When will I get to play with this? The demos blow me away each I time I see them.


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Posted in 3d, flash9. 5 Comments »

Flex as a DSL for Graphics

One of my colleagues, John Dunne, forwarded me this interesting article by Bruce Eckel about Flex as the universal platform for application development. The article talks about the state of affairs with web technologies. It talks about HTML, Javascript, AJAX and Java Applets and describes the problems like installation issues, browser incompatibilities and UI-richness. Finally he settles on Flex as being the solution that overcomes all of the said problems. Check it out.


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Flex and WPF: A comparison – Part 2b

In part-2a we saw some of the building blocks of WPF and how they are used to achieve the content-style separation. This post is a continuation and will focus on Flex’s approach.

Flex has adopted the web standards right from ground up. So if you are a web developer who has had any experience with HTML, CSS, Javascript it should be very natural to program in Flex. A direct one-one mapping looks like this:



Javascript –> Actionscript


MXML markup + code-behind

    MXML is the markup language that is used to describe the User Interface components in Flex. In concept it looks very similar to XAML, but since it is also like HTML, it has the familiar tags like <mx:Script/> and <mx:Style/>. The <mx:Script/> tag is used for inline code-behind whereas the <mx:Style/> describe the stylesheet elements for the UI elements. In a previous post I did a comparison of the code-behind models in Flex and WPF, which is slightly more detailed. Actionscript is the language used for the code-behind and its syntax is very similar to Javascript.

    The standard practice for describing UIs is to use MXML. Even the event-handlers are specified directly inside MXML. However it is also possible to bypass MXML completely and define the UI component completely in Actionscript. In the former approach, the MXML compiler will generate the Actionscript for you, which in turn gets compiled to SWF.


    Databinding is definitely my favorite feature in Flex and it is extremely powerful. It is also very easy to make your model objects bindable by including a simple metadata tag like [Bindable] against the model property or object. This is simpler than the WPF approach where you will have to implement the INotifyPropertyChanged interface for the model object and manually fire the property change notifications. In Flex just by attaching the [Bindable] metadata tag you get FREE property change notifications. Of course if you want to customize the notifications, that too is possible. What I find really interesting is the part where you bind a UI element to a model object. Say I want to bind the text property of a <mx:Label/> to the firstName property of the Person object. The syntax would look something like this:



    var _person:Person = new Person(“Pavan”, “Podila”);



    <mx:Label text=”{_person.firstName}”/>



Since I made the _person instance [Bindable], the Label will automatically update its text property whenever _person.firstName changes. What is exciting is the fact that the curly braces syntax for data-binding can actually include Actionscript inside it! This is very useful in cases where I want to do a little bit of more formatting to the text. If I wanted my text to appear like “FirstName: PAVAN”, I could do this right inside the {}:

    <mx:Label text=”FirstName: {_person.firstName.toUpperCase()}“/>

Doing the same in WPF would have required me to write a class that implements the IValueConverter and then pass in the instance of my model as a parameter. So the syntax would look like below.

[Note that I am following the C#/XAML naming conventions, so my property names have the first letter of each word capitalized: FirstName, LastName.]


    <local:StringToUpperCaseConverter x:Key=”stringConverter”/>

    <local:Person x:Key=”person” FirstName=”Pavan” LastName=”Podila”/>


<Label Content=”{Binding Path=FirstName, Converter={StaticResource stringConverter}}”/>

Also my converter class would look like this:

    [ValueConversion(typeof (string), typeof (string))]

    public class StringToUpperCaseConverter : IValueConverter


        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)


            string firstName = (string)value;

            return “FirstName: “ + firstName.ToUpper();



        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)


            return null;



That definitely is a lot more typing to do than the Flex’s approach. I see this is as a major benefit in Flex which I would love to have in WPF.


MXML components

    MXML components are reusable views in your application. Say you have a photo-viewer application that shows a list of photos. Each photo could be wrapped inside a MXML component that shows other information about the photo when you click on it. The view of the photo and the interaction can be captured inside this component and can be reused throughout your application. These components can also behave like the DataTemplates in WPF. You can create a property on the MXML component and set that as the model object. Inside the component you can data-bind to the properties of the model thus simulating the DataTemplate. Note that the property you create on your MXML component is your DataContext.

    ItemTemplates in WPF have a direct one-one mapping with the ItemRenderers in Flex. List based controls use the ItemRenderers to render each item in the list. Here is a simple example of an inline ItemRenderer.

<mx:DataGridColumn headerText=”Online” dataField=”online”>



<mx:CheckBox click=”!” selected=”{}”/>





You can also create a separate MXML file, say Checkbox.mxml, that encapsulates the component. In that case you provide the name of the component to the itemRenderer property (itemRenderer=”Checkbox”, assuming Checkbox.mxml is in the same directory).

CSS styles, skins

    Styling a Flex application means creating a CSS file (or a group of CSS files) that provides the appearance for the different UI elements. Most CSS files would include customized values for many of the UI element properties. But if you want more fine-grained control on the appearance one can also create something called the Skins. A skin file is also a CSS file but can include components from a compiled SWF file. The SWF may have custom rendered skins, also called programmatic skins. Here is a nice article on creating skins for Flex.

Packaging into SWC, SWF and modules

    SWF is the final version of a compiled Flex application. It may also have a bunch of dependencies on Actionscript/Component libraries. These libraries are compiled as SWC. The other option for packaging is to use Flex Modules. Bruce Phillips did a good post on this topic and covers all the necessary material.


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