Add new helper API

docs/CHANGELOG.md

@@ -10,6 +10,10 @@ nav_order: 5
 
 ## main
 
+* Add `use_helper` API.
+
+    *Reegan Viljoen*
+
 * Fix bug where the `Rails` module wasn't being searched from the root namespace.
 
     *Zenéixe*

docs/guide/helpers.md

@@ -51,6 +51,26 @@ class UserComponent < ViewComponent::Base
 end
 ```
 
+## UseHelpers setter
+
+By default, ViewComponents don't have access to helper methods defined externally. The `use_helpers` method allows external helpers to be called from the component.
+
+To use the `use_helpers` method, include `ViewComponent::UseHelpers`.
+`UseHelpers` defines the helper on the component and is similar in use to using `delegate` on helpers.
+
+```ruby
+class UseHelpersComponent < ViewComponent::Base
+  include ViewComponent::UseHelpers
+  use_helpers :icon
+
+  erb_template <<-ERB
+    <div class="icon">
+      <%= icon :user %>
+    </div>
+  ERB
+end
+```
+
 ## Nested URL helpers
 
 Rails nested URL helpers implicitly depend on the current `request` in certain cases. Since ViewComponent is built to enable reusing components in different contexts, nested URL helpers should be passed their options explicitly:

lib/view_component/base.rb

@@ -12,6 +12,7 @@
 require "view_component/slotable"
 require "view_component/translatable"
 require "view_component/with_content_helper"
+require "view_component/use_helpers"
 
 module ViewComponent
   class Base < ActionView::Base

lib/view_component/use_helpers.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module ViewComponent::UseHelpers
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def use_helpers(*args)
+      args.each do |helper_method|
+        class_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+          def #{helper_method}(*args, &block)
+            raise HelpersCalledBeforeRenderError if view_context.nil?
+            __vc_original_view_context.#{helper_method}(*args, &block)
+          end
+        RUBY
+
+        ruby2_keywords(helper_method) if respond_to?(:ruby2_keywords, true)
+      end
+    end
+  end
+end

test/sandbox/app/components/use_helpers_component.html.erb

@@ -0,0 +1,3 @@
+<div class="helper__message">
+  <%= message %>
+</div>

test/sandbox/app/components/use_helpers_component.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class UseHelpersComponent < ViewComponent::Base
+  include ViewComponent::UseHelpers
+
+  use_helpers :message
+end

test/sandbox/test/rendering_test.rb

@@ -1094,4 +1094,9 @@ def test_content_security_policy_nonce
 
     assert_selector("script", text: "\n//<![CDATA[\n  \"alert('hello')\"\n\n//]]>\n", visible: :hidden)
   end
+
+  def test_use_helper
+    render_inline(UseHelpersComponent.new)
+    assert_selector ".helper__message", text: "Hello helper method"
+  end
 end