Add deferred partial rendering with Declarative Partial Updates

Introduce an `after` modifier for the render tag using Google's
proposed Declarative Partial Updates format. Initial rendering emits
processing-instruction markers, queues isolated partial renders, and
flushes `<template for=...>` replacement patches from the context.

lib/liquid/context.rb

@@ -64,6 +64,33 @@ module Liquid
       @warnings ||= []
     end
 
+    def after_render_jobs
+      @registers.static[:after_render_jobs] ||= []
+    end
+
+    def next_after_render_id
+      @registers.static[:after_render_sequence] ||= 0
+      @registers.static[:after_render_sequence] += 1
+      "liquid-after-#{@registers.static[:after_render_sequence]}"
+    end
+
+    def enqueue_after_render(job)
+      after_render_jobs << job
+    end
+
+    def render_after_tags_to_output_buffer(output)
+      while (job = after_render_jobs.shift)
+        output << %(<template for="#{job[:id]}">)
+        job[:renderer].call(output)
+        output << %(</template>)
+      end
+      output
+    end
+
+    def render_after_tags
+      render_after_tags_to_output_buffer(+'')
+    end
+
     def strainer
       @strainer ||= @environment.create_strainer(self, @filters)
     end

lib/liquid/tags/render.rb

@@ -27,7 +27,11 @@ module Liquid
   # @liquid_syntax_keyword filename The name of the snippet to render, without the `.liquid` extension.
   class Render < Tag
     FOR = 'for'
-    SYNTAX = /(#{QuotedString}+)(\s+(with|#{FOR})\s+(#{QuotedFragment}+))?(\s+(?:as)\s+(#{VariableSegment}+))?/o
+    AFTER = 'after'
+    AFTER_MARKUP = /\s+#{AFTER}(?=\s|,|\z)/o
+    WITH_OR_FOR_MARKUP = /\s+(with|#{FOR})\s+(#{QuotedFragment}+)/o
+    ALIAS_MARKUP = /\s+(?:as)\s+(#{VariableSegment}+)/o
+    SYNTAX = /(#{QuotedString}+)(#{AFTER_MARKUP})?(#{WITH_OR_FOR_MARKUP})?(#{ALIAS_MARKUP})?/o
 
     disable_tags "include"
 
@@ -39,10 +43,11 @@ module Liquid
       raise SyntaxError, options[:locale].t("errors.syntax.render") unless markup =~ SYNTAX
 
       template_name = Regexp.last_match(1)
-      with_or_for = Regexp.last_match(3)
-      variable_name = Regexp.last_match(4)
+      @after = !!Regexp.last_match(2)
+      with_or_for = Regexp.last_match(4)
+      variable_name = Regexp.last_match(5)
 
-      @alias_name = Regexp.last_match(6)
+      @alias_name = Regexp.last_match(7)
       @variable_name_expr = variable_name ? parse_expression(variable_name) : nil
       @template_name_expr = parse_expression(template_name)
       @is_for_loop = (with_or_for == FOR)
@@ -61,6 +66,10 @@ module Liquid
       render_tag(context, output)
     end
 
+    def after?
+      @after
+    end
+
     def render_tag(context, output)
       # The expression should be a String literal, which parses to a String object
       template_name = @template_name_expr
@@ -74,26 +83,43 @@ module Liquid
 
       context_variable_name = @alias_name || template_name.split('/').last
 
-      render_partial_func = ->(var, forloop) {
+      evaluated_attributes = @attributes.transform_values { |value| context.evaluate(value) }
+
+      render_partial_func = ->(var, forloop, render_output) {
         inner_context               = context.new_isolated_subcontext
         inner_context.template_name = partial.name
         inner_context.partial       = true
         inner_context['forloop']    = forloop if forloop
 
