public class ModifyCustomFieldFunction implements FunctionProvider {
	
	public static final String VALUES_SUFFIX = ".newvalues";
	
	public void execute(Map transientVars, Map args, PropertySet ps)
			throws WorkflowException {

		Set<String> newValues = new HashSet<String>();
		
		// Lookup of the custom field to be modified
		String fieldId = (String) args.get("field.id");	
		CustomFieldManager cfm = ComponentManager.getInstance().getCustomFieldManager();
		CustomField field = cfm.getCustomFieldObject(fieldId);
		
		// the name which will be used for the lookup of the new values
		String fieldName = field.getName().toLowerCase().replace(' ', '_'); 
		
		// lookup of the current workflow
		MutableIssue mIssue = (MutableIssue) transientVars.get("issue");			
		JiraWorkflow wflow = null; 
		try {
			wflow = ManagerFactory.getWorkflowManager().getWorkflow(mIssue);
		}
		catch(com.atlassian.jira.workflow.WorkflowException we) {
			throw new WorkflowException("ModifyCustomFieldFunction: Unable to lookup the workflow associated with this issue",we); 
		}
		
		if (wflow == null) {
			throw new WorkflowException("ModifyCustomFieldFunction: Unable to lookup the workflow associated with this issue (null workflow)");
		}
		else {
			
			// Once we have the workflow, we retrieve the action descriptor associated
			// with the current transition			
			int curTransitionId = (Integer) transientVars.get("actionId");
			
			Collection actions = wflow.getAllActions();			
			for (Object o : actions) {
				ActionDescriptor ad = (ActionDescriptor) o;											
				if (curTransitionId == ad.getId()) {
							
					// transition properties lookup to identify the newvalues to be 
					// assigned to the customfield					
					boolean found = false;
					for (Object attr : ad.getMetaAttributes().entrySet()) {
						Map.Entry m = (Map.Entry) attr;
						if (m.getKey().toString().equals(fieldName + VALUES_SUFFIX)) {
							for (String s : m.getValue().toString().split(",")) {
								newValues.add(s.trim());
							}											
							found = true;
						}
						
					}
					
					if (!found) {
						StringBuilder sb = new StringBuilder();
						sb.append("Unable to find new values to be added to custom field " + field.getName() + " (id=" + fieldId + ") ");
						sb.append("using property " + fieldName + VALUES_SUFFIX + " for lookup." );
						sb.append("Plese contact your administrators");
						throw new WorkflowException(sb.toString());
					}
					
				}

			}
		} 
		
		
		// Integrate the custom field values
		if (newValues.size() > 0) {
			if (mIssue.getCustomFieldValue(field) != null) {
				
				// WARNING: this operation will fail if the custom field does not
				// use a collection of String to store its values !! 
				// This has been tested only for multiselect fields.
				
				newValues.addAll((Collection) mIssue.getCustomFieldValue(field));
			}
		}
		
		// Update the custom field
		mIssue.setCustomFieldValue(field, newValues);
			
	}

}