Check if a node field has changed upon save

Question

I am parsing pdf's in a field for a custom content type in a cron job. I have a boolean on each node that determines whether their pdf has already been parsed. I grab the ones I need like so:

$query = \Drupal::entityQuery('node')
    ->condition('status',1)
    ->condition('type', 'issue')
    ->exists('field_pdf')
    ->condition('field_indexed',false)
    ->sort('created','ASC');
if($range > 0){
    $query->range(0,$range);
}

$issue_nids = $query->execute();
$issue_nodes = \Drupal::entityTypeManager()->getStorage('node')->loadMultiple($issue_nids);

foreach ($issue_nodes as $issue_node){
    //do my pdf parsing magic on field_pdf
    $node->field_indexed->value = true;
    $node->save();
}

This works fine but if the user replaces field_pdf and uploads a new one I want to make field_indexed false again so I know to reindex it. The problem is I don't really have a way of knowing whether the field changed.

I considered using a presave hook but I didn't see a way to compare the form state to what was currently on the node.

Is there a way I can detect when a field has changed when a node has saved?

Answer 1

Clive is correct. $node->original does give access to the original node values. I was able to accomplish what I needed like so

function my_module_node_presave($node) {
    $type = $node->getType();
    if($type == 'issue'){
            if(isset($node->field_pdf->entity) && !isset($node->original->field_pdf->entity) || empty($node->original->field_pdf->entity) || $node->field_pdf->entity->id() != $node->original->field_pdf->entity->id()){
                $node->field_indexed->value = false;
            } else {
                $node->field_indexed->value = true;
            }
    }