Add experimental support for default slot values (#2063)

* Add support for default slot values

Closes #1861 #614

* Fix final line endings

* add test case for overriding slot default value

* lints

* look up slot default method at compile time

* fix indent

* lints

---------

Co-authored-by: GitHub Actions Bot <41898282+github-actions[bot]@users.noreply.github.com>

Author: joelhawksley

docs/CHANGELOG.md

@@ -10,6 +10,10 @@ nav_order: 5
 
 ## main
 
+* Add experimental `SlotableDefault` module, allowing components to define a `default_SLOTNAME` method to provide a default value for slots.
+
+    *Joel Hawksley*
+
 * Add documentation on rendering ViewComponents outside of the view context.
 
     *Joel Hawksley*

docs/guide/slots.md

@@ -349,3 +349,36 @@ end
 ```
 
 The setters are now `#with_icon_visual` and `#with_avatar_visual` instead of the default `#with_visual_icon` and `#with_visual_avatar`. The slot getter remains `#visual`.
+
+## `#default_SLOT_NAME`
+
+Since 3.14.0
+{: .label }
+
+To provide a default value for a slot, include the experimental `SlotableDefault` module and define a `default_SLOT_NAME` method:
+
+```ruby
+class SlotableDefaultComponent < ViewComponent::Base
+  include SlotableDefault
+
+  renders_one :header
+
+  def default_header
+    "Hello, World!"
+  end
+end
+```
+
+`default_SLOT_NAME` can also return a component instance to be rendered:
+
+```ruby
+class SlotableDefaultInstanceComponent < ViewComponent::Base
+  include SlotableDefault
+
+  renders_one :header
+
+  def default_header
+    MyComponent.new
+  end
+end
+```

lib/view_component/base.rb

@@ -10,6 +10,7 @@
 require "view_component/inline_template"
 require "view_component/preview"
 require "view_component/slotable"
+require "view_component/slotable_default"
 require "view_component/translatable"
 require "view_component/with_content_helper"
 require "view_component/use_helpers"

lib/view_component/compiler.rb

@@ -86,6 +86,12 @@ def #{method_name}
         define_render_template_for
       end
 
+      component_class.registered_slots.each do |slot_name, config|
+        config[:default_method] = component_class.instance_methods.find { |method_name| method_name == :"default_#{slot_name}" }
+
+        component_class.registered_slots[slot_name] = config
+      end
+
       component_class.build_i18n_backend
 
       CompileCache.register(component_class)

lib/view_component/slotable.rb

@@ -266,10 +266,7 @@ def register_slot(slot_name, **kwargs)
       end
 
       def define_slot(slot_name, collection:, callable:)
-        # Setup basic slot data
-        slot = {
-          collection: collection
-        }
+        slot = {collection: collection}
         return slot unless callable
 
         # If callable responds to `render_in`, we set it on the slot as a renderable

lib/view_component/slotable_default.rb

@@ -0,0 +1,20 @@
+module ViewComponent
+  module SlotableDefault
+    def get_slot(slot_name)
+      @__vc_set_slots ||= {}
+
+      return super unless !@__vc_set_slots[slot_name] && (default_method = registered_slots[slot_name][:default_method])
+
+      renderable_value = send(default_method)
+      slot = Slot.new(self)
+
+      if renderable_value.respond_to?(:render_in)
+        slot.__vc_component_instance = renderable_value
+      else
+        slot.__vc_content = renderable_value
+      end
+
+      slot
+    end
+  end
+end

test/sandbox/app/components/slotable_default_component.rb

@@ -0,0 +1,13 @@
+class SlotableDefaultComponent < ViewComponent::Base
+  include ViewComponent::SlotableDefault
+
+  erb_template <<~ERB
+    <h1><%= header %></h1>
+  ERB
+
+  renders_one :header
+
+  def default_header
+    "hello,world!"
+  end
+end

test/sandbox/app/components/slotable_default_instance_component.rb

@@ -0,0 +1,13 @@
+class SlotableDefaultInstanceComponent < ViewComponent::Base
+  include ViewComponent::SlotableDefault
+
+  erb_template <<~ERB
+    <h1><%= header %></h1>
+  ERB
+
+  renders_one :header
+
+  def default_header
+    MyComponent.new
+  end
+end

test/sandbox/test/slotable_test.rb

@@ -764,4 +764,25 @@ def test_forwarded_slot_renders_correctly
 
     assert_text "Target content", count: 1
   end
+
+  def test_slotable_default
+    render_inline(SlotableDefaultComponent.new)
+
+    assert_text "hello,world!", count: 1
+  end
+
+  def test_slotable_default_override
+    component = SlotableDefaultComponent.new
+    component.with_header_content("foo")
+
+    render_inline(component)
+
+    assert_text "foo", count: 1
+  end
+
+  def test_slotable_default_instance
+    render_inline(SlotableDefaultInstanceComponent.new)
+
+    assert_text "hello,world!", count: 1
+  end
 end