EventHandlerResult classes in request or response scenarios

Delegate methods and delegate handler methods can be declared to support a request/response scenario, where the delegate calling logic requests the subscribers to provide a response. To support this scenario the EventHandlerResult class is most often passed as a parameter, and the delegate handler methods provide their result using one of the result methods on the class. However, the EventHandlerResult class can only contain a single result, so if multiple subscribers provide their individual result, the last respondent wins, and the results from the previous subscribers are overwritten.

Before the functionality described in this article was introduced (platform update 5), there was no mechanism to ensure that, at most, a single subscriber provided a result, and that no results were lost if there were multiple subscribers.

Ensuring, at most, one response

In platform update 5, the EventHandlerResult class has an additional static constructor which ensures that the logic fails if more than one subscriber provides a result. The new constructor is named newSingleResponse. When instantiating an EventHandlerResult object using this method, the framework will throw an exception as soon as a second delegate handler method attempts to provide a result.

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EventHandlerResult result = EventHandlerResult::newSingleResponse();
this.validateWarehouseTypeDelegate(this.WarehouseType, result);

IEventHandlerResultValidator interface

The validation in the EventHandlerResult class is handled by injecting an object of a type that implements the IEventHandlerResultValidator interface. When instantiating the EventHandlerResult object using the newSingleResponse static constructor, an EventHandlerSingleResponseValidator object is instantiated and injected into the EventHandlerResult object, and the injected object becomes responsible for validating any result provided to the EventhandlerResult object. Other validation classes can be implemented by having the class implement the IEventHandlerResultValidator interface, and injecting it into the EventHandlerResult class by instantiating the EventHandlerResult object using another new static constructor named newWithResultValidator. The constructor takes an argument of type IEventHandlerResultValidator, which makes it possible to inject any validator object as long as it implements the IEventHandlerResultValidator interface.

For example, the newSingleResponse static constructor simply delegates the instantiation to the newWithResultValidator static constructor like this.

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return EventHandlerResult::newWithResultValidator(EventHandlerSingleResponseValidator::construct());

Accept and reject request/response scenarios

In certain request/response scenarios, the subscriber is only expected to provide their acceptance or rejection. Using the EventHandlerResult class to request acceptance/rejection can be confusing, if the subscriber is only expected to respond with a Boolean value. In a validation scenario, for example, should the subscriber only respond with Boolean false, when validation fails, or should the subscriber also respond with Boolean true, if validation succeeds? If the response is gathered using an EventHandlerResult object, then the second subscriber that validates and replies with Boolean true, might overwrite the Boolean false from the first subscriber.

To mitigate this confusion, two new result type classes have been introduced in Platform update 5: EventHandlerAcceptResult and EventHandlerRejectResult.

When using the EventHandlerAcceptResult class, the delegate handler method can only respond by calling the accept method. When using the EventHandlerRejectResult class, only the reject method can be called.

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[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 two new classes also contain a newSingleResponse static constructor for use in scenarios where, at most, one subscriber is allowed to respond with their rejection or acceptance. Whether any subscriber has responded can still be answered by querying the hasResult method, and the acceptance/rejection is queried by calling either the isAccepted or isRejected methods for the EventHandlerAcceptResult and EventHandlerRejectResult classes, respectively.

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boolean ret = false;
EventHandlerAcceptResult result = EventHandlerAcceptResult::newSingleResponse(); 
this.validateWarehouseTypeDelegate(this.WarehouseType, result);
if (result.hasResult())
{
    ret = result.isAccepted();
}