Server-Side Events

TXDataServerModule published several events that you can use to implement additional server-side logic, customize XData behavior, among other tasks. You can use events, for example, to:

Implement custom authorization, refusing or accepting an operation based on the user credentials or any other context information;
Restrict/change the data returned to the client by TMS Aurelius CRUD Endpoints, by adding more filters to a query made by the user, for example;
Implement additional server-side logic, for example, performing extra operations after a resource is saved.

Events in XData are available in the Events property of the TXDataServerModule object. Such property refers to a TXDataModuleEvents (declared in unit XData.Module.Events) object with several subproperties, each to them related to an event.

Read Using Events for more detailed info. You can also see real-world usage of events by checking the Authentication Example using JSON Web Token (JWT).

Events in TXDataModuleEvents

General-purpose events

Name	Description
OnModuleException	Occurs when an exception is raised during request processing. You can use it to provide custom error-handling.
OnManagerCreate	Occurs when a TObjectManager instance is created to be used during request processing. You can use it to customize settings for the TObjectManager.

Events of TMS Aurelius CRUD Endpoints

Name	Description
OnEntityGet	Occurs after an entity is retrieved, right before being sent to the client.
OnEntityList	Occurs when the client queries an entity collection.
OnEntityInserting	Occurs right before an entity creation.
OnEntityInserted	Occurs right after an entity creation.
OnEntityModifying	Occurs right before an entity update.
OnEntityModified	Occurs right after an entity update.
OnEntityDeleting	Occurs right before an entity delete.
OnEntityDeleted	Occurs right after an entity delete.

Using Events

For example, to access the OnEntityInserting event:

uses {...}, XData.Server.Module, XData.Module.Events;

// Module is an instance of TXDataServerModule object
Module.Events.OnEntityInserting.Subscribe(
  procedure(Args: TEntityInsertingArgs)
  begin
    // Use Args.Entity to retrieve the entity being inserted
  end
);

In a less direct way, using method reference instead of anonymous method:

uses {...}, XData.Server.Module, XData.Module.Events;

procedure TSomeClass.MyEntityInsertingProc(Args: TEntityInsertingArgs);
begin
  // Use Args.Entity to retrieve the entity being inserted
end;

procedure TSomeClass.RegisterMyEventListeners(Module: TXDataServerModule);
var
  Events: TXDataModuleEvents;
begin
  Events := Module.Events;
  Events.OnEntityInserting.Subscribe(MyEntityInsertingProc);
end;

Listeners are method references that receive a single object as a parameter. Such object has several properties containing relevant information about the event, and differ for each event type. Names of event properties, method reference type and arguments follow a standard. The event property is named "On<event>", method reference type is "T<event>Proc" and parameter object is "T<event>Args". For example, for the "EntityInserting" event, the respective names will be "OnEntityInserting", "TEntityInsertingProc" and "TEntityInsertingArgs".

All events in XData are multicast events, which means you can add several events handlers (listeners) to the same event. When an event occurs, all listeners will be notified. This allows you to add a listener in a safe way, without worrying if it will replace an existing listener that might have been set by other part of the application.

It's always safe to set the events before adding the module and running the server.

OnEntityGet Event

Occurs after an entity is retrieved, right before being sent to the client. This event is also fired when the client requests part of that entity, for example, individual properties of the entity, blob data, or associated entities.

Example:

Module.Events.OnEntityGet.Subscribe(
  procedure(Args: TEntityGetArgs)
  begin
    // code here
  end
);

TEntityGetArgs Properties:

Name	Description
Entity: TObject	The retrieved entity.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityList Event

Occurs when the client queries an entity collection. This event is fired after the TCriteria object is built based on the client request, and right before the criteria is actually executed to retrieve the entities and sent to the client.

Example:

Module.Events.OnEntityList.Subscribe(
  procedure(Args: TEntityListArgs)
  begin
    // code here
  end
);

TEntityListArgs Properties:

Name	Description
Criteria: TCriteria	The Aurelius criteria built based on client request that will be used to retrieve the collections. You can modify the request here, adding extra filters, orders, etc., before it's executed and results are sent to the client.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityInserting Event

Occurs right before an entity creation. This event happens in the middle of a database transaction, right before the entity is about to be created in the database.

Example:

Module.Events.OnEntityInserting.Subscribe(
  procedure(Args: TEntityInsertingArgs)
  begin
    // code here
  end
);

TEntityInsertingArgs Properties:

Name	Description
Entity: TObject	The entity being inserted.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityInserted Event

Occurs after an entity is created. Note that, unlike OnEntityInserting Event, this event happens after the transaction is committed. There is no way to rollback the insertion of the entity, and any database operation here will be performed with no active transaction (unless you begin one manually).

Example:

Module.Events.OnEntityInserted.Subscribe(
  procedure(Args: TEntityInsertedArgs)
  begin
    // code here
  end
);

TEntityInsertedArgs Properties:

Name	Description
Entity: TObject	The entity which was created (inserted).
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityModifying Event

Occurs right before an entity update. This event happens in the middle of a database transaction, right before the entity is about to be updated in the database.

Example:

Module.Events.OnEntityModifying.Subscribe(
  procedure(Args: TEntityModifyingArgs)
  begin
    // code here
  end
);

TEntityModifyingArgs Properties:

Name	Description
Entity: TObject	The entity being modified.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityModified Event

Occurs right after an entity update. Note that, unlike OnEntityModifying Event, this event happens after the transaction is committed. There is no way to rollback the update of the entity, and any database operation here will be performed with no active transaction (unless you begin one manually).

Example:

Module.Events.OnEntityModified.Subscribe(
  procedure(Args: TEntityModifiedArgs)
  begin
    // code here
  end
);

TEntityModifiedArgs Properties:

Name	Description
Entity: TObject	The entity which was modified.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityDeleting Event

Occurs right before an entity delete. This event happens in the middle of a database transaction, right before the entity is about to be deleted in the database.

Example:

Module.Events.OnEntityDeleting.Subscribe(
  procedure(Args: TEntityDeletingArgs)
  begin
    // code here
  end
);

TEntityDeletingArgs Properties:

Name	Description
Entity: TObject	The entity being deleted.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnEntityDeleted Event

Occurs after an entity is deleted. Note that, unlike OnEntityDeleting event, this event happens after the transaction is committed. There is no way to rollback the deletion of the entity, and any database operation here will be performed with no active transaction (unless you begin one manually).

Example:

Module.Events.OnEntityDeleted.Subscribe(
  procedure(Args: TEntityDeletedArgs)
  begin
    // code here
  end
);

TEntityDeletedArgs Properties:

Name	Description
Entity: TObject	The entity which has been deleted.
Handler: TXDataBaseRequestHandler	The XData request processor object.

OnModuleException Event

Occurs when an unexpected exception is raised during server request processing. By default, when that happens XData will send a response to the client with the property HTTP status code (usually 500 but other codes might be provided as well) and a JSON response with details of the error (a JSON object with properties ErrorCode and ErrorMessage). You can use this event to change that behavior when a specific exception occurs.

Example:

Module.Events.OnModuleException.Subscribe(
  procedure(Args: TModuleExceptionArgs)
  begin
    // code here, for example:
    if Args.Exception is EInvalidJsonProperty then
      Args.StatusCode := 400;
  end
);

TModuleExceptionArgs Properties:

Name	Description
Exception: Exception	The Exception object raised while processing the requested.
StatusCode: Integer	The HTTP status code to be returned to the client. You can change this property to tell XData to send a different status code.
ErrorCode: string	The value of the ErrorCode property in the JSON response to be sent to the client. You can modify this value.
ErrorMessage: string	The value of the ErrorMessage property in the JSON response to be sent to the client. You can modify this value.
Action: TModuleExceptionAction	The action to be performed: `TModuleExceptionAction = (SendError, RaiseException, Ignore)` Default value is SendError which means XData will send the HTTP response to the client with the specified StatusCode and with a JSON response that includes ErrorCode and ErrorMessage. You can optionally use RaiseException, which means re-raising the original exception and let it propagate through the code. This gives an opportunity for some Sparkle middleware to catch the raise exception. If that doesn't happen, the exception will be handled by the Sparkle dispatcher. The third option is simply choose Ignore. Use this option if you want to send a custom HTTP response yourself. In this case XData will simply do nothing and finish processing request silently.

OnManagerCreate Event

Occurs when a TObjectManager instance is internally created by XData during request processing. You can use this event to initialize settings in the object manager, or change some behavior. For example, you can use this event to enable filters.

Example:

Module.Events.OnManagerCreate.Subscribe(
  procedure(Args: TManagerCreateArgs)
  begin
    // code here, for example:
    Args.Manager.EnableFilter('Multitenant')
      .SetParam('tenantId', 123);
  end
);

TManagerCreateArgs Properties:

Name	Description
Manager: TObjectManager	The TObjectManager instance that has just been created by XData.

Authentication Example using JSON Web Token (JWT)

Please refer to Authentication and Authorization guide for more information about how to secure your XData API.