruby
+event = ProfileCompleted.new(user: user)
+
+# then publish the event
+Downstream.publish(event)
+```
+
+That's it! Your event has been stored and propagated.
+
+### Subscribe to events
+
+To subscribe a handler to an event you must use `Downstream.subscribe` method.
+
+You should do this in your app or engine initializer:
+
+```ruby
+# some/engine.rb
+
+initializer "my_engine.subscribe_to_events" do
+  # To make sure event store is initialized use load hook
+  # `store` == `Downstream`
+  ActiveSupport.on_load "downstream-events" do |store|
+    store.subscribe MyEventHandler, to: ProfileCreated
+
+    # anonymous handler (could only be synchronous)
+    store.subscribe(to: ProfileCreated) do |name, event|
+      # do something
+    end
+
+    # you can omit event if your subscriber follows the convention
+    # for example, the following subscriber would subscribe to
+    # ProfileCreated event
+    store.subscribe OnProfileCreated::DoThat
+  end
+end
+```
+
+**NOTE:** event handler **must** be a callable object.
+
+Although subscriber could be any callable Ruby object, that have specific input format (event); thus we suggest putting subscribers under `app/subscribers/on_<event_type>/<subscriber.rb>`, e.g. `app/subscribers/on_profile_created/create_chat_user.rb`).
+
+## Testing
+
+You can test subscribers as normal Ruby objects.
+
+To test that a given subscriber exists, you can do the following:
+
+```ruby
+it "is subscribed to some event" do
+  allow(MySubscriberService).to receive(:call)
+
+  event = MyEvent.new(some: "data")
+
+  Downstream.publish event
+
+  expect(MySubscriberService).to have_received(:call).with(event)
+end
+```
+
+To test publishing use `have_published_event` matcher:
+
+```ruby
+expect { subject }.to have_published_event(ProfileCreated).with(user: user)
+```
+
+**NOTE:** `have_published_event` only supports block expectations.
+
+**NOTE 2** `with` modifier works like `have_attributes` matcher (not `contain_exactly`);

Rakefile

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+# frozen_string_literal: true
+
+require "bundler/setup"
+require "downstream"
+require_relative "spec/rails_helper"
+
+begin
+  require "rspec/core/rake_task"
+  RSpec::Core::RakeTask.new(:spec)
+rescue LoadError
+end

bin/rails

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+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "bundler/setup"
+
+ENGINE_ROOT = File.expand_path("..", __dir__)
+ENGINE_PATH = File.expand_path("../lib/downstream/engine", __dir__)
+
+require_relative "../spec/rails_helper"
+
+require "rails/engine/commands"

dip.yml

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+version: '7'
+
+compose:
+  files:
+    - docker-compose.yml
+
+interaction:
+  shell:
+    description: Open the Bash shell in app's container
+    service: ruby
+    command: /bin/bash
+
+  bundle:
+    description: Run Bundler commands
+    service: ruby
+    command: bundle
+
+  rails:
+    description: Open Rails console
+    service: ruby
+    command: ./bin/rails
+
+  appraisal:
+    description: Run Appraisal commands
+    service: ruby
+    command: bundle exec appraisal
+
+  rspec:
+    description: Run Rspec commands
+    service: ruby
+    command: bundle exec rspec
+
+  standardrb:
+    description: Run Standard linter
+    service: ruby
+    command: bundle exec standardrb
+
+provision:
+  - cp -f lefthook-local.dip_example.yml lefthook-local.yml
+  - dip compose down --volumes
+  - rm -f Gemfile.lock gemfiles/*
+  - dip bundle install
+  - dip appraisal install

docker-compose.yml

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+services:
+  ruby:
+    image: ruby:${RUBY_IMAGE:-3.0}-buster
+    environment:
+      - HISTFILE=/app/tmp/.bash_history
+      - BUNDLE_PATH=/bundle
+    command: bash
+    working_dir: /app
+    volumes:
+      - .:/app:cached
+      - bundler_data:/bundle
+    tmpfs:
+      - /tmp
+
+volumes:
+  bundler_data: