Application event in Lightning AURA component

Lightning framework is based on event-driven architecture which allows to communicate between different events. Lightning events are fired from JavaScript controller actions that are triggered by a user interacting with the user interface.

Note:-The handler for an application event won’t work if you set the name attribute in <aura:handler>. Use the name attribute only when you’re handling component events.

The communication between components are handled by events. There are two types of custom events in the Lightning Framework:

• Component Events
• Application Events

Application Events are handled by any component have handler defined for event.These events are essentially a traditional publish-subscribe model. link

Application Event:

SampleApplicationEvent.evt

<!--SampleApplicationEvent.evt-->
<aura:event type="Application" description="Sample Application Event">
    <aura:attribute name="message" type="String" />
</aura:event>

Component1.cmp:

<!--Component1.cmp-->
<aura:component>
    <aura:registerEvent name="SampleApplicationEvent" type="c:SampleApplicationEvent"/>
    <lightning:button label="Click to fire the event" variant="brand" onclick="{!c.component1Event}"/>
</aura:component>

Component1 JS Controller: Use $A.get(“e.myNamespace:myAppEvent”) in JavaScript to get an instance of the myAppEvent event in the myNamespace namespace.
To set the attribute values of event, call event.setParam() or event.setParams().

({
    component1Event : function(cmp, event,helper) { 
        //Get the event using event name. 
        var appEvent = $A.get("e.c:SampleApplicationEvent"); 
        //Set event attribute value
        appEvent.setParams({"message" : "Welcome "}); 
        appEvent.fire(); 
    }
})

Component2: The application event handled by the Component2 that fired using aura:handler in the markup.
The action attribute of aura:handler sets the client-side controller action to handle the event.

<!--Component2.cmp-->
<aura:component>
    <aura:attribute name="eventMessage" type="String"/> 
    <aura:handler event="c:SampleApplicationEvent" action="{!c.component2Event}"/>
    <div class="slds-m-around_xx-large">
        <p>{!v.eventMessage}</p> 
    </div>
</aura:component>

Component2 JS Controller:

({
    component2Event : function(cmp, event) { 
        //Get the event message attribute
        var message = event.getParam("message"); 
        //Set the handler attributes based on event data 
        cmp.set("v.eventMessage", message + 'Biswajeet');         
    }
})

Lightning Application:

<aura:application extends="force:slds">
    <c:Component1/>
    <c:Component2/>
</aura:application>

link