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Respond by using EventHandlerResult

  • Article

Some delegate methods are implemented so that they can request a response from subscribing delegate handler methods. The delegate calling logic then uses the response from a potential subscriber when it continues execution after the response has been received. These delegate methods usually have a signature that has an EventHandlerResult parameter as the last parameter. However, because of the support for the EventHandlerAcceptResult and EventHandlerRejectResult types, the parameter can be of any type that implements the IEventHandlerResult interface.

  • In general, the logic that is implemented in the delegate handler method should contain a condition that verifies that the subscribing logic is responsible for providing a response. It should also include logic to provide the response in the form of a result.
  • When the delegate handler method must provide the response to an EventHandlerResult object parameter, the subscribing logic might also contain logic to calculate or retrieve the result.
  • When the condition and the response logic are implemented, the calculation of the result must occur only when the condition is evaluated to true.
  • All the subscribing delegate handler methods are run when a delegate is called. Therefore, you should make sure that the overhead of running your method is as low as possible when the method isn't responsible for providing a response. Therefore, make sure that the condition is evaluated to false as quickly as possible when your delegate handler method isn't responsible for providing a result.

Examples

The following example shows a delegate handler that has a condition in the form of a switch statement. The delegate handler also has logic to provide a response in the form of the result. The responding logic is run only when the condition is evaluated to true.

[SubscribesTo(tableStr(InventWarehouseEntity), delegateStr(InventWarehouseEntity, validateWarehouseTypeDelegate))]
public static void validateWarehouseTypeIsSupportedStandardDelegateHandler(InventLocationType _inventLocationType, EventHandlerResult _result)
{
    switch (_inventLocationType)
    {
        case InventLocationType::Standard:
        case InventLocationType::Quarantine:
        case InventLocationType::Transit:
            _result.result(true);
            break;
    }
}

When the delegate method requests a response by using an EventHandlerAcceptResult or an EventHandlerRejectResult object parameter, the subscriber is expected to respond only with an accept or a reject. The subscribing logic might also add messages to the Infolog.

The following example resembles the previous example. However, the delegate method now requests a response by using an EventHandlerAcceptResult object and by calling the accept method.

[SubscribesTo(tableStr(InventWarehouseEntity), delegateStr(InventWarehouseEntity, validateWarehouseTypeDelegate))]
public static void validateWarehouseTypeIsSupportedStandardDelegateHandler(InventLocationType _inventLocationType, EventHandlerAcceptResult _result)
{
    switch (_inventLocationType)
    {
        case InventLocationType::Standard:
        case InventLocationType::Quarantine:
        case InventLocationType::Transit:
            _result.accept();
            break;
    }
}

The following example shows a delegate handler method that responds by using an EventHandlerRejectResult object. To respond by using an EventHandlerRejectResult object, you can call the reject method or the checkFailed extension method. If you use the checkFailed method, you can add a warning message to the Infolog. Internally, the checkFailed method calls the reject method.

[SubscribesTo(classStr(ProdTableType), delegateStr(ProdTableType, validateWriteProdTableInventRefTypeDelegate))]
public static void validateWriteProdTableInventRefTypeDelegateHandler(ProdTable _prodTable, EventHandlerRejectResult _result)
{
    if (_prodTable.InventRefType == InventRefType::ProdLine)
    {
        if (! _prodTable.InventRefId || !_prodTable.InventRefTransId)
        {
            _result.checkFailed("@SYS19558");
        }
        ProdBOM prodBOM;
        select prodBOM
            where prodBOM.InventTransId  == _prodTable.InventRefTransId;
        if (! _prodTable.checkRefProdBOM(prodBOM))
        {
            _result.reject();
        }
    }
}

Guidelines

In addition to the previously described practices, the following general guidelines apply:

  • Respond only when the subscribing logic is responsible for responding. The delegate handler methods were implemented to provide a response when a specific condition is met. Therefore, the subscribing logic must provide a result when a specific condition is met. Before the subscribing logic responds, it should not evaluate whether the result object parameter already contains a result. For example, a delegate handler method should not contain logic that resembles the logic in the following example. This logic evaluates whether the EventHandlerResult object parameter already contains a result when the method is run.

    Warning

    This example is an example of code that you should not write.

    [SubscribesTo(tableStr(InventWarehouseEntity), delegateStr(InventWarehouseEntity, validateWarehouseTypeDelegate))]
  public static void validateWarehouseTypeIsSupportedStandardDelegateHandler(InventLocationType _inventLocationType, EventHandlerResult _result)
  {
      // this if statement is an example of the bad practice.
      if (_result.hasResult())
      {
           return;
      }
      switch (_inventLocationType)
      {
          case InventLocationType::Standard:
          case InventLocationType::Quarantine:
          case InventLocationType::Transit:
               _result.result(true);
              break;
      }
  }

  • Don't provide a response on behalf of other subscribers. If the delegate handler method isn't responsible for providing a response, the method must not provide a response. If the method provides a response when the condition isn't met, it provides a response on behalf of other subscribers. The requesting logic must be responsible for handling situations where no subscribers have responded. The delegate handler method must not contain logic that resembles the logic in the following example. This logic which provides a result when the condition is evaluated to false.

    Warning

    This example is an example of code that you should not write.

      [SubscribesTo(tableStr(InventWarehouseEntity), delegateStr(InventWarehouseEntity, validateWarehouseTypeDelegate))]
  public static void validateWarehouseTypeIsSupportedStandardDelegateHandler(InventLocationType _inventLocationType, EventHandlerResult _result)
  {
      switch (_inventLocationType)
      {
          case InventLocationType::Standard:
          case InventLocationType::Quarantine:
          case InventLocationType::Transit:
              _result.result(true);
              break;
  	    // this default block is an example of the bad practice
          default:
              _result.result(false);
              break;
      }
  }