DEV: Add hook for plugins modify how slugs are computed (#16907)

Allows plugins to hook into and change how topic slugs are computed.

app/models/topic.rb

@@ -1191,11 +1191,26 @@ class Topic < ActiveRecord::Base
     }
   end
 
+  cattr_accessor :slug_computed_callbacks
+  self.slug_computed_callbacks = []
+
+  def slug_for_topic(title)
+    return '' unless title.present?
+    slug = Slug.for(title)
+
+    # this is a hook for plugins that need to modify the generated slug
+    self.class.slug_computed_callbacks.each do |callback|
+      slug = callback.call(self, slug, title)
+    end
+
+    slug
+  end
+
   # Even if the slug column in the database is null, topic.slug will return something:
   def slug
     unless slug = read_attribute(:slug)
       return '' unless title.present?
-      slug = Slug.for(title)
+      slug = slug_for_topic(title)
       if new_record?
         write_attribute(:slug, slug)
       else
@@ -1216,7 +1231,7 @@ class Topic < ActiveRecord::Base
   end
 
   def title=(t)
-    slug = Slug.for(t.to_s)
+    slug = slug_for_topic(t.to_s)
     write_attribute(:slug, slug)
     write_attribute(:fancy_title, nil)
     write_attribute(:title, t)

spec/models/topic_spec.rb

@@ -270,6 +270,37 @@ describe Topic do
         end
       end
     end
+
+    context 'slug computed hooks' do
+      before do
+        invert_slug = ->(topic, slug, title) { slug.reverse }
+        Topic.slug_computed_callbacks << invert_slug
+      end
+
+      let!(:title) { "hello test topic" }
+      let!(:slug) { "hello-test-topic".reverse }
+      let!(:other_title) { "other title" }
+      let!(:other_slug) { "other-title".reverse }
+      let!(:topic) { Fabricate.build(:topic, title: title) }
+
+      it "returns a reversed slug for a title" do
+        expect(topic.title).to eq(title)
+        expect(topic.slug).to eq(slug)
+      end
+
+      it "returns a reversed slug after the title is changed" do
+        expect(topic.title).to eq(title)
+        expect(topic.slug).to eq(slug)
+
+        topic.title = other_title
+        expect(topic.title).to eq(other_title)
+        expect(topic.slug).to eq(other_slug)
+      end
+
+      after do
+        Topic.slug_computed_callbacks.clear
+      end
+    end
   end
 
   context "updating a title to be shorter" do