PureMVC uses mediators for all communication from the model and controller to the view. Out of the box PureMVC normally creates the view mediators after creationComplete has fired and the model has been initialized. The reason for this is that mediators require a reference to a view component to work. In order to construct a mediator you first need an object or view component to pass to the constructor.

There are basically three different ways to handle this, two of which I don’t recommend using.

1. Use dummy objects to initialize mediators
If you know that your view component is not on the stage yet you could still create it’s mediator by passing a POASO (plain old actionscript object) to it’s constructor. I have two problems with this solution; One you’re unnecessarily using memory and two you have not treated the whole problem, you still need some type of event schema to reinitialize the mediator once the component has been created. I don’t recommend using this solution.

2. Use creationPolicy=”all” to avoid deferred views
By default components like ViewStack and TabNavigator on create their top level children at runtime. The remainder are created as the user navigates to them. You can use the creationPolicy=”all” attribute to override this behavior. This causes the application to create all the view components at runtime. This is a terrible idea and I cannot recommend it. What you are in effect saying to your user is “I don’t care about your memory or performance because I’m to lazy to code a decent solution” I know this is a bit dramatic but I hate when people use this attribute.

3. Use a custom Event subclass
The idea here is to create an event subclass that the application mediator or another mediator can listen for and use to create new mediators. This event class has an extra property which holds a reference to the newly created component, other mediators can use this reference to construct a mediator for the component. This approach not only keeps your memory footprint smaller but it allows you to easily create and destroy mediators. Here’s an example of an Event subclass:

You create your own constant names based on the event type you want your mediator to listen for.

[as]
package com.tomcornilliac.myproject.events
{
import flash.events.Event;

public class ComponentCreationEvent extends Event
{
// Publically accessible properties
public var component:Object;

//Constants used for routing withing PureMVC mediators
public static const SOME_EVENT:String = “gatewayCreated”;
public static const SOME_OTHER_EVENT:String = “gatewayStackCreated”;

public function ComponentCreationEvent(type:String, component:Object, bubbles:Boolean=false, cancelable:Boolean=false)
{
super(type, bubbles, cancelable);
//Probably should have a getter/setter for this property
this.component = component;
}

//You must override the clone method
override public function clone():Event
{
return new ComponentCreationEvent(type, component, bubbles, cancelable);
}
}
}
[/as]

An instance of this event is dispatched from your view component with a reference to its self. Note that “bubbles” is set to true, this is required for the event to make it up the chain to whatever mediator is listening.
[xml]

width="100%" height="100%" creationComplete="init(event)">

[Event(name="gatewayCreated", type="org.deschutes.grizzly.events.ComponentCreationEvent")]

import com.tomcornilliac.myproject.events.ComponentCreationEvent;

private function init(e:Event):void
{
dispatchEvent(new ComponentCreationEvent(ComponentCreationEvent.GATEWAY_CREATED, this, true));
}
]]>

[/xml]

The listening mediator (in this case the ApplictionMediator) will respond to the event and create and register a new mediator for the newly created view component. Now it’s important to note that while I’m using the ApplicationMediator in this example, you can use any mediator you want, so long as the event will bubble-up to it.

[as]
package com.tomcornilliac.myproject.view
{
import com.tomcornilliac.myproject.events.ComponentCreationEvent;
import com.tomcornilliac.myproject.view.components.SomeComponent;
import org.puremvc.interfaces.IMediator;
import org.puremvc.interfaces.INotification;
import org.puremvc.patterns.mediator.Mediator;

public class ApplicationMediator extends Mediator implements IMediator
{
//Canonical name of the mediator
public static const NAME:String = ‘ApplicationMediator’;

//Constructor
public function ApplicationMediator(viewComponent:Object)
{
super(viewComponent);

/**
* Setup event listeners for components created dynamically
*/
app.addEventListener(ComponentCreationEvent.SOME_EVENT, createNewMediator);
}

private function createNewMediator(e:ComponentCreationEvent):void
{
//Create the new Mediator and initialize it with the component from our event
var mediator:SomeComponentMediator = new SomeComponentMediator(e.component);
//Register the mediator with the Application Facade
facade.registerMediator(mediator)
//That’s it now your view component is hooked-up and ready to handle I/O
}

….other methods
[/as]

This technique is not only good for component creation but you could just as easily create a custom ComponentDestructionEvent and use it to remove Mediators off the stack when no longer needed.

If you would like more information about the PureMVC framework I encourage you to visit the PureMVC web site. The documentation is excellent and there’s even a training course to help you on your way.