Prevent double Imports (Implement Import Filters)

Sure thing, thats a missing feature. Some thoughts about this…

The implementation should be independent and outside of the existing PostImporterInterface structure. So it wont go into WpPostImporter
This could better go into a new ImportFilterInterface which checks each Type\ImportPostInterface whether it should be imported or not:

namespace W2M\Import\Filter;

use
    W2M\Import\Type;

interface PostImportFilterInterface {

    /**
     * Checks if a post should be imported or not
     * 
     * @param Type\ImportPostInterface $import_post
     *
     * @return bool
     */
    public function post_to_import( Type\ImportPostInterface $import_post );
}

To connect the Filter with the import logic, the Import\Service\PostProcessor depends additionaly on the Filter, and asks for each post before passing it to the importer.
A basic implementor of PostFilterInterface has two tasks: Of course it implements post_to_import() method. Furthermore it listens to the w2m_post_imported action with a second method. The strategy to identify already imported posts could be to save post_guids/~~origin_ids~~ as post-meta for the imported posts.

namespace W2M\Import\Filter;

use
    W2M\Import\Type,
    WP_Post;

class DuplicatePostFilter implements PostFilterInterface {

    /**
     * Checks if a post should be imported or not
     * 
     * @param Type\ImportPostInterface $import_post
     *
     * @return bool
     */
    public function post_to_import( Type\ImportPostInterface $import_post ) {
        // check for existing post here
    }

    /**
     * Records new imported post
     * 
     * @wp-hook w2m_post_imported
     *
     * @param WP_Post $wp_post
     * @param Type\ImportPostInterface $import_post
     */
    public function mark_imported_post( WP_Post $wp_post, Type\ImportPostInterface $import_post ) {
        // save post meta to identify the post here
    }
}

Update It's important to have a separate Interface for the w2m_{TYPE}_imported action listeners ( #54 )

inpsyde / wpml2mlp

Prevent double Imports (Implement Import Filters) #49