Introduce Turbo::SystemTestHelper

Introduce the `Turbo::SystemTestHelper` module to be included into
[ActionDispatch::SystemTestCase][] when it's available.

The module is named to mimic [ActionText::SystemTestHelper][].

The module defines a `#wait_for_turbo_cable_stream_sources` helper
method extracted from this project's System Test suite. It aims to
synchronize the test harness with Turbo's Action Cable-powered broadcast
support. The method will find all `<turbo-cable-stream-source>` elements
that are present but not yet `[connected]` (returning the results
immediately with Capybara's `:wait`), then wait for them to connect
(using whatever Capybara's configured wait value).

[ActionDispatch::SystemTestCase]: https://edgeapi.rubyonrails.org/classes/ActionDispatch/SystemTestCase.html
[ActionText::SystemTestHelper]: https://edgeapi.rubyonrails.org/classes/ActionText/SystemTestHelper.html

lib/turbo/engine.rb

@@ -111,5 +111,12 @@ class TurboStreamEncoder < IdentityEncoder
         end
       end
     end
+
+    initializer "turbo.system_test_helper" do
+      ActiveSupport.on_load(:action_dispatch_system_test_case) do
+        require "turbo/system_test_helper"
+        include Turbo::SystemTestHelper
+      end
+    end
   end
 end

lib/turbo/system_test_helper.rb

@@ -0,0 +1,28 @@
+module Turbo::SystemTestHelper
+  extend ActiveSupport::Concern
+
+  # Delay until every `<turbo-cable-stream-source>` element present in the page
+  # is ready to receive broadcasts
+  #
+  #   test "Page receives new Messages broadcast over Action Cable" do
+  #     visit "/"
+  #     click_link "All Messages"
+  #
+  #     wait_for_turbo_cable_stream_sources
+  #     create_message_record_then_broadcast # do server-side broadcasting
+  #
+  #     assert_text "Hello from Action Cable"
+  #   end
+  #
+  # By default, calls to `#visit` will wait for all `<turbo-cable-stream-source>`
+  # elements to connect
+  def wait_for_turbo_cable_stream_sources(**options)
+    all(:css, "turbo-cable-stream-source:not([connected])", wait: 0).each do |element|
+      element.assert_matches_selector(:css, "[connected]", visible: :all, **options)
+    end
+  end
+
+  def visit(...) # :nodoc:
+    super.tap { wait_for_turbo_cable_stream_sources }
+  end
+end

test/system/broadcasts_test.rb

@@ -5,7 +5,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts Turbo Streams" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     assert_broadcasts_text "Message 1", to: :messages do |text, target|
       Message.create(content: text).broadcast_append_to(target)
@@ -14,7 +13,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts with html: render option" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     assert_broadcasts_text "Hello, with html: option", to: :messages do |text, target|
       Message.create(content: "Ignored").broadcast_append_to(target, html: text)
@@ -23,25 +21,22 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts with renderable: render option" do
     visit messages_path
-    wait_for_stream_to_be_connected
-
+
     assert_broadcasts_text "Test message", to: :messages do |text, target|
       Message.create(content: "Ignored").broadcast_append_to(target, renderable: MessageComponent.new(text))
     end
   end
-  
+
   test "Does not render the layout twice when passed a component" do
     visit messages_path
-    wait_for_stream_to_be_connected
-
+
     Message.create(content: "Ignored").broadcast_append_to(:messages, renderable: MessageComponent.new("test"))
-  
+
     assert_selector("title", count: 1, visible: false, text: "Dummy")
   end
 
   test "Message broadcasts with extra attributes to turbo stream tag" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     assert_broadcasts_text "Message 1", to: :messages do |text, target|
       Message.create(content: text).broadcast_action_to(target, action: :append, attributes: { "data-foo": "bar" })
@@ -50,7 +45,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts with correct extra attributes to turbo stream tag" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     assert_forwards_turbo_stream_tag_attribute attr_key: "data-foo", attr_value: "bar", to: :messages do |attr_key, attr_value, target|
       Message.create(content: text).broadcast_action_to(target, action: :test, attributes: { attr_key => attr_value })
@@ -59,7 +53,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts with no rendering" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     assert_forwards_turbo_stream_tag_attribute attr_key: "data-foo", attr_value: "bar", to: :messages do |attr_key, attr_value, target|
       Message.create(content: text).broadcast_action_to(target, action: :test, render: false, partial: "non_existant", attributes: { attr_key => attr_value })
@@ -68,7 +61,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts later with extra attributes to turbo stream tag" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     perform_enqueued_jobs do
       assert_broadcasts_text "Message 1", to: :messages do |text, target|
@@ -80,7 +72,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts later with correct extra attributes to turbo stream tag" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     perform_enqueued_jobs do
       assert_forwards_turbo_stream_tag_attribute attr_key: "data-foo", attr_value: "bar", to: :messages do |attr_key, attr_value, target|
@@ -91,7 +82,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Message broadcasts later with no rendering" do
     visit messages_path
-    wait_for_stream_to_be_connected
 
     perform_enqueued_jobs do
       assert_forwards_turbo_stream_tag_attribute attr_key: "data-foo", attr_value: "bar", to: :messages do |attr_key, attr_value, target|
@@ -102,7 +92,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "Users::Profile broadcasts Turbo Streams" do
     visit users_profiles_path
-    wait_for_stream_to_be_connected
 
     assert_broadcasts_text "Profile 1", to: :users_profiles do |text, target|
       Users::Profile.new(id: 1, name: text).broadcast_append_to(target)
@@ -111,7 +100,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   test "passing extra parameters to channel" do
     visit section_messages_path
-    wait_for_stream_to_be_connected
 
     assert_broadcasts_text "In a section", to: :messages do |text|
       Message.create(content: text).broadcast_append_to(:important_messages)
@@ -120,10 +108,6 @@ class BroadcastsTest < ApplicationSystemTestCase
 
   private
 
-  def wait_for_stream_to_be_connected
-    assert_selector "turbo-cable-stream-source[connected]", visible: false
-  end
-
   def assert_broadcasts_text(text, to:, &block)
     within(:element, id: to) { assert_no_text text }