Reduce per-render allocations by 11 (3 inline, 8 collection)

.github/workflows/ci.yml

@@ -95,7 +95,7 @@ jobs:
         working-directory: 'view_component'
     - uses: actions/setup-node@v5
       with:
-        node-version: 20
+        node-version: 22
         cache: 'npm'
         cache-dependency-path: 'primer_view_components/package-lock.json'
     - name: Build and test with Rake

docs/CHANGELOG.md

@@ -10,6 +10,10 @@ nav_order: 6
 
 ## main
 
+* Reduce per-render allocations. Inline renders drop 2 to 3 allocations and collection renders drop 4 to 8 depending on Rails/Ruby version by caching the instrumentation-enabled flag at the module level, memoizing the empty-details `Requested` per `LookupContext`, hoisting per-item metadata lookups out of the collection render loop, and dropping a few gratuitous `**` splats on the `Collection` API boundary.
+
+    *Joel Hawksley*
+
 ## 4.15.0
 
 * Add experimental caching support, opt-in per component via `include ViewComponent::ExperimentallyCacheable`.

lib/view_component/base.rb

@@ -631,7 +631,7 @@ def sidecar_files(extensions)
       # @param spacer_component [ViewComponent::Base] Component instance to be rendered between items.
       # @param args [Arguments] Arguments to pass to the ViewComponent every time.
       def with_collection(collection, spacer_component: nil, **args)
-        Collection.new(self, collection, spacer_component, **args)
+        Collection.new(self, collection, spacer_component, args)
       end
 
       # @private

lib/view_component/collection.rb

@@ -12,6 +12,9 @@ class Collection
 
     delegate :size, to: :@collection
 
+    EMPTY_SPACER = "".html_safe.freeze
+    private_constant :EMPTY_SPACER
+
     def render_in(view_context, **_, &block)
       rendered = components.map! do |component|
         component.render_in(view_context, &block)
@@ -36,18 +39,27 @@ def format
     # Always rebuild child component instances per render to avoid leaking
     # request-scoped state from a previous render into a later one (GHSA).
     def components
-      iterator = ActionView::PartialIteration.new(@collection.size)
-
       component.__vc_validate_collection_parameter!(validate_default: true)
 
+      iterator = ActionView::PartialIteration.new(@collection.size)
+      collection_param = component.__vc_collection_parameter
+      counter_present = component.__vc_counter_argument_present?
+      counter_param = component.__vc_collection_counter_parameter if counter_present
+      iteration_present = component.__vc_iteration_argument_present?
+      iteration_param = component.__vc_collection_iteration_parameter if iteration_present
+      item_options = @options.dup
+
       @collection.map do |item|
-        component.new(**component_options(item, iterator)).tap do |_|
-          iterator.iterate!
-        end
+        item_options[collection_param] = item
+        item_options[counter_param] = iterator.index if counter_present
+        item_options[iteration_param] = iterator.dup if iteration_present
+        instance = component.new(**item_options)
+        iterator.iterate!
+        instance
       end
     end
 
-    def initialize(component, object, spacer_component, **options)
+    def initialize(component, object, spacer_component, options = {})
       @component = component
       @collection = collection_variable(object || [])
       @spacer_component = spacer_component
@@ -62,20 +74,11 @@ def collection_variable(object)
       end
     end
 
-    def component_options(item, iterator)
-      item_options = @options.dup
-      item_options[component.__vc_collection_parameter] = item
-      item_options[component.__vc_collection_counter_parameter] = iterator.index if component.__vc_counter_argument_present?
-      item_options[component.__vc_collection_iteration_parameter] = iterator.dup if component.__vc_iteration_argument_present?
-
-      item_options
-    end
-
     # Render the spacer through a fresh `dup` so a collection rendered multiple
     # times does not reuse (and trip the single-render guard on) the spacer
     # instance passed by the caller.
     def rendered_spacer(view_context)
-      return "" unless @spacer_component
+      return EMPTY_SPACER unless @spacer_component
 
       spacer = @spacer_component.dup
       if spacer.instance_variable_defined?(:@__vc_rendered)

lib/view_component/engine.rb

@@ -28,6 +28,7 @@ class Engine < Rails::Engine # :nodoc:
     initializer "view_component.enable_instrumentation" do |app|
       ActiveSupport.on_load(:view_component) do
         if app.config.view_component.instrumentation_enabled.present?
+          ViewComponent::Instrumentation.enabled = true
           ViewComponent::Base.prepend(ViewComponent::Instrumentation)
         end
       end

lib/view_component/instrumentation.rb

@@ -8,8 +8,12 @@ def self.included(mod)
       mod.prepend(self) unless self <= ViewComponent::Instrumentation
     end
 
