Add ability to define templates that take parameters

```ruby
class TestComponent < ViewComponent::Base
  template_arguments :list, :multiple # call_list now takes a `multiple` keyword argument
  def initialize(mode:)
    @mode = mode
  end

  def call
    case @mode
    when :list
      call_list multiple: false
    when :multilist
      call_list multiple: true
    when :summary
      call_summary
    end
  end
end
```

docs/guide/templates.md

@@ -126,4 +126,26 @@ class TestComponent < ViewComponent::Base
     end
   end
 end
+```
+
+You can define templates that take parameters, using the `template_arguments` method.
+
+```ruby
+class TestComponent < ViewComponent::Base
+  template_arguments :list, :multiple # call_list now takes a `multiple` keyword argument
+  def initialize(mode:)
+    @mode = mode
+  end
+
+  def call
+    case @mode
+    when :list
+      call_list multiple: false
+    when :multilist
+      call_list multiple: true
+    when :summary
+      call_summary
+    end
+  end
+end
 ```

lib/view_component/base.rb

@@ -393,6 +393,26 @@ def compiler
         @__vc_compiler ||= Compiler.new(self)
       end
 
+      # Contains the arguments for additional templates
+      # @return [Hash{String => Array<Symbol>}]
+      # @private
+      def template_arguments_hash
+        @template_arguments_hash ||= {}
+      end
+
+      # Declares arguments that need to be passed to a template.
+      #
+      # If a sidecar template needs additional locals that need to be passed
+      # at call site, then this method should be used.
+      #
+      # The signature for the resulting method will use keyword arguments
+      #
+      # @param [Symbol, String] template The template that needs arguments
+      # @param [Array<Symbol>] *args The arguments for the template
+      def template_arguments(template, *args)
+        compiler.add_template_arguments(template, *args)
+      end
+
       # we'll eventually want to update this to support other types
       # @private
       def type

lib/view_component/compiler.rb

@@ -41,29 +41,21 @@ def compile(raise_errors: false)
       end
 
       templates.each do |template|
+        method_name, method_args = method_name_and_args_for template
+
         # Remove existing compiled template methods,
         # as Ruby warns when redefining a method.
-        pieces = File.basename(template[:path]).split(".")
-
-        method_name =
-          # If the template matches the name of the component,
-          # set the method name with call_method_name
-          if pieces.first == component_class.name.demodulize.underscore
-            call_method_name(template[:variant])
-          # Otherwise, append the name of the template to
-          # call_method_name
-          else
-            "#{call_method_name(template[:variant])}_#{pieces.first.to_sym}"
-          end
-
         if component_class.instance_methods.include?(method_name.to_sym)
           component_class.send(:undef_method, method_name.to_sym)
         end
 
-        component_class.class_eval <<-RUBY, template[:path], -1
-          def #{method_name}
+        component_class.class_eval <<-RUBY, template[:path], -2
+          def #{method_name}(#{method_args.join(", ")})
+            old_buffer = @output_buffer if defined? @output_buffer
             @output_buffer = ActionView::OutputBuffer.new
             #{compiled_template(template[:path])}
+          ensure
+            @output_buffer = old_buffer
           end
         RUBY
       end
@@ -75,10 +67,27 @@ def #{method_name}
       CompileCache.register(component_class)
     end
 
+    def add_template_arguments(template, *args)
+      template = template.to_s
+      raise ArgumentError, "Arguments already defined for template #{template}" if template_arguments.key?(template)
+
+      template_exists = templates.any? { |t| t[:base_name].split(".").first == template }
+      raise ArgumentError, "Template does not exist: #{template}" unless template_exists
+
+      template_arguments[template] = args
+    end
+
     private
 
     attr_reader :component_class
 
+    # Contains the arguments for additional templates
+    # @return [Hash{String => Array<Symbol>}]
+    # @private
+    def template_arguments
+      @template_arguments ||= {}
+    end
+
     def define_render_template_for
       if component_class.instance_methods.include?(:render_template_for)
         component_class.send(:undef_method, :render_template_for)
@@ -199,6 +208,18 @@ def inline_calls
         end
     end
 
+    def method_name_and_args_for(template)
+      pieces = template[:base_name].split(".")