-        @attributes.each do |key, value|
-          inner_context[key] = context.evaluate(value)
+        evaluated_attributes.each do |key, value|
+          inner_context[key] = value
         end
         inner_context[context_variable_name] = var unless var.nil?
-        partial.render_to_output_buffer(inner_context, output)
+        partial.render_to_output_buffer(inner_context, render_output)
         forloop&.send(:increment!)
       }
 
       variable = @variable_name_expr ? context.evaluate(@variable_name_expr) : nil
-      if @is_for_loop && variable.respond_to?(:each) && variable.respond_to?(:count)
+
+      if @after
+        id = context.next_after_render_id
+        context.enqueue_after_render(
+          id: id,
+          renderer: ->(after_output) {
+            if @is_for_loop && variable.respond_to?(:each) && variable.respond_to?(:count)
+              forloop = Liquid::ForloopDrop.new(template_name, variable.count, nil)
+              variable.each { |var| render_partial_func.call(var, forloop, after_output) }
+            else
+              render_partial_func.call(variable, nil, after_output)
+            end
+          }
+        )
+        output << %(<?marker name="#{id}">)
+      elsif @is_for_loop && variable.respond_to?(:each) && variable.respond_to?(:count)
         forloop = Liquid::ForloopDrop.new(template_name, variable.count, nil)
-        variable.each { |var| render_partial_func.call(var, forloop) }
+        variable.each { |var| render_partial_func.call(var, forloop, output) }
       else
-        render_partial_func.call(variable, nil)
+        render_partial_func.call(variable, nil, output)
       end
 