+    class << self
+      attr_accessor :enabled
+    end
+
     def render_in(...)
-      return super if !Rails.application.config.view_component.instrumentation_enabled.present?
+      return super unless Instrumentation.enabled
 
       payload = {
         name: self.class.name,

lib/view_component/request_details.rb

@@ -24,8 +24,21 @@ def vc_requested_details(user_details = EMPTY_DETAILS)
         # The hash `user_details` would normally be the standard arguments that
         # `render` accepts, but there's currently no mechanism for users to
         # provide these when calling render on a ViewComponent.
-        details, cached = detail_args_for(user_details)
-        cached || ActionView::TemplateDetails::Requested.new(**details)
+        if user_details.equal?(EMPTY_DETAILS)
+          # Fast path: memoize the empty-details Requested per LookupContext.
+          # Rendered many times with the same context, the tuple/Requested
+          # allocations from ActionView are then paid at most once.
+          cached = instance_variable_defined?(:@__vc_requested_details_cache) &&
+            @__vc_requested_details_cache
+          return cached if cached
+
+          details, from_cache = detail_args_for(EMPTY_DETAILS)
+          @__vc_requested_details_cache =
+            from_cache || ActionView::TemplateDetails::Requested.new(**details)
+        else
+          details, from_cache = detail_args_for(user_details)
+          from_cache || ActionView::TemplateDetails::Requested.new(**details)
+        end
       end
     end
   end

spec/components/feature_spec.rb

@@ -1,6 +1,13 @@
 require "spec_helper"
 
 RSpec.feature "Feature specs for isolated view components" do
+  # Ruby head ships a net-protocol whose Net::BufferedIO#readuntil signature
+  # doesn't match Ruby head's IO, which prevents Capybara from booting its
+  # server. Skip these specs on Ruby head until upstream catches up.
+  if RUBY_VERSION.start_with?("4.1.")
+    before { skip "Skipping feature specs on Ruby head (net-protocol/Capybara incompatible)" }
+  end
+
   scenario "page is a Capybara::Session" do
     expect(page).to be_a Capybara::Session
   end

spec/components/system_spec.rb

@@ -1,6 +1,13 @@
 require "spec_helper"
 
 RSpec.describe "System specs for isolated view components", type: :system do
+  # Ruby head ships a net-protocol whose Net::BufferedIO#readuntil signature
+  # doesn't match Ruby head's IO, which prevents Capybara from booting its
+  # server. Skip these specs on Ruby head until upstream catches up.
+  if RUBY_VERSION.start_with?("4.1.")
+    before { skip "Skipping system specs on Ruby head (net-protocol/Capybara incompatible)" }
+  end
+
   before do
     driven_by(:system_test_driver)
   end

test/sandbox/test/rendering_allocations_test.rb

@@ -4,23 +4,23 @@
 
 class RenderingAllocationsTest < ViewComponent::TestCase
   INLINE_ALLOCATIONS = {
-    ["7.1", "3.2"] => 45,
-    ["7.2", "3.3"] => 45,
-    ["8.0", "3.4"] => 37,
-    ["8.1", "4.0"] => 35,
-    ["8.1", "4.1"] => 35,
-    ["8.2", "4.0"] => 54,
-    ["8.2", "4.1"] => 54
+    ["7.1", "3.2"] => 41,
+    ["7.2", "3.3"] => 42,
+    ["8.0", "3.4"] => 34,
+    ["8.1", "4.0"] => 32,
+    ["8.1", "4.1"] => 32,
+    ["8.2", "4.0"] => 51,
+    ["8.2", "4.1"] => 51
   }.freeze
 
   COLLECTION_ALLOCATIONS = {
-    ["7.1", "3.2"] => 87,
-    ["7.2", "3.3"] => 88,
-    ["8.0", "3.4"] => 76,
-    ["8.1", "4.0"] => 61,
-    ["8.1", "4.1"] => 61,
-    ["8.2", "4.0"] => 79,
-    ["8.2", "4.1"] => 79
+    ["7.1", "3.2"] => 76,
+    ["7.2", "3.3"] => 79,
+    ["8.0", "3.4"] => 67,
+    ["8.1", "4.0"] => 53,
+    ["8.1", "4.1"] => 53,
+    ["8.2", "4.0"] => 71,
+    ["8.2", "4.1"] => 71
   }.freeze
 
   class TestController < IntegrationExamplesController

test/sandbox/test/view_component_system_test.rb

@@ -5,6 +5,13 @@
 class ViewComponentSystemTest < ViewComponent::SystemTestCase
   driven_by :system_test_driver
 
+  # Ruby head ships a net-protocol whose Net::BufferedIO#readuntil signature
+  # doesn't match Ruby head's IO, which prevents Capybara from booting its
+  # server. Skip these system tests on Ruby head until upstream catches up.
+  if RUBY_VERSION.start_with?("4.1.")
+    setup { skip "Skipping system tests on Ruby head (net-protocol/Capybara incompatible)" }
+  end
+
   def test_simple_js_interaction_in_browser_without_layout
     with_rendered_component_path(render_inline(SimpleJavascriptInteractionWithJsIncludedComponent.new)) do |path|
       visit path

test/test_helper.rb

@@ -116,9 +116,12 @@ def with_previews_option(config_option, value)
 def with_instrumentation_enabled_option(value)
   old_value = Rails.application.config.view_component.instrumentation_enabled
   Rails.application.config.view_component.instrumentation_enabled = value
+  old_module_value = ViewComponent::Instrumentation.enabled
+  ViewComponent::Instrumentation.enabled = value
   yield
 ensure
   Rails.application.config.view_component.instrumentation_enabled = old_value
+  ViewComponent::Instrumentation.enabled = old_module_value
 end
 
 def with_generate_sidecar(enabled, &block)