Add A/B Testing Proxy Block Plugin

[e0ipso]

Add A/B Testing Proxy Block Plugin

Summary

Create a new block plugin that acts as a proxy for other blocks, allowing A/B testing by dynamically selecting which block to render based on the configured A/B test decision. This will enable testing different block types/configurations or testing presence vs absence of blocks.

Problem Statement

Currently, A/B testing with blocks is limited to testing different configurations of the same block type. There's no way to:

1. Test completely different block types against each other (e.g. a Custom HTML block vs a View block)
2. Test having a block vs not having any block at all
3. Easily swap between different pre-configured blocks in an A/B test

Proposed Solution

Create a new block plugin called AbTestProxyBlock that:

Core Functionality

• Block Selection: Configuration form allows selecting an existing block from the system
• Empty Rendering: Option to render nothing (empty/hidden state)
• Proxy Behavior: Renders the selected block with its original configuration
• A/B Integration: Works seamlessly with the existing ab_blocks A/B testing system

Configuration Options

1. Target Block: Dropdown/autocomplete to select which block to proxy
   • Should list all available block plugins
   • Include block instance selection for blocks that have multiple instances
2. Render Mode:
   • "Render selected block" - Shows the proxied block
   • "Render empty" - Shows nothing (for testing block presence vs absence)
3. Block Configuration:
   • If proxying a block, allow configuring the target block's settings
   • Or reference an existing configured block instance

A/B Testing Integration

The proxy block will work with existing A/B testing infrastructure:

• Decider plugins can choose between different proxy configurations
• One variant might proxy Block A, another might proxy Block B, another might render empty
• Analytics tracking continues to work on the proxy block container

Technical Implementation

Block Plugin Structure

modules/ab_blocks/src/Plugin/Block/AbTestProxyBlock.php

Key Methods to Implement

• blockForm(): Configuration form for block selection and render mode
• blockSubmit(): Save proxy configuration
• build(): Render logic to instantiate and build the target block
• getCacheContexts(), getCacheTags(), getCacheMaxAge(): Proxy cache metadata from target block

Configuration Schema

# In ab_blocks.schema.yml
block.settings.ab_test_proxy_block:
  type: mapping
  mapping:
    target_block_plugin:
      type: string
      label: 'Target block plugin ID'
    target_block_config:
      type: sequence
      label: 'Target block configuration'
    render_mode:
      type: string
      label: 'Render mode (block|empty)'

Dependencies

• Depends on existing ab_blocks module
• Uses Drupal's Block Plugin API
• Integrates with Layout Builder (if target blocks support it)

Use Cases

Use Case 1: Block Type A/B Testing

Test a custom "Featured Product" block vs a "Product Carousel" view block:

• Variant A: Proxy renders Featured Product block
• Variant B: Proxy renders Product Carousel block
• Variant C: Proxy renders empty (no product promotion)

Use Case 2: Content Strategy Testing

Test different approaches to newsletter signup:

• Variant A: Proxy renders Newsletter Signup form block
• Variant B: Proxy renders Custom HTML block with social links
• Variant C: Proxy renders empty (no call-to-action)

Use Case 3: Layout Optimization

Test sidebar content effectiveness:

• Variant A: Proxy renders Related Articles block
• Variant B: Proxy renders Advertisement block
• Variant C: Proxy renders empty (cleaner layout)

Acceptance Criteria

• New block plugin AbTestProxyBlock is created in ab_blocks module
• Configuration form allows selecting target block plugin and configuration
• "Render empty" option is available and functional
• Target block is rendered with proper cache contexts, tags, and max-age
• Works with existing A/B testing decider and analytics plugins
• Proper error handling when target block fails to render
• Unit and functional tests are included
• Documentation updated to describe the proxy block functionality

Testing Requirements

Unit Tests

• Test block configuration form submission
• Test render logic for different render modes
• Test cache metadata propagation

Functional Tests

• Test block placement and configuration via Layout Builder
• Test A/B testing integration with different proxy configurations
• Test empty rendering mode
• Test error handling for missing/invalid target blocks

JavaScript Tests (if applicable)

• Verify A/B testing JavaScript works with proxy blocks
• Test analytics tracking on proxy block container

Documentation Needs

• Update README with proxy block usage examples
• Add configuration examples to CLAUDE.md
• Include proxy block in A/B testing workflow documentation

Implementation Notes

• The proxy block should handle access control by respecting the target block's access restrictions
• Cache metadata must be properly bubbled up from the target block to ensure correct caching behavior
• Consider performance implications of instantiating target blocks dynamically
• Ensure compatibility with contrib modules that extend block functionality

Related Issues

• Extends functionality from existing ab_blocks A/B testing system
• May relate to future block management improvements
• Consider integration with Block Content module for reusable block instances