Enable Altering Content During Pattern Updates

Problem/Motivation

When blocks are created using existing patterns on a site, the pattern's schema and template are cached in the database for use with that block moving forward until it is updated to use a new pattern version. This process helps to maintain stability in the content by preventing breakage to existing content when a pattern may be updated later.

This also introduces an issue, however, when we consider the types of changes that may be introduced when a pattern is updated. If new fields are added to a pattern's schema in new versions, updates to existing content face no issues. If instead, an existing field is changed that may have content in existing blocks, the content saved for those blocks would also need to be changed to match the new schema structure. Unfortunately, there is no easy way to do this at this time.

This issue proposes the introduction of event dispatching during the pattern update process to allow event subscribers to react to, and alter the content of an affected block. If this process is effectively consolidated to always fire when a block's pattern instance is updated, then the same operation for altering a specific pattern's content during updates could be supported regardless how the block's update process was triggered whether that be from the block edit form or a Drush command.

See the related change record for an example implementation.

Proposed resolution

• Unify the pattern update operation into the block plugin
• Implement a pattern update event supporting pattern content alteration
• Dispatch and apply changes from the event during pattern updates

Remaining tasks

• Code review
• Testing
• Provide example event subscriber

User interface changes

None

API changes

• Additional "--pattern" filter may be provided to drush patternkit:libUpdate to limit updates to only a single pattern ID.
  • Example: drush pklu --pattern='@patternkit/atoms/example/src/example'
• New getData() method added to the Entity\PatternkitBlock class which returns the block entity's content decoded into an array.
• New PatternUpdateEvent named patternkit.pattern_update is dispatched when a block plugin's pattern entity is being updated.
• Plugin\Block\PatternkitBlock now exposes an updatePattern() method for updating the block's pattern entity to the latest or a provided base pattern version. This method is responsible for dispatching the new pattern update event.
• UpdateHelper::updateBlockComponents() now expects an array of filter parameters instead of only the library name to filter by.
• UpdateHelper::updateBlockComponentPattern() no longer accepts a library name parameter for filtering. All filtering of components should be performed beforehand using the new UpdateHelper::filterComponent() method.

Data model changes

None