Working With Event Handlers

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Event handlers are the controller layer in ColdBox and is what you will be executing via the URLor a FORMpost. All event handlers are singletons, which means they are cached for the duration of the application, so always remember to var scope your variables in your functions.
Tip: For development we highly encourage you to turn handler caching off or you will have to reinit the application in every request, which is annoying. Open the config/ColdBox.cfc and look for the coldbox.handlerCaching setting.  
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By default this is already done for you on the application templates.
config/Coldbox.cfc
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/**
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 * Development environment
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 */
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function development() {
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    coldbox.customErrorTemplate = "/coldbox/system/exceptions/Whoops.cfm"; // interactive bug report
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    coldbox.handlerCaching = false;
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    coldbox.handlersIndexAutoReload = true;
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    coldbox.eventCaching = false;
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    coldbox.viewCaching = false;
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}
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Handler Code
Go open the handlers/main.cfc and let's explore the code.
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component extends="coldbox.system.EventHandler" {
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​
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    /**
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     * Default Action
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     */
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    function index( event, rc, prc ) {
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        prc.welcomeMessage = "Welcome to ColdBox!";
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        event.setView( "main/index" );
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    }
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​
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    /**
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     * Produce some restfulf data
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     */
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    function data( event, rc, prc ) {
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        return [
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            { "id" : createUUID(), name : "Luis" },
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            { "id" : createUUID(), name : "JOe" },
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            { "id" : createUUID(), name : "Bob" },
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            { "id" : createUUID(), name : "Darth" }
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        ];
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    }
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​
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    /**
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     * Relocation example
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     */
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    function doSomething( event, rc, prc ) {
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        relocate( "main.index" );
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    }
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​
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    /************************************** IMPLICIT ACTIONS *********************************************/
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​
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    function onAppInit( event, rc, prc ) {
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    }
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​
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    function onRequestStart( event, rc, prc ) {
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    }
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​
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    function onRequestEnd( event, rc, prc ) {
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    }
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​
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    function onSessionStart( event, rc, prc ) {
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    }
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​
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    function onSessionEnd( event, rc, prc ) {
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        var sessionScope     = event.getValue( "sessionReference" );
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        var applicationScope = event.getValue( "applicationReference" );
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    }
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​
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    function onException( event, rc, prc ) {
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        event.setHTTPHeader( statusCode = 500 );
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        // Grab Exception From private request collection, placed by ColdBox Exception Handling
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        var exception = prc.exception;
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        // Place exception handler below:
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    }
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​
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}
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Let's recap: Every action in ColdBox receives three arguments:
event - An object that models and is used to work with the current request, called the request context.
rc - A struct that contains both URL/FORM variables (unsafe data)
prc - A secondary struct that is private only settable from within your application (safe data)
Setting Views - Default Layout
This line event.setView( "main/index" ) in the index action told ColdBox to render a view back to the user found in views/main/index.cfm.  
ColdBox also has the concepts of layouts, which are essentially reusable views that can wrap other views or layouts.  They allow you to reuse content so you can render views/layouts inside in a specific location in the CFML content.  By convention, ColdBox looks for a layout called layouts/Main.cfm.  This is yet another convention, the default layout.  Your application can have many layouts or non layouts at all.
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Executing Events
We have now seen how to add handlers via CommandBox using the coldbox create handler command and also execute them by convention by leveraging the following URL pattern:
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http://localhost:{port}/folder/handler/action
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http://localhost:{port}/handler/action
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http://localhost:{port}/handler
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Also remember, that if no action is defined in the incoming URL then the default action of index will be used.
Remember that the URL mappings support in ColdBox is what allows you to execute events in such a way from the URL. These are controlled by your application router: config/Router.cfc
Working With Incoming Data
Now, let's open the handler we created before called handlers/hello.cfc and add some public and private variables to it so our views can render the variables.
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function index( event, rc, prc ){
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    // param an incoming variable.
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    event.paramValue( "name", "nobody" );
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    // set a private variable
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    prc.when = dateFormat( now(), "full" );
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    // set the view to render
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    event.setView( "hello/index" );
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}
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Let's open the view now: views/hello/index.cfm and change it to this:
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<cfoutput>
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<p>Hello #encodeForHTML( rc.name )#, today is #prc.when#</p>
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</cfoutput>
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Please note that we used the ColdFusion function encodeForHTML() (https://cfdocs.org/encodeforhtml) on the public variable. Why? Because you can never trust the client and what they send, make sure you use the built-in ColdFusion encoding functions in order to avoid XSS hacks or worse on incoming public (rc) variables.
If you execute the event now: http://localhost:{port}/hello/index you will see a message of Hello nobody.
Now change the incoming URL to this: http://localhost:{port}/hello/index?name=ColdBox and you will see a message of Hello ColdBox.
Tip: Please see the layouts and views section for in-depth information about them.
Linking Events Together
Adding A Layout
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Contents
Handler Code
Setting Views - Default Layout
Executing Events
Working With Incoming Data