Snap Event Manager

The Snap Event Manager is available on each controller via controller.eventManager and is used to create events and attach middleware to them. Events are recommended to be configured via Configuration Middleware to hook into controller events at critical times in the life cycle. It also allows for custom events to be used throughout your implementation.

Controller usage

Snap Event Manager is a dependency of Snap Controller and it is recommended to use methods of the controller to attach events to the EventManager. Additionally, different events exist for the different controllers - see the Controller documentation for more details.

Standalone usage

$3

Used to attach middleware to an event. If the event name previously had middleware attached, it will add to the middleware stack.

``js
controller.eventManager.on('interestingEvent', async (eventData, next) => {
// do something with the eventData

// pass control to the next middleware attached to the event
await next();

// do something after other middleware has fired
});
`

If a middleware returns false the entire middleware flow is interrupted and any remaining middleware is not executed.

$3


Invoke custom event. Data passed into the second parameter gets handed off to the middleware attached with the on method.

`js
controller.eventManager.fire('interestingEvent', { data: { some: 'string' } });
`

Middleware


Middleware provide a way for mutating or modifying the data passed in during the fire method.

$3

The first middleware attached with the on method is the first to execute. When calling await next(), control flows to the next attached middleware. This happens until the final middleware has been called after which control flows back up to the first middleware attached. The first middleware gets the first, and last opportunity to modify the data.

$3

`js
controller.eventManager.on('interestingEvent', async (data, next) => {
console.log('first middleware start');
await next();
console.log('first middleware end');
});

controller.eventManager.on('interestingEvent', async (data, next) => {
console.log('second middleware start');
await next();
console.log('second middleware end');
});

controller.eventManager.on('interestingEvent', async (data, next) => {
console.log('third middleware start');
await next();
console.log('third middleware end');
});

controller.eventManager.fire('interestingEvent', { data: { some: 'string' } } );

`

After firing the interestingEvent event, the following would be displayed in the console:
`text
first middleware start
second middleware start
third middleware start
third middleware end
second middleware end
first middleware end
``