Add an iterate process plugin

This works like a charm! Thank you!

I think that for the specific use case of having an array with values this is the simpler and better solution than the approach of ✨ Add wrapper process plugin to wrap/unwrap values in arrays Needs review .

Sorry, probably that needs a better review ;)

I think you can get the same effect using the build_array process plugin from ✨ Process plugin: build an array from source, destination, pipeline Needs work .

Instead of this example from the doc block for iterate

process:
  field_dates:
    plugin: iterate
    source: field_timestamps
    process:
      plugin: format_date
      from_format: 'U'
      to_format: 'Y-m-d\TH:i:s'

you can use build_array to prepare the array of arrays that sub_process expects:

process:
  field_dates:
    - plugin: build_array
      source: field_timestamps
      template:
        - 'pipeline:'
    - plugin: sub_process
      process:
        plugin: format_date
        from_format: 'U'
        to_format: 'Y-m-d\TH:i:s'
        source: '0'

As @danflanagan8 pointed out in #3, it is much simpler in this case to let the Migrate API loop through the array for you, but I think this example shows that any time you want to use iterate, you can get the same effect with build_array and sub_process.

Many simple uses of sub_process can also be handled by build_array. The second example from #3 becomes

field_node_references:
  -
    plugin: explode
    delimiter: '/'
    source: field_links
  -
    plugin: build_array
    template:
      - pipeline:2
  -
    plugin: migration_lookup
    migration: node

The build_array plugin uses array_walk_recursive(), which is a lot simpler than sub_process and iterate, both of which create a separate process pipeline. So I prefer to use build_array when it can replace one of the others.