Learning Flex 4 – Spark Components and Skinning Basics

One of the key changes to Flex moving to version 4 is the introduction of the Spark component set.

The components in Flex 3 (Button, TextInput etc) were known under the code name Halo and they’re still available in Flex 4. These components are distinguished by the mx namespace prefix you are used to. The new Spark components have an s prefix and require the addition of a new namespace definition – xmlns:s="library://ns.adobe.com/flex/spark" (The mx namespace definition also changes to¬† xmlns:mx="library://ns.adobe.com/flex/halo").

There are Spark equivalents of most but not all mx or Halo components (DataGrid, AdvancedDataGrid and Tree are some of the currently missing).

The key difference in the component sets is their construction. Halo components contain everything required for that object including styling, behavior and layout. Spark components on the other hand, separate out the behavior of a component from the display aspects. The information corresponding to component display is kept in a separate skin class.

The upshot of this change is that the look and feel of an application can be separated from it’s behavior and more readily changed. With the introduction of the Catalyst tool in CS5, designers can deal with the display skins while developers deal with the underlying behavior.

You can specify the skin a component should use with:

  • CSS: S|Button {skinClass: ClassReference ("com.example.skins.MyButtonSkin");}
  • MXML: <s:Button skinClass="com.example.MyButtonSkin"/>
  • ActionScript: myButton.setStyle("skinClass",Class(MyButtonSkin);

The skin class is generally written in MXML and defines the states (with transitions) the skin reacts to, graphical elements and any component parts the skin is constructed from. The skin may also specify the HostComponent in a metadata tag. The HostComponent is the component class that will use the skin. This is useful for the skin to be able to gain a reference to properties of the component instance.

A skin class must declare state properties that are specified for it’s host component. For example, a Spark Button has states up, over, down and disabled so the skin must declare what it looks like for each of these states. The component will then set the relevant state at runtime.

Skin parts are sub components used to define a larger component – for example, a scrollbar has increment and decrement button parts together with a track and thumb part whereas a button just has a label.

The graphical elements of a skin are defined using a new graphics tag library cald FXG which includes graphics and text primitives including tags for creating basic shapes, fills, gradients and bitmaps. FXG is also used by Catalyst and other tools to transfer graphical data. The specification for FXG can be found here.

A great way to begin working with Skins is to modify the Adobe supplied skin class for a component. Flash Builder 4 has a workflow for creating skin classes that allows you to work from a supplied class. With FlashDevelop, you’ll have to go off and find the class for yourself and copy it in to your project under the name you want. You’ll find the skin class source files under your SDK install in the frameworks\projects\spark\src\spark\skins\spark directory.

For further detail on skinning Spark components, take a look at these Adobe articles.

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