DEV: Introduce plugin api for conditionally rendering assets (#9200)

app/views/common/_discourse_stylesheet.html.erb

@@ -12,6 +12,6 @@
   <%= discourse_stylesheet_link_tag(mobile_view? ? :mobile_theme : :desktop_theme) %>
 <%- end %>
 
-<%- Discourse.find_plugin_css_assets(include_official: allow_plugins?, include_unofficial: allow_third_party_plugins?, mobile_view: mobile_view?, desktop_view: !mobile_view?).each do |file| %>
+<%- Discourse.find_plugin_css_assets(include_official: allow_plugins?, include_unofficial: allow_third_party_plugins?, mobile_view: mobile_view?, desktop_view: !mobile_view?, request: request).each do |file| %>
   <%= discourse_stylesheet_link_tag(file) %>
 <%- end %>

app/views/layouts/application.html.erb

@@ -28,7 +28,7 @@
     <%= preload_script "vendor" %>
     <%= preload_script "pretty-text-bundle" %>
     <%= preload_script "application" %>
-    <%- Discourse.find_plugin_js_assets(include_official: allow_plugins?, include_unofficial: allow_third_party_plugins?).each do |file| %>
+    <%- Discourse.find_plugin_js_assets(include_official: allow_plugins?, include_unofficial: allow_third_party_plugins?, request: request).each do |file| %>
       <%= preload_script file %>
     <%- end %>
     <%- if staff? %>

lib/discourse.rb

@@ -271,6 +271,10 @@ module Discourse
   def self.find_plugin_css_assets(args)
     plugins = self.find_plugins(args)
 
+    plugins = plugins.select do |plugin|
+      plugin.asset_filters.all? { |b| b.call(:css, args[:request]) }
+    end
+
     assets = []
 
     targets = [nil]
@@ -289,9 +293,15 @@ module Discourse
   end
 
   def self.find_plugin_js_assets(args)
-    self.find_plugins(args).find_all do |plugin|
+    plugins = self.find_plugins(args).select do |plugin|
       plugin.js_asset_exists?
-    end.map { |plugin| "plugins/#{plugin.directory_name}" }
+    end
+
+    plugins = plugins.select do |plugin|
+      plugin.asset_filters.all? { |b| b.call(:js, args[:request]) }
+    end
+
+    plugins.map { |plugin| "plugins/#{plugin.directory_name}" }
   end
 
   def self.assets_digest

lib/plugin/instance.rb

@@ -49,6 +49,7 @@ class Plugin::Instance
    :styles,
    :themes,
    :csp_extensions,
+   :asset_filters
  ].each do |att|
     class_eval %Q{
       def #{att}
@@ -413,6 +414,14 @@ class Plugin::Instance
     csp_extensions << extension
   end
 
+  # Register a block to run when adding css and js assets
+  # Two arguments will be passed: (type, request)
+  # Type is :css or :js. `request` is an instance of Rack::Request
+  # When using this, make sure to consider the effect on AnonymousCache
+  def register_asset_filter(&blk)
+    asset_filters << blk
+  end
+
   # @option opts [String] :name
   # @option opts [String] :nativeName
   # @option opts [String] :fallbackLocale

plugins/discourse-internet-explorer/plugin.rb

@@ -20,22 +20,9 @@ DiscourseEvent.on(:after_plugin_activation) do ||
 end
 
 after_initialize do
-
-  # Conditionally load the stylesheet. There is unfortunately no easy way to do this via
-  # Plugin API.
-  reloadable_patch do |plugin|
-    ApplicationHelper.module_eval do
-      alias_method :previous_discourse_stylesheet_link_tag, :discourse_stylesheet_link_tag
-      def discourse_stylesheet_link_tag(name, opts = {})
-
-        if name == 'discourse-internet-explorer'
-          return unless SiteSetting.discourse_internet_explorer_enabled?
-          return unless request.env['HTTP_USER_AGENT'] =~ /MSIE|Trident/
-        end
-
-        previous_discourse_stylesheet_link_tag(name, opts)
-      end
-    end
+  # Conditionally load the stylesheet
+  register_asset_filter do |type, request|
+    request.nil? || request.env['HTTP_USER_AGENT'] =~ /MSIE|Trident/
   end
 
   register_anonymous_cache_key(:ie) do

spec/components/discourse_spec.rb

@@ -75,12 +75,17 @@ describe Discourse do
       end
     end
 
-    let(:plugin1) { plugin_class.new.tap { |p| p.enabled = true } }
-    let(:plugin2) { plugin_class.new.tap { |p| p.enabled = false } }
+    let(:plugin1) { plugin_class.new.tap { |p| p.enabled = true; p.path = "my-plugin-1" } }
+    let(:plugin2) { plugin_class.new.tap { |p| p.enabled = false; p.path = "my-plugin-1" } }
 
     before { Discourse.plugins.append(plugin1, plugin2) }
     after { Discourse.plugins.clear }
 
+    before do
+      plugin_class.any_instance.stubs(:css_asset_exists?).returns(true)
+      plugin_class.any_instance.stubs(:js_asset_exists?).returns(true)
+    end
+
     it 'can find plugins correctly' do
       expect(Discourse.plugins).to contain_exactly(plugin1, plugin2)
 
@@ -90,6 +95,19 @@ describe Discourse do
       # Include disabled plugins when requested
       expect(Discourse.find_plugins(include_disabled: true)).to contain_exactly(plugin1, plugin2)
     end
+
+    it 'can find plugin assets' do
+      plugin2.enabled = true
+
+      expect(Discourse.find_plugin_css_assets({}).length).to eq(2)
+      expect(Discourse.find_plugin_js_assets({}).length).to eq(2)
+      plugin1.register_asset_filter do |type, request|
+        false
+      end
+      expect(Discourse.find_plugin_css_assets({}).length).to eq(1)
+      expect(Discourse.find_plugin_js_assets({}).length).to eq(1)
+    end
+
   end
 
   context 'authenticators' do