       output

proposals/render-after.md

@@ -0,0 +1,208 @@
+# Proposal: Deferred `render` with `after`
+
+## Summary
+
+Add an optional `after` modifier to Liquid's `{% render %}` tag:
+
+```liquid
+{% render 'product-card' after, product: product %}
+```
+
+When `after` is present, Liquid does not render the partial inline. Instead, it emits a stable HTML placeholder marker into the output and records enough information on the current `Liquid::Context` to render the partial later. A new context API then renders all deferred partials, ideally as a stream of out-of-order HTML replacement patches.
+
+This is inspired by Chrome's Declarative Partial Updates proposal: https://developer.chrome.com/blog/declarative-partial-updates. The browser-side idea is to let HTML declare patch targets and stream their replacement content later, enabling the initial shell to be sent quickly while slower islands arrive when ready.
+
+For Liquid, the equivalent is server-side syntax for declaring that a snippet can be delayed without changing template structure.
+
+## Motivation
+
+Liquid templates often have a mix of cheap layout work and expensive isolated snippets. Today, an expensive snippet blocks all subsequent output because `{% render %}` is synchronous and inline.
+
+`render after` would allow templates to produce the main document quickly, reserve exact DOM locations for deferred snippets, and render those snippets later using the same Liquid render semantics.
+
+Example use cases:
+
+- Product recommendations below the fold.
+- Expensive merchandising or personalization blocks.
+- Analytics or SEO metadata fragments that can be patched into known locations.
+- App blocks where the outer page shell should not wait on the block.
+
+## Goals
+
+- Add a small, Liquid-native API for deferring isolated snippet rendering.
+- Preserve existing `{% render %}` isolation semantics.
+- Emit processing-instruction placeholders that can be targeted by a later replacement patch.
+- Store deferred render work in `Liquid::Context`.
+- Add a context method to flush/enumerate/render deferred work.
+- Keep the first prototype simple and non-streaming, while shaping the API so true streaming can be added later.
+
+## Non-goals
+
+- Implement browser support for Declarative Partial Updates.
+- Require JavaScript for the Liquid-side primitive.
+- Make arbitrary tags asynchronous.
+- Allow deferred snippets to mutate the parent scope after the placeholder is emitted.
+- Solve scheduling, prioritization, cancellation, or parallel execution in the first prototype.
+
+## Syntax
+
+The proposed syntax is:
+
+```liquid
+{% render 'snippet' after %}
+{% render 'snippet' after, product: product %}
+{% render 'snippet' after with product as item %}
+{% render 'snippet' after for products as product %}
+```
+
+`after` is a render modifier with no value. It is intentionally boolean and reserved in this position.
+
+The prototype supports bare `after` immediately after the rendered template name, because it reads like a render modifier rather than data passed into the snippet. `after: value` remains a normal named argument passed to the snippet.
+
+## Output shape
+
+When a deferred render is encountered, Liquid emits a Chrome-style processing-instruction placeholder marker with a unique id:
+
+```html
+<?marker name="liquid-after-1">
+```
+
+Later, flushing the deferred renders produces replacement patches. The target shape should be compatible with the direction of Declarative Partial Updates. For example:
+
+```html
+<template for="liquid-after-1">
+  ...rendered snippet HTML...
+</template>
+```
+
+The exact patch attribute names should track the platform proposal as it evolves. Until browser APIs stabilize, Liquid can expose a server-side patch format behind a small formatter object.
+
+For the prototype, the replacement payload is a concatenated HTML patch string:
+
+```ruby
+context.render_after_tags
+# => "<template for=...>...</template>"
+```
+
+## Semantics
+
+### Evaluation timing
+
+When `{% render 'snippet' after ... %}` is encountered:
+
+1. Liquid evaluates the snippet name expression.
+2. Liquid evaluates the `with` / `for` expression, if present.
+3. Liquid evaluates all named render arguments.
+4. Liquid records a deferred render job containing the evaluated values and render metadata.
+5. Liquid emits a placeholder marker.
+
+This means deferred renders capture values at enqueue time, not flush time. That avoids surprising behavior when variables change later in the template.
+
+### Isolation
+
+Deferred render jobs should use the same isolation semantics as normal `{% render %}`:
+
+- The snippet receives only explicitly-passed variables plus globals/environments available to render today.
+- Variables assigned inside the snippet do not leak into the parent template.
+- The `include` tag remains disabled inside rendered snippets.
+
+### Ordering
+
+The queue is FIFO by default. Placeholder ids are monotonically increasing per context render:
+
+```html
+<?marker name="liquid-after-1">
+<?marker name="liquid-after-2">
+```
+
+The streaming API may later render jobs as they become ready, but the prototype can preserve source order.
+
+### Error handling
+
+Deferred renders should use Liquid's existing error handling through `Context#handle_error` and `exception_renderer`.
+
+Open question: if an error occurs while flushing deferred renders after the main template was already sent, should the replacement patch contain the rendered error string, an empty patch, or an out-of-band error? The prototype should match inline render behavior and place the rendered error into the patch body.
+
+## Proposed API
+
+Add queue APIs to `Liquid::Context`:
+
+```ruby
+context.enqueue_after_render(job) # internal
+context.after_render_jobs         # inspection/testing
+context.render_after_tags         # prototype: returns a string of patches
+context.render_after_tags_to_output_buffer(output) # streaming-ready shape
+```
+
+Possible streaming-oriented API:
+
+```ruby
+context.each_after_render_patch do |patch|
+  response.write(patch)
+end
+```
+
+or:
+
+```ruby
+context.render_after_tags_to_output_buffer(response_stream)
+```
+
+The first implementation may buffer each snippet internally. The API should still write to an output object so callers can later stream each completed patch without changing template code.
+
+## Example
+
+Template:
+
+```liquid
+<h1>{{ product.title }}</h1>
+
+{% render 'price', product: product %}
+
+<section>
+  {% render 'recommendations' after, product: product %}
+</section>
+```
+
+Initial output:
+
+```html
+<h1>Snowboard</h1>
+
+<span>$699.00</span>
+
+<section>
+  <?marker name="liquid-after-1">
+</section>
+```
+
+Deferred patch output:
+
+```html
+<template for="liquid-after-1">
+  <ul class="recommendations">...</ul>
+</template>
+```
+
+A Rack-like integration could do:
+
+```ruby
+context = Liquid::Context.build(...)
+body = template.render!(context)
+response.write(body)
+context.render_after_tags_to_output_buffer(response)
+```
+
+The prototype can buffer `body` first. A production integration would stream `body` immediately, then stream each deferred patch as soon as it completes.
+
+## Compatibility
+
+Existing templates are unaffected unless they use bare `after` immediately after the rendered template name.
+
+Because bare `after` becomes reserved syntax for the render tag in that position, this could conflict with unusual templates that currently rely on that token being ignored. Snippets currently receiving an `after:` keyword argument continue to work:
+
+```liquid
+{% render 'divider', after: 'label' %}
+```
+
+This proposal only reserves bare `after`; `after: value` continues to be passed as a normal snippet attribute. That minimizes compatibility risk.

