How can I get data from the rows being created in a list view before saving?

Is there any way to access to the items beign added in a list view before saving the form and writing them? Scenario: product.product has a o2m relation (packaging field) with product.packaging. If I edit a product in its form view and add a packaging, is there any way to access to all the packaging rows (the ones it had previously and those not saved yet) before saving the product form? Edit: The moment I need to retrieve the data of the unsaved rows is when adding a new row. I need to set a default value for a field in the new row taking into account the previous packagings the product has, whether they are saved or beign just added. Thanks in advance