How to fire a Rule when an image is removed from Image field with unlimited values?

Question

Working with Drupal 7.58 and File Entity 2.19 and Rules 2.10

First I needed to fire a rule when saving a new file and I found a solution (here).

Now I would like to fire a rule when an user removes an image (file) from a content (Image field with multiple values).

I can not find any event to start working after an image is removed?

EDIT 1 :

Following @Pierre.Vriens "check list approach", I have tried to create a simple rule fired when an image is removed :

{ "rules_verify_pictures_removed" : {
    "LABEL" : "Verify pictures removed",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [ "image" ],
    "REQUIRES" : [ "rules_conditional", "rules" ],
    "ON" : { "node_update--article" : { "bundle" : "article" } },
    "DO" : [
      { "LOOP" : {
          "USING" : { "list" : [ "node-unchanged:field-pictures" ] },
          "ITEM" : { "list_item" : "Previous list item" },
          "DO" : [
            { "CONDITIONAL" : [
                {
                  "IF" : { "NOT list_contains" : { "list" : [ "node:field-pictures" ], "item" : [ "list-item" ] } },
                  "DO" : [
                    { "drupal_message" : { "message" : "Picture has been removed.", "type" : "warning" } }
                  ]
                }
              ]
            }
          ]
        }
      }
    ]
  }
}

Unfortunately, condition is always False and I get the message even if no item has been removed (One message for each item, unchanged, added or removed)

Answer 1

To my knowledge there is no Rules Event like the one you are looking for. However, as illustrated in my answer to the question you linked to, you can use an approach like so:

• iterate over all field values of your field of the unchanged node.
• within each iteration step, check if the list of items in the changed node does NOT contain the field value being processed in the current iteration step ... If it is NOT contained in it anymore, it means the values is removed.

Note: processing image fields in Rules is a bit special. You may have to use a token like [node:field-pictures:file:url] and [list-item:file:url]. And if they appear to be empty, then retry by adding a Rules Condition like "content is of type = article"