+      if pieces.first == component_class.name.demodulize.underscore
+        [call_method_name(template[:variant]), []]
+      else
+        [
+          "#{call_method_name(template[:variant])}_#{pieces.first.to_sym}",
+          (template_arguments || {}).fetch(pieces.first, []).map { |arg| "#{arg}:" }
+        ]
+      end
+    end
+
     def inline_calls_defined_on_self
       @inline_calls_defined_on_self ||= component_class.instance_methods(false).grep(/^call/)
     end

test/sandbox/app/components/multiple_templates_component.rb

@@ -1,18 +1,10 @@
 # frozen_string_literal: true
 
 class MultipleTemplatesComponent < ViewComponent::Base
-  def initialize(mode:)
-    @mode = mode
-
+  def initialize
     @items = ["Apple", "Banana", "Pear"]
   end
 
-  def call
-    case @mode
-    when :list
-      call_list
-    when :summary
-      call_summary
-    end
-  end
+  template_arguments :list, :number
+  template_arguments :summary, :string
 end

test/sandbox/app/components/multiple_templates_component/list.html.erb

@@ -1,4 +1,4 @@
-<ul>
+<ul data-number="<%= number %>">
   <% @items.each do |item| %>
     <li><%= item %></li>
   <% end %>

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

@@ -0,0 +1,4 @@
+<div class="container">
+  <%= call_summary string: "foo" %>
+  <%= call_list number: 1 %>
+</div>

test/sandbox/app/components/multiple_templates_component/summary.html.erb

@@ -1 +1 @@
-<div>The items are: <%= @items.to_sentence %></div>
+<div>The items are: <%= @items.to_sentence %>, <%= string %></div>

test/view_component/base_test.rb

@@ -77,4 +77,35 @@ def test_sidecar_files
       TranslatableComponent._sidecar_files(["yml"])
     )
   end
+
+  def test_template_arguments_validates_existence
+    error = assert_raises ArgumentError do
+      Class.new(ViewComponent::Base) do
+        def self._sidecar_files(*)
+          [
+            "/Users/fake.user/path/to.templates/component/test_component/test_component.html.erb",
+            "/Users/fake.user/path/to.templates/component/test_component/sidecar.html.erb",
+          ]
+        end
+        template_arguments :non_existing, [:foo]
+      end
+    end
+    assert_equal "Template does not exist: non_existing", error.message
+  end
+
+  def test_template_arguments_validates_duplicates
+    error = assert_raises ArgumentError do
+      Class.new(ViewComponent::Base) do
+        def self._sidecar_files(*)
+          [
+            "/Users/fake.user/path/to.templates/component/test_component/test_component.html.erb",
+            "/Users/fake.user/path/to.templates/component/test_component/sidecar.html.erb",
+          ]
+        end
+        template_arguments :sidecar, [:foo]
+        template_arguments :sidecar, [:bar]
+      end
+    end
+    assert_equal "Arguments already defined for template sidecar", error.message
+  end
 end

test/view_component/view_component_test.rb

@@ -749,15 +749,13 @@ def test_collection_component_with_trailing_comma_attr_reader
   end
 
   def test_render_multiple_templates
-    render_inline(MultipleTemplatesComponent.new(mode: :list))
+    render_inline(MultipleTemplatesComponent.new)
 
+    assert_selector("div", text: "The items are: Apple, Banana, and Pear, foo")
     assert_selector("li", text: "Apple")
     assert_selector("li", text: "Banana")
     assert_selector("li", text: "Pear")
-
-    render_inline(MultipleTemplatesComponent.new(mode: :summary))
-
-    assert_selector("div", text: "Apple, Banana, and Pear")
+    assert_selector("div.container")
   end
 
   def test_renders_component_using_rails_config