Anatomy of a Feathers user interface component (Starling version)

The following properties and methods are core parts of FeathersControl and all custom component developers should be intimately familiar with them. Please be sure to read Feathers Component Lifecycle for additional, related information.

The Constructor

Any code appearing within the constructor should be kept to a minimum. In general, most initialization code should appear within initialize() instead.


The initialize() method is typically called the first time that the component is added to the display list. It will only be called once in the component's lifetime. You may override this method to create children and handle other tasks that should be run when the component is first initialized.

If validate() is called manually before the component is added to the display list, initialize() will be called immediately.


The invalidate() method should be called to tell the component that a property has changed and that it needs to update itself. Typically, this method is called within a setter function. It takes one or more flags as arguments to inform the component what has changed. The component may use these flags to focus on redrawing only a subset of itself if some parts are able to remain the same.


The draw() method is called when a component validates. Typically, this happens shortly before the component is rendered by Starling. You should override it to commit property changes, calculate an ideal size, and layout children.

Please read Creating a draw() function to validate a custom Feathers component for more detailed information about the draw() method.


The isInvalid() method is used to determine if a specific flag has been set with invalidate(). Call this in draw() to determine which parts of the component need to be redrawn. If you call it with no arguments, the result will be true if invalidate() has been called regardless of which flags have been passed in.


The saveMeasurements() method is called to set a component's dimensions during validation, if the width and height properties have not been set manually. In this case, the component needs to automatically measure its own ideal dimensions, possibly based on the dimensions of skins or sub-components and styles like padding and gap.

See below for more detailed information on the various properties for a component's dimensions.


The isQuickHitAreaEnabled property is similar to mouseChildren from the classic display list. However, it takes things a step further and limits the component's hit area to a simple rectangle, which can greatly improve performance of touch hit tests. The rectangular hit area is automatically calculated based on the component's actualWidth and actualHeight member variables (see below). This is most useful in buttons, but any component where the children don't need to receive touch events can benefit from this optimization.

styleName and styleNameList

A component's styleNameList is used by Feathers themes to provide separate skins to different types of the same component. It is most often used by components that have child components that need to be skinned, such as a Slider has track and a thumb sub-components that are buttons.

For more information about component style names, please read Introduction to Feathers Themes.


Properties that affect the visual appearance of a Feathers component are called "styles". When a style is set outside of a theme, the component needs to set a flag that ensures that the theme cannot replace the style's new value. The processStyleRestriction() method should be called inside the style's setter function to determine if a style should be ignored or not.

public function set customStyle(value:Object):void
	// handle the new value here

Calling processStyleRestriction() requires one argument to identify the style uniquely. Any object is allowed, but a reference to the setter function with arguments.callee works very nicely.

Display objects, such as for a background skins, should be disposed if the style property is considered restricted. Otherwise, you may have memory leaks.

public function set customStyle(value:DisplayObject):void
		if(value != null)
	// handle the new value here

Variables and properties for width and height

The FeathersControl class provides a number of useful variables and properties for its dimensions. Fully understanding what each one is used for and when they should be changed or accessed is important for maximizing the performance of custom Feathers components and getting the most out of the framework's architecture.

The width and height getters and setters expose the component's dimensions