Support for an HTML builder that can create dynamic HTML

app/controllers/application_controller.rb

@@ -434,6 +434,10 @@ class ApplicationController < ActionController::Base
         data.merge! DiscoursePluginRegistry.custom_html
       end
 
+      DiscoursePluginRegistry.html_builders.each do |name, blk|
+        data[name] = blk.call
+      end
+
       MultiJson.dump(data)
     end
 

app/helpers/application_helper.rb

@@ -316,6 +316,11 @@ module ApplicationHelper
     end
   end
 
+  def build_plugin_html(name)
+    return "" unless allow_plugins?
+    DiscoursePluginRegistry.build_html(name) || ""
+  end
+
   def theme_lookup(name)
     lookup = Theme.lookup_field(theme_key, mobile_view? ? :mobile : :desktop, name)
     lookup.html_safe if lookup

lib/discourse_plugin_registry.rb

@@ -13,7 +13,6 @@ class DiscoursePluginRegistry
     attr_writer :handlebars
     attr_writer :serialized_current_user_fields
     attr_writer :seed_data
-
     attr_accessor :custom_html
 
     def plugins
@@ -60,6 +59,10 @@ class DiscoursePluginRegistry
     def seed_data
       @seed_data ||= HashWithIndifferentAccess.new({})
     end
+
+    def html_builders
+      @html_builders ||= {}
+    end
   end
 
   def register_js(filename, options={})
@@ -127,6 +130,14 @@ class DiscoursePluginRegistry
     self.seed_data[key] = value
   end
 
+  def self.register_html_builder(name, &block)
+    html_builders[name] = block
+  end
+
+  def self.build_html(name)
+    html_builders[name]&.call
+  end
+
   def javascripts
     self.class.javascripts
   end
@@ -169,6 +180,7 @@ class DiscoursePluginRegistry
     sass_variables.clear
     serialized_current_user_fields
     asset_globs.clear
+    html_builders.clear
   end
 
   def self.setup(plugin_class)

lib/plugin/instance.rb

@@ -238,6 +238,10 @@ class Plugin::Instance
     DiscoursePluginRegistry.custom_html.merge!(hash)
   end
 
+  def register_html_builder(name, &block)
+    DiscoursePluginRegistry.register_html_builder(name, &block)
+  end
+
   def register_asset(file, opts=nil)
     full_path = File.dirname(path) << "/assets/" << file
     assets << [full_path, opts]

spec/components/discourse_plugin_registry_spec.rb

@@ -44,6 +44,15 @@ describe DiscoursePluginRegistry do
     end
   end
 
+  context '.register_html_builder' do
+    it "can register and build html" do
+      DiscoursePluginRegistry.register_html_builder(:my_html) { "<b>my html</b>" }
+      expect(DiscoursePluginRegistry.build_html(:my_html)).to eq('<b>my html</b>')
+      DiscoursePluginRegistry.reset!
+      expect(DiscoursePluginRegistry.build_html(:my_html)).to be_blank
+    end
+  end
+
   context '.register_css' do
     before do
       registry_instance.register_css('hello.css')