Implement the base method for deleting artefacts from a blockinstance.

This method assumes that either 'artefactid' contains a single ID or 'artefactids' contains an array of artefact IDs, of which one could be the one to delete. This means that as long as the children store there artefact IDs this way, there's no need for them to re-implement this method.

htdocs/blocktype/lib.php

@@ -488,6 +488,43 @@ class BlockInstance {
         call_static_method(generate_class_name('blocktype', $this->get('blocktype')), 'delete_instance', $this);
     }
 
+    /**
+     * Deletes an artefact from the blockinstance.
+     *
+     * This is implemented in the baseclass by looking for arrays in the block 
+     * instance configuration called 'artefactid' or 'artefactids', and 
+     * removing the one we were looking to delete. This means two things:
+     *
+     * 1) In order to not have to re-implement this method for new blocktypes, 
+     *    your blocktype should ALWAYS store its artefact IDs in the config data 
+     *    value 'artefactid' or in the array 'artefactids'
+     * 2) The block must ALWAYS continue to work even when artefacts are 
+     *    removed from it
+     */
+    public function delete_artefact($artefact) {
+        $configdata = $this->get('configdata');
+        $changed = false;
+
+        if (isset($configdata['artefactid'])) {
+            if ($configdata['artefactid'] == $artefact) {
+                $configdata['artefactid'] = null;
+            }
+            $changed = true;
+        }
+
+        if (isset($configdata['artefactids']) && is_array($configdata['artefactids'])) {
+            $configdata['artefactids'] = array_diff($configdata['artefactids'], array($artefact));
+            $changed = true;
+        }
+
+        if ($changed) {
+            $this->set('configdata', $configdata);
+
+            // We would commit here but we don't want to rebuild the artefact list
+            set_field('block_instance', 'configdata', serialize($configdata), 'id', $this->get('id'));
+        }
+    }
+
 }