Implement ifchanged Liquid tag

[sebastienros]

Summary

Implements the {% ifchanged %} Liquid tag, which outputs content only when it has changed from the previous invocation.

Changes

• Fluid/Ast/IfChangedStatement.cs - New AST node that:

  • Renders inner statements to a buffer
  • Compares with the previous output stored in context.AmbientValues
  • Only outputs content if it differs from the last ifchanged output

• Fluid/FluidParser.cs - Registers the ifchanged/endifchanged block tag

• Fluid/Ast/AstVisitor.cs & AstRewriter.cs - Added visitor methods

• Fluid.Tests/GoldendLiquidTests.cs - Enabled golden liquid tests (removed skip entry)

• Fluid.Tests/IfChangedStatementTests.cs - 9 new unit tests

Behavior

All {% ifchanged %} blocks share a single "previous value" in the render context. This matches Shopify Liquid's behavior:

{% assign foo = 'hello' %}
{% ifchanged %}{{ foo }}{% endifchanged %}  {# outputs 'hello' #}
{% ifchanged %}{{ foo }}{% endifchanged %}  {# no output (unchanged) #}
{% assign foo = 'goodbye' %}
{% ifchanged %}{{ foo }}{% endifchanged %}  {# outputs 'goodbye' #}

Common use case - grouping in loops:

{% for item in list | sort %}
  {% ifchanged %}{{ item }}{% endifchanged %}
{% endfor %}

Tests

• All 5 golden liquid tests for ifchanged tag pass
• 9 new unit tests pass
• All 2415 existing tests pass (20 skipped for unrelated features)

[hishamco]

Is this an offical tag by Shopify?

[sebastienros]

@hishamco yes https://github.com/Shopify/liquid/blob/d897899f6654c476e58e884bc8e24924600e5801/lib/liquid/tags/ifchanged.rb

[hishamco]

Are there any missing tags or filters, I need to put my hands dirty again in Fluid :) It has been awhile

[sebastienros]

AFAIK everything is implemented, that's why I added Golden Test Suite

There are a few skipped tests but mostly because implementing these things could be complex or too expensive for such niche feature. You can always check in the open issues though is there is something intereseting.