EVENT LISTNER & HANDLERS:

When click on a page sub-pages will open --->internal servlet is called by granite.js-- Approchable for 3rd party.

What if i want some functionalities internally??

Eg: When  a page is created i want to set few properties automatically.


When to use??
To assign some default functionalities when some event occurred.


UseCase::

-->When  a node is created felix console throws some event (Internally :: 'A node created' event is created by event manager)


-->Whiteboard pattern is a memory in aem

When we load  a bundle  it goes into active state. What happen  in this process??
All components in the bundle will get activated. At time of Activation.
Anything with @Component is a singleton, So Object is created  for that component.

While creating that Object.
If it is a servlet, in memory it created  a list of servlets.
If it is a Event listener, in memory it created  a list of Event listener.
If it is a Event handler, in memory it created  a list of Event handler.
If it is a Workflow, in memory it created  a list of Workflow.

It will separate/Categorize --> This is called whiteboard pattern.

Whiteboard pattern is a design pattern implemented by apache felix.

In a nutshell: Whiteboard pattern brings the objects into the  memory & Segregate them.

Eg:  In Jcr  any small changes then triggers an event like -->craete node+save triggers an event  & felix console catches the event. Now felix will go to whiteboard  & check for the eventListener.

Event-- Event Listener binding/registering/tagging is done by ObservationManager.

If it is registered then it executes that class.


Case2:
:: @Reference -->While code execution it will search for that singleton object -->goes to the registered service object  & executes it.

-->
All this happens  in runtime
1. Whiteboard pattern
2.EventListener pattern.
3.Singleton Pattern



Q) What happens when  a singleton object is requested by multiple events??
A) for Each call a thread is created.




EVENT LISTENER:


Adv:

Its very fast in repository level events & single level nodes.
Eg:tag remove or add.

Dis Adv:


1. When a node is created with automatic child nodes also created then event listener will applied child nodes also(eg: page is created -> event is triggered 2 times).

2. When dealing with more nodes say 5000 nodes then 5000 event will be triggered which consumes a lot  of time & memory . System will hang.

3. Its In-memory, When restared there  might be a data loss so UN RELIABLE.

-->EventListener class should implement EventListener.

--> get ListenerSession when active is called.

--> From Session-->get workspace -->get observation manager (in repository i.e in JCR  level).
For this observation manager  add an EventListener.

When create a tag it is stored as cq:tags string[] namespace:tag

Here comes in Event Handler

Event Handler::

-->Based on node type event will be triggered.

For page -->event will be triggered single time.

For Asset -->event will be triggered single time.

--> Better than Event Listener but still expensive.

--> Doesn't have observation manager but works on EVENT_TOPIC

-->EventConstansts: We can manually  register our own events.

--> EVENT_TOPIC : implemented by sling-->to bind/register  to events---> Here  Dam & page events

--> One event can be converted in to another by formEvent()--->take the details  in one event & converts into another

There are some functionalities which should be executed even if system hangs out (renditions etc)then the savior is: