Event Handlers MAKE INVISIBLE NOT USED

Event Handler artefacts (.eh.lua) are the Event Sourcing implementation in TENUM®. With event handlers, you can react to certain events in your app. Events are dispatched asynchronously, and in your event handlers you can filter for the events you want to react to. There are multiple event types that are dispatched by default. For now, you cannot create your own custom events and dispatch them - since this is still under development. However, you can react to filesystem events.

Before we go any further into event handlers, we have to explain another native API - the filesystem API. The filesystem API is how you do persistence in your application. We will take a deep dive into this API later, but for now I want to show you 2 functions of this API in action - the filesystem.createOrUpdateFile and filesystem.delete. Let's go ahead and test them out in the Tutorial project. Let's first add a save button, which will save whatever the user has typed into the input field to a file:

If we now go ahead and enter something in the input and click "Save":

Then we should be able to see the file in the production instance filesystem:

Success! Now if we change the input again, the file should be updated:

Showing the filesystem.delete function is trivial. We only to pass in a path argument to the function and the file will be deleted.

Note: This IDE that lets you see the filesystem of the production/test instances is located on a different route. More on this later.

Now to get back to event handlers. Whenever you create, update or delete files in you running application, TENUM® dispatches a filesystem event containing the path of the file that was created/updated/deleted. Let's see how we can catch these events and react on them. First lets create the necessary folder structure that TENUM® will look for in order to invoke your event handler artefacts.

Now, if we add event handlers in the FileCreated folder, each of them will be invoked whenever a file is created. This is similar for FileUpdated and FileDeleted. TENUM® will invoke the event handlers with the path of the file as an argument, it is up to you - the application developer to filter the events you are interested in and react on them.

Let's see an example. We will now add an event handler in FileUpdated, and we will print the props to see what TENUM® is providing us. Keep in mind in the above code, we already create test.txt, so next time we call click on the "Save" button we will actually update the existing file - hence getting a FileUpdated event.

Whether you should use Event Handlers in you application is up to you. There are pros and cons to using Event Sourcing, but it's important that you know that TENUM® provides this pattern for you. Next, let's take a look at Decorators.