Add docs about QA page objects and pages DSL

qa/qa/page/README.md

+# Page objects in GitLab QA
+
+In GitLab QA we are using a known pattern, called _Page Objects_.
+
+This means that we have built an abstraction for all GitLab pages that we use
+to drive GitLab QA scenarios. Whenever we do something on a page, like filling
+in a form, or clicking a button, we do that only through a page object
+associated with this area of GitLab.
+
+For example, when GitLab QA test harness signs in into GitLab, it needs to fill
+in a user login and user password. In order to do that, we have a class, called
+`Page::Main::Login` and `sign_in_using_credentials` methods, that is the only
+piece of the code, that has knowledge about `user_login` and `user_password`
+fields.
+
+## Why do we need that?
+
+We need page objects, because we need to reduce duplication and avoid problems
+whenever someone changes some selectors in GitLab's source code.
+
+Imagine that we have a hundred specs in GitLab QA, and we need to sign into
+GitLab each time, before we make assertions. Without page object one would need
+to rely on volatile helpers or invoke Capybara methods directly. Imagine
+invoking `fill_in :user_login` in every `*_spec.rb` file / test example.
+
+When someone later changes `t.text_field :login` in the view associated with
+this page to `t.text_field :username` it will generate different field ID,
+what would effectively break all 100 tests.
+
+Because we are now using `Page::Main::Login.act { sign_in_using_credentials }`
+everywhere, where we want to sign into GitLab, page object is the single source
+of truth, and we will need to update `fill_in :user_login`
+to `fill_in :user_username` only in a one place as well.
+
+## What problems did we have in the past?
+
+We do not run QA tests for every commit, because of performance reasons, and
+the time it would take to build packages and test everything.
+
+That is why when someone changes `t.text_field :login` to
+`t.text_field :username` in new session view we won't know about this change
+until our GitLab QA nightly pipeline runs, or someone triggers `package-qa`
+action in their merge request.
+
+Obviously such a change would break all tests. We call this problem a _fragile
+tests problem_.
+
+In order to make GitLab QA more reliable and robust, we had to solve this
+problem by introducing coupling between GitLab CE / EE views and GitLab QA.
+
+## How did we solve fragile tests problem?
+
+Currently, when you add a new `Page::Base` derived class, you will also need to
+define all selectors that your page objects depends on.
+
+Whenever your push your code to CE / EE repository, `qa:selectors` sanity test
+job is going to be run as a part of a CI pipeline.
+
+This test is going to validate all page objects that we have implemented in
+`qa/page` directory. When if fails, you will be notified about missing
+or invalid views / selectors definition.
+
+## How to properly implement a page object?
+
+We have built a DSL to define coupling between a page object and GitLab views
+it is actually implemented by. See an example below.
+
+```ruby
+module Page
+  module Main
+    class Login < Page::Base
+      view 'app/views/devise/passwords/edit.html.haml' do
+        element :password_field, 'password_field :password'
+        element :password_confirmation, 'password_field :password_confirmation'
+        element :change_password_button, 'submit "Change your password"'
+      end
+
+      view 'app/views/devise/sessions/_new_base.html.haml' do
+        element :login_field, 'text_field :login'
+        element :passowrd_field, 'password_field :password'
+        element :sign_in_button, 'submit "Sign in"'
+      end
+
+      # ...
+  end
+end
+```
+
+It is possible to use `element` DSL method without value, with a String value
+or with a Regexp.
+
+```ruby
+view 'app/views/my/view.html.haml' do
+  # Require `f.submit "Sign in"` to be present in `my/view.html.haml
+  element :my_button, 'f.submit "Sign in"'
+
+  # Match every line in `my/view.html.haml` against
+  # `/link_to .* "My Profile"/` regexp.
+  element :profile_link, /link_to .* "My Profile"/
+
+  # Implicitly require `.qa-logout-button` CSS class to be present in the view
+  element :logout_button
+end
+```
+
+## Where to ask for help?
+
+If you need more information, ask for help on `#qa` channel on Slack (GitLab
+Team only).
+
+If you are not a Team Member, and you still need help to contribute, please
+open an issue in GitLab QA issue tracker.