Add ActiveSupport::Notifications instrumentation to compiler

Emit 'compile.view_component' notification when components are compiled,
allowing gems to extend ViewComponent without monkey-patching.

The notification payload includes:
- component: The component class being compiled
- compiler: The compiler instance

Author: jsolas

docs/CHANGELOG.md

@@ -10,6 +10,12 @@ nav_order: 6
 
 ## main
 
+## 4.0.0.rc6
+
+* Add `compile.view_component` ActiveSupport::Notifications instrumentation, enabling gems to safely extend the compiler without monkey-patching.
+
+    *Jose Solás*
+
 * Setup Trusted Publishing to RubyGems to improve software supply chain safety.
 
     *Hans Lemuet*

lib/view_component/compiler.rb

@@ -24,34 +24,40 @@ def compile(raise_errors: false, force: false)
       return if @component == ViewComponent::Base
 
       @lock.synchronize do
-        # this check is duplicated so that concurrent compile calls can still
-        # early exit
+        # This check is duplicated so that concurrent compile calls can still
+        # early exit before we instrument.
         return if compiled? && !force
 
-        gather_templates
+        ActiveSupport::Notifications.instrument(
+          "compile.view_component",
+          component: @component,
+          compiler: self
+        ) do |payload|
+          gather_templates
 
-        if self.class.__vc_development_mode && @templates.any?(&:requires_compiled_superclass?)
-          @component.superclass.__vc_compile(raise_errors: raise_errors)
-        end
-
-        if template_errors.present?
-          raise TemplateError.new(template_errors) if raise_errors
-
-          # this return is load bearing, and prevents the component from being considered "compiled?"
-          return false
-        end
+          if self.class.__vc_development_mode && @templates.any?(&:requires_compiled_superclass?)
+            @component.superclass.__vc_compile(raise_errors: raise_errors)
+          end
 
-        if raise_errors
-          @component.__vc_validate_initialization_parameters!
-          @component.__vc_validate_collection_parameter!
+          if template_errors.present?
+            raise TemplateError.new(template_errors) if raise_errors
+  
+            # this return is load bearing, and prevents the component from being considered "compiled?"
+            return false
+          end
+  
+          if raise_errors
+            @component.__vc_validate_initialization_parameters!
+            @component.__vc_validate_collection_parameter!
+          end
+  
+          define_render_template_for
+  
+          @component.__vc_register_default_slots
+          @component.__vc_build_i18n_backend
+  
+          CompileCache.register(@component)
         end
-
-        define_render_template_for
-
-        @component.__vc_register_default_slots
-        @component.__vc_build_i18n_backend
-
-        CompileCache.register(@component)
       end
     end
 

test/sandbox/test/base_test.rb

@@ -195,4 +195,48 @@ def test_uses_module_configuration
     assert_equal false, TestAlreadyConfigurableModule::SomeComponent.instrumentation_enabled
     assert_equal false, TestAlreadyConfiguredModule::SomeComponent.instrumentation_enabled
   end
+
+  def test_compiler_emits_notification_on_compile
+    notification_received = false
+    component_received = nil
+    compiler_received = nil
+
+    # Subscribe to the notification
+    subscription = ActiveSupport::Notifications.subscribe("compile.view_component") do |name, start, finish, id, payload|
+      notification_received = true
+      component_received = payload[:component]
+      compiler_received = payload[:compiler]
+    end
+
+    begin
+      # Compile a component to trigger the notification (force to ensure it runs)
+      ViewComponent::Compiler.new(EmptyComponent).compile(force: true)
+
+      # Verify the notification was emitted
+      assert notification_received, "Expected compile.view_component notification to be emitted"
+      assert_equal EmptyComponent, component_received
+      assert_instance_of ViewComponent::Compiler, compiler_received
+    ensure
+      # Clean up the subscription
+      ActiveSupport::Notifications.unsubscribe(subscription)
+    end
+  end
+
+  def test_compiler_does_not_emit_notification_when_already_compiled
+    # Ensure the component is compiled once
+    ViewComponent::Compiler.new(EmptyComponent).compile
+
+    notification_count = 0
+    subscription = ActiveSupport::Notifications.subscribe("compile.view_component") do
+      notification_count += 1
+    end
+
+    begin
+      # This should not emit a notification as it's already compiled (no `force`)
+      ViewComponent::Compiler.new(EmptyComponent).compile
+      assert_equal 0, notification_count, "Expected no notification for an already compiled component"
+    ensure
+      ActiveSupport::Notifications.unsubscribe(subscription)
+    end
+  end
 end