Add compile.view_component ActiveSupport::Notifications event for eager compilation at boot time.

docs/CHANGELOG.md

@@ -10,6 +10,10 @@ nav_order: 6
 
 ## main
 
+* Add `compile.view_component` ActiveSupport::Notifications event for eager compilation at boot time.
+
+    *Joel Hawksley*, *GitHub Copilot*
+
 ## 4.7.0
 
 * Fix stale content cache when slots are accessed before `render_in`.

docs/guide/instrumentation.md

@@ -28,6 +28,20 @@ end
 
 _Note: Enabling instrumentation negatively impacts the performance of ViewComponent._
 
+## Compile instrumentation
+
+Since 4.8.0
+{: .label }
+
+ViewComponent also instruments eager compilation at boot time via the `compile.view_component` event. This event is always emitted (no configuration needed) when `config.eager_load` is `true`:
+
+```ruby
+ActiveSupport::Notifications.subscribe("compile.view_component") do |event|
+  event.name     # => "compile.view_component"
+  event.duration # => 123.45 (milliseconds)
+end
+```
+
 ## Viewing instrumentation sums in the browser developer tools
 
 When using `render.view_component` with `config.server_timing = true` (default in development) in Rails 7, the browser developer tools display the sum total timing information in Network > Timing under the key `render.view_component`.

lib/view_component/engine.rb

@@ -79,7 +79,11 @@ class Engine < Rails::Engine # :nodoc:
 
     initializer "view_component.eager_load_actions" do
       ActiveSupport.on_load(:after_initialize) do
-        ViewComponent::Base.descendants.each(&:__vc_compile) if Rails.application.config.eager_load
+        if Rails.application.config.eager_load
+          ActiveSupport::Notifications.instrument("compile.view_component") do
+            ViewComponent::Base.descendants.each(&:__vc_compile)
+          end
+        end
       end
     end
 

test/sandbox/test/instrumentation_test.rb

@@ -32,4 +32,26 @@ def test_instrumentation_disabled
       assert_equal(events.size, 0)
     end
   end
+
+  def test_compile_instrumentation
+    events = []
+    subscriber = ActiveSupport::Notifications.subscribe("compile.view_component") do |*args|
+      events << ActiveSupport::Notifications::Event.new(*args)
+    end
+
+    old_eager_load = Rails.application.config.eager_load
+    Rails.application.config.eager_load = true
+
+    ViewComponent::CompileCache.invalidate!
+
+    # Execute the same block that the view_component.eager_load_actions initializer registers
+    initializer = ViewComponent::Engine.initializers.find { |i| i.name == "view_component.eager_load_actions" }
+    initializer.run(Rails.application)
+
+    assert_equal(1, events.size)
+    assert_equal("compile.view_component", events[0].name)
+  ensure
+    Rails.application.config.eager_load = old_eager_load
+    ActiveSupport::Notifications.unsubscribe(subscriber)
+  end
 end