Workshop 203, Tutorial 3, Step 5, Apex

HandleOrderUpdate.cls

trigger HandleOrderUpdate on Invoice__c (after update) {
  
  // Create a map of IDs to all of the *old* versions of sObjects
  // updated by the call that fires the trigger.
    Map<ID, Invoice__c> oldMap =
    new Map<ID, Invoice__c>(Trigger.old);

  // Create an empty list of Ids
  List<Id> invoiceIds = new List<Id>();

  // Interate through all of the *new* versions of Invoice 
  // sObjects updated by the call that fires the trigger, adding 
  // corresponding Ids to the invoiceIds List, but *only* when an
  // sObject's status changed from a non-"Closed" value to "Closed".
  for (Invoice__c invoice: Trigger.new) {
    if (invoice.status__c == 'Closed' &&
      oldMap.get(invoice.Id).status__c != 'Closed') {
        invoiceIds.add(invoice.Id);
      }
    }

  // If the list of Ids is not empty, call Integration.postOrder,
  // supplying the list of Ids for fulfillment.
  if (invoiceIds.size() > 0) {
    Integration.postOrder(invoiceIds);
  }
}