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Wednesday, January 28, 2009

I finally found the time to finish my FolderVisualizer application. To recap: FolderVisualizer draws a Tree Map of a selected folder which makes it easy to see which folders/files take up the most space. You can also delete these folders from within the application. I started this project with a colleague to do a comparison between Adobe Air and JavaFX.

Version 2.0.0RC1 is down-loadable from the project's website. The following screenshot gives an impression of the application:

The application implements the following functionality:

  • Select a folder which must be visualized in the tree map

  • drill down to other folders

  • change the number (density) of the folders in the tree map

  • provide a breadcrumb navigation path for easy navigation

  • Delete the selected folder/file

  • Visual effects when changing view stacks

  • More appealing theme/colors than the previous version

Version 2 is a complete rewrite of the whole application. Because a lot of new functionality is added, I restructured the application to make it more maintainable and transparent. As a guideline, I used the principles of MVCS, which is basically a set of design guidelines to implement the Model View Controller Service architecture in Flex applications. It is NOT a framework unlike for example Cairngorm. I Used Cairngorm on another project but I had the impression that the framework gets in your way. I found myself writing to much boilerplate code like commands, custom events, controllers etc. I'm NOT saying that Cairngorm is a bad framework, but I found it to complicated for the task at hand: structuring a Flex application in a well defined matter.

On the other hand, I really like the clarity and simplicity of the MVCS approach and the freedom to adapt the principles to your application's needs. The FolderVisualizer application consists of a couple controllers, a model class which contains references to the application's model objects, a FileService and components which make up the view. To handle user interaction, I used the event bubbling approach. In the event bubbling approach, when the user clicks on a button, a UIEvent is dispatched. This event "bubbles" up to the parent who handles it, usually a controller. This controller is responsible for handling the business logic associated with this specific user action. Consider the following example, taken from the FolderVisualizer application:
<?xml version="1.0" encoding="utf-8"?>
<mx:HBox xmlns:mx="">

... Some code omitted

public var model : Model;

private function handleBack(event:Event):void {
dispatchEvent(new UIEvent(UIEventKind.BACK))

... Some code omitted

<mx:Spacer width="5"/>
<mx:Button label="Back" click="handleBack(event)"/>
... Some code omitted
The above code is taken from the NavigationControlBar.mxml which implements the navigational actions a user can perform. When a user clicks on the back button, a UIEvent is dispatched of kind UIEventKind.BACK. Also note that this component defines an event "uiEvent" which parent containers can use to add an eventListener which listens for UIEvents. See the following code-snippet taken from FolderVisualizer.mxml, which is the parent container of the NavigationControlBar:
<?xml version="1.0" encoding="utf-8"?>
<mx:HBox xmlns:mx=""

... Some code omitted

<!-- Controllers used in the application, could also use dependency injection here (for example Prana) -->
<controller:AppController id="appController" model="{model}"/>
<controller:NavigationController id="navController" model="{model}"/>

<view:ControlBar width="100" height="100%" uiEvent="appController.handleUIEvent(event);"/>

<mx:VBox width="100%" height="100%" verticalGap="0">
<view:NavigationControlBar height="35" width="100%" verticalAlign="middle" horizontalAlign="left" horizontalGap="0" model="{model}" uiEvent="navController.handleUIEvent(event);"/>
... Some code omitted
As you can see in the above code, the navController.handleUIEvent method handles all UIEvents dispatched from the NavigationControlBar. One way to handle the events in the handleUIEvent method is the use of a switch statement. For every UIEventKind which the controller must handle, a separate case in the switch statement is used which calls the correct method. Consider the following example:
public function handleUIEvent(event:UIEvent) : void {
switch (event.kind) {
case UIEventKind.BACK:
// handle event
case UIEvent.SHUTDOWN:
// Handle event
trace("No event handler defined")
What I don't like about this implementation is the excessive amount of switch-case code to handle the events. For every controller in your application which must handle UIEvents, a switch block must be implemented. Instead of using a switch block, I used a different approach for handling UIEvents in controllers. I implemented a MultiActionController which acts as a base class of all controllers which must be able to handle UIEvents. The following code snippet comes from the MultiActionController (some code omitted):
public class MultiActionController extends BaseController
* Mappings between UIEvent.kind (see UIEventKind) to functions. Those functions are defined in
* the subclass.
protected var _handlerMappings:HashMap = new HashMap();

public function MultiActionController()

public function handleUIEvent(event:UIEvent) : void {
var funct:Function = _handlerMappings.getValue(event.kind)
if (funct==null) {
trace("No function for [" + event.kind + "] is defined in the handlerMappings. Make sure you define this event to function mapping in the constructor of you base class. See the ASDocs of the MultiActionController.")
throw new IllegalOperationError("No function for [" + event.kind + "] is defined in the handlerMappings. Make sure you define this event to function mapping in the constructor of you base class. See the ASDocs of the MultiActionController.")
}, event)
The MultiActionController defines a _handlerMapping variable of type HashMap. This hashmap contains the mapping between UIEventKind and functions (functions are objects in ActionScript 3). When the handleUIEvent is called, this method looks up the function belonging to the UIEvent.kind property. If this function exists, it is called passing the UIEvent as a parameter and stops propagation for this event. Otherwise an IllegalOperationError is thrown. Now take a look at the NavigationController which extends MultiActionController:
public class NavigationController extends MultiActionController
private var _model:Model;

public function set model(model:Model):void {
_model = model

public function NavigationController()
super._handlerMappings.put(UIEventKind.BACK, this.handleBack)
super._handlerMappings.put(UIEventKind.DIRECT_LINK, this.handleDirect)
super._handlerMappings.put(UIEventKind.DENSITY_CHANGE, this.handleDensityChanged)
super._handlerMappings.put(UIEventKind.DRILLDOWN, this.handleDrilldown)

private function handleBack(event:UIEvent):void {
... Do actual work here

... Some code omitted

The constructor of the NavigationController defines the mappings between the UIEvents this controller handles, and the corresponding functions to call. As you can see, there is no need to write blocks of switch statements in every controller to handle UIEvents. Just define the mapping between the UIEvents and functions in the constructor of the corresponding controller. One thing to be aware of is that all functions that are called by the MultiActionController accept a single argument of type UIEvent. This should not be a problem because the only way to call these functions is by dispatching a UIEvent. The UIEvent.payload property should contain all data required for the function to do its work.

Note that this is NOT a framework but an implementation variant of the MVCS principle for handling UIEvents. Feel free to use this code in your own project.


Structuring and organizaing a Flex (and any other!) application gives you the following benefits:

  • easier to maintain the application

  • easier to extend the application with new funcitonality

  • easier to test the application

  • easier to understand the application for new team members

  • ...

Implementing the principles of the MVCS approach is an effective way of organizing and structuring Flex applications. The principles of MVCS are not restricted to a particular implementation. Instead, they can be implemented in various ways that best suites the needs of your application. In this post, I showed you a variation of handling UIEvents which reduces the amount of code necessary to handle these events. When you work on moderate to large Flex applications, have a look at the MVCS guidelines to structure the application. The time you invest does get pay back. The complete source code of the FolderVisualizer application can be found here.

One final note: although I did not use a DI framework, for example Prana, using a DI framework can be an effective way to decrease the coupling between components and increase testability. At the moment, my DI framework of choice for Flex applications is Prana.


Folder Visualizer
Tree map component
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