Add Asciinema Liquid tag (#4152)

* Add Asciinema Liquid tag

Closes #4135

* Fix Iframe URL

* Remove unnecessary regex modifier

* Re-add accidentally deleted closing tag and set iframe height

* Improve error message for invalid ID

Co-Authored-By: Andy Zhao <andyzhao.zhao@gmail.com>

app/liquid_tags/asciinema_tag.rb

@@ -0,0 +1,32 @@
+class AsciinemaTag < LiquidTagBase
+  PARTIAL = "liquids/asciinema".freeze
+
+  def initialize(tag_name, id, tokens)
+    super
+    @id = parse_id(id)
+  end
+
+  def render(_context)
+    ActionController::Base.new.render_to_string(
+      partial: PARTIAL,
+      locals: {
+        id: @id
+      },
+    )
+  end
+
+  private
+
+  def parse_id(input)
+    input_no_space = input.delete(" ")
+    raise StandardError, "Invalid Asciinema ID: {% asciinema #{input_no_space} %}" unless valid_id?(input_no_space)
+
+    input_no_space
+  end
+
+  def valid_id?(id)
+    id.match?(/\A\d+\Z/)
+  end
+end
+
+Liquid::Template.register_tag("asciinema", AsciinemaTag)

app/views/liquids/_asciinema.html.erb

@@ -0,0 +1,8 @@
+<div class="ltag_asciinema">
+  <iframe frameborder="0"
+    scrolling="no"
+    id="asciinema_<%= id %>"
+    src="https://asciinema.org/a/<%= id %>/iframe"
+    style="width:100%;height:50vh;">
+  </iframe>
+</div>

app/views/pages/_editor_guide_text.html.erb

@@ -321,6 +321,9 @@
 {% stackexchange <span style="color: aquamarine;">170185</span> <span style="color: orange;">diy</span> %}
         </pre>
         </p>
+        <h3><strong>Asciinema Embed</strong></h3>
+        <p>All you need is the Asciinema id:</p>
+        <code>{% asciinema 239367 %}</code>
         <h3><strong>Parsing Liquid Tags as a Code Example</strong></h3>
         <p>To parse Liquid tags as code, simply wrap it with a single backtick or triple backticks.</p>
         <p><code>`{% mytag %}{{ site.SOMETHING }}{% endmytag %}`</code></p>

spec/liquid_tags/asciinema_tag_spec.rb

@@ -0,0 +1,21 @@
+require "rails_helper"
+
+RSpec.describe AsciinemaTag, type: :liquid_template do
+  describe "#id" do
+    let(:valid_id)      { "1234" }
+    let(:invalid_id)    { "inv@lid" }
+
+    def generate_tag(id)
+      Liquid::Template.register_tag("asciinema", AsciinemaTag)
+      Liquid::Template.parse("{% asciinema #{id} %}")
+    end
+
+    it "rejects invalid ids" do
+      expect { generate_tag(invalid_id) }.to raise_error(StandardError)
+    end
+
+    it "accepts a valid id" do
+      expect { generate_tag(valid_id) }.not_to raise_error
+    end
+  end
+end