SVG Chooser Block Fix Plan

Problem Description

The SVG chooser block allows selecting an SVG file and saving it once, but when saving again, the SVG is removed from the page as if it was left blank. This suggests the JavaScript/code doesn't maintain the previous file context after page reload.

Investigation Steps

1. Analyze the current codebase structure - Understand how the SVG chooser block works
2. Examine the widget implementation - Check the AdminSvgChooser widget for state management issues
3. Review form state handling - Investigate how form state is preserved between saves
4. Check JavaScript functionality - Look for client-side code that might be clearing the selection
5. Test the current behavior - Run the application to reproduce the issue
6. Identify the root cause - Determine why the SVG selection is lost on subsequent saves
7. Implement the fix - Apply the necessary changes to maintain state
8. Test the fix - Verify the issue is resolved

Current Status

• Initial investigation started
• Root cause identified
• Fix implemented
• Testing completed

Findings

1. Widget State Issue: The render_html method in AdminSvgChooser converts empty string values to None, which may cause the widget to lose its state on subsequent saves.

2. Message Type Mismatch: The chooser templates are using a custom message type 'svg_chooser:choose' but Wagtail's standard chooser expects a different message format.

3. Value Data Handling: The get_value_data method returns empty string for "value" when instance is None, but render_html converts this to None, potentially causing state loss.

4. Debug Logging: There's already debug logging in place that shows the issue - the value_from_datadict method is being called and should help identify when values are being lost.

5. CRITICAL: Block Form State Issue: The SvgChooserBlock.get_form_state method was returning a processed value_data dictionary instead of the raw value (SVG ID). This caused StreamFields to lose the SVG selection on re-save because the widget expected the raw ID value, not a processed dictionary.

Solution

1. Fixed Widget State Issue: Removed the conversion of empty string to None in the render_html method. The widget now preserves the original value from get_value_data to maintain state between saves.

2. Fixed Message Type Mismatch: Updated the chooser templates to use the standard Wagtail message type 'chooser:choose' instead of the custom 'svg_chooser:choose' type.

3. Improved Debug Logging: Enhanced the debug logging in both get_value_data and value_from_datadict methods to better track state changes.

4. Fixed URL References: Updated the chooser templates to use the correct URL namespace 'wagtailsvg:edit' instead of 'svg_chooser:edit'.

5. Fixed Template Inheritance: Updated chooser templates to properly extend Wagtail's base chooser templates (wagtailadmin/generic/chooser/choose.html and wagtailadmin/generic/chooser/results.html).

6. Switched to Standard Chooser Interface: Disabled custom chooser templates and switched to using Wagtail's standard chooser interface, which handles JavaScript communication properly.

7. Fixed JavaScript Constructor: Updated the widget's js_constructor to use the standard Wagtail chooser constructor.

8. Added Missing Methods: Added get_instance and get_chooser_modal_url methods to ensure proper widget functionality.

9. Enhanced Block Value Handling: Added value_from_form method to the block to properly handle form value conversion.

10. CRITICAL FIX: Fixed Block Form State: Fixed the get_form_state method to return the raw SVG ID instead of a processed value_data dictionary. This was the root cause of the StreamField state loss issue.

11. Enhanced Error Handling: Added proper error handling in get_form_state to handle various input types (None, empty string, invalid values) gracefully.

12. Added Database Value Conversion: Added to_python, get_prep_value, and get_db_prep_value methods to ensure proper value conversion during database operations.

The root cause was that the widget was converting empty string values to None in the render_html method, which caused the widget to lose its state on subsequent saves. Additionally, the chooser templates were not properly extending the base Wagtail chooser templates, which could cause JavaScript communication issues. Most critically, the SvgChooserBlock.get_form_state method was returning a processed dictionary instead of the raw value, causing StreamFields to lose SVG selections on re-save. By preserving the original value, fixing the message type, ensuring proper template inheritance, and fixing the block's form state handling, the chooser should now maintain its state correctly.