Fetch data trigger.

DeskTriggerHelper.class

public class DeskTriggerHelper
{
    private static DeskClient CLIENT = new DeskClient(new Map<String, String>{
        'token' => 'TOKEN',
        'tokenSecret' => 'TOKEN_SECRET',
        'consumerKey' => 'CONSUMER_KEY',
        'consumerSecret' => 'CONSUMER_SECRET',
        'endpoint' => 'https://example.desk.com'
    });
    
    public static void fetchData(List<Deskcom__Case__c> cases)
    {
        for (Deskcom__Case__c c : cases) {
            DeskTriggerHelper.fetchData(c.Id);
        }
    }
    
    @future(callout=true)
    public static void fetchData(Id cId)
    {
        // get the case object
        Deskcom__Case__c c = [Select Deskcom__display_id__c From Deskcom__Case__c Where Id = :cId];
        
        // fetch the latest changes
        DeskResource res = CLIENT.getResource('cases').find(Integer.valueOf(c.Deskcom__display_id__c));
        
        // get the custom fields
        Map<String, Object> custom_fields = (Map<String, Object>)res.get('custom_fields');
        
        // get a timestamp as ISO8601 string
        String firstResponseAt = String.valueOf(custom_fields.get('first_response_at'));
        // make sure we don't just have a blank string
        if (String.isNotBlank(firstResponseAt)) {
            // convert the ISO8601 string to an actual DateTime object
            c.FirstResponseAt__c = DeskTriggerHelper.convertISO8601(firstResponseAt);
        }
        
        String severity = String.valueOf(custom_fields.get('severity'));
        if (String.isNotBlank(severity)) {
            c.Severity__c = severity;
        }
        
        // update the case
        update(c);
    }
    
    public static DateTime convertISO8601(String iso8601_ts)
    {
        // use JSON.deserialize method to get a DateTime object
        DateTime dt = (DateTime)JSON.deserialize('"' + iso8601_ts + '"', DateTime.class);
        // Bug in JSONParser or DateTime object results in a malformed DateTime,
        // so convert to Long and back to DateTime.  Without the conversion,
        // methods that access timeGmt() and its components will actually get
        // local time instead of GMT.
        return DateTime.newInstance(dt.getTime());
    }
}

FetchData.trigger

trigger FetchData on Deskcom__Case__c (after update)
{
    List<Deskcom__Case__c> cases = new List<Deskcom__Case__c>();
    
    for (Deskcom__Case__c c : Trigger.new) {
      if (c.Deskcom__updated_at__c != Trigger.oldMap.get(c.Id).Deskcom__updated_at__c) {
        cases.add(c);
      }
    }
    
    // this has to be a future method to run the API calls
    DeskTriggerHelper.fetchData(cases);
}