test/integration/tags/render_tag_test.rb

@@ -46,6 +46,70 @@ class RenderTagTest < Minitest::Test
     )
   end
 
+  def test_render_after_defers_partial_until_context_flush
+    file_system = StubFileSystem.new('snippet' => 'deferred {{ value }}')
+    environment = Liquid::Environment.build(file_system: file_system)
+    template = Liquid::Template.parse('before {% render "snippet" after, value: 1 %} after', environment: environment)
+    registers = Liquid::Registers.new(file_system: file_system)
+    context = Liquid::Context.build(registers: registers, environment: environment)
+
+    assert_equal('before <?marker name="liquid-after-1"> after', template.render(context))
+    assert_equal('<template for="liquid-after-1">deferred 1</template>', context.render_after_tags)
+    assert_empty(context.after_render_jobs)
+  end
+
+  def test_render_after_evaluates_attributes_when_enqueued
+    file_system = StubFileSystem.new('snippet' => '{{ value }}')
+    environment = Liquid::Environment.build(file_system: file_system)
+    template = Liquid::Template.parse('{% assign value = 1 %}{% render "snippet" after, value: value %}{% assign value = 2 %}', environment: environment)
+    registers = Liquid::Registers.new(file_system: file_system)
+    context = Liquid::Context.build(registers: registers, environment: environment)
+
+    assert_equal('<?marker name="liquid-after-1">', template.render(context))
+    assert_equal('<template for="liquid-after-1">1</template>', context.render_after_tags)
+  end
+
+  def test_render_after_supports_with_and_as
+    file_system = StubFileSystem.new('snippet' => '{{ item }}')
+    environment = Liquid::Environment.build(file_system: file_system)
+    template = Liquid::Template.parse('{% render "snippet" after with value as item %}', environment: environment)
+    registers = Liquid::Registers.new(file_system: file_system)
+    context = Liquid::Context.build(static_environments: { 'value' => 'captured' }, registers: registers, environment: environment)
+
+    assert_equal('<?marker name="liquid-after-1">', template.render(context))
+    assert_equal('<template for="liquid-after-1">captured</template>', context.render_after_tags)
+  end
+
+  def test_render_after_supports_for_and_as
+    file_system = StubFileSystem.new('snippet' => '{{ forloop.index }}:{{ item }};')
+    environment = Liquid::Environment.build(file_system: file_system)
+    template = Liquid::Template.parse('{% render "snippet" after for values as item %}', environment: environment)
+    registers = Liquid::Registers.new(file_system: file_system)
+    context = Liquid::Context.build(static_environments: { 'values' => ['a', 'b'] }, registers: registers, environment: environment)
+
+    assert_equal('<?marker name="liquid-after-1">', template.render(context))
+    assert_equal('<template for="liquid-after-1">1:a;2:b;</template>', context.render_after_tags)
+  end
+
+  def test_render_after_preserves_fifo_order
+    file_system = StubFileSystem.new('snippet' => '{{ value }}')
+    environment = Liquid::Environment.build(file_system: file_system)
+    template = Liquid::Template.parse('{% render "snippet" after, value: 1 %}{% render "snippet" after, value: 2 %}', environment: environment)
+    registers = Liquid::Registers.new(file_system: file_system)
+    context = Liquid::Context.build(registers: registers, environment: environment)
+
+    assert_equal('<?marker name="liquid-after-1"><?marker name="liquid-after-2">', template.render(context))
+    assert_equal('<template for="liquid-after-1">1</template><template for="liquid-after-2">2</template>', context.render_after_tags)
+  end
+
+  def test_render_after_colon_remains_a_named_argument
+    assert_template_result(
+      'later',
+      '{% render "snippet", after: "later" %}',
+      partials: { 'snippet' => '{{ after }}' },
+    )
+  end
+
   def test_render_does_not_inherit_parent_scope_variables
     assert_template_result(
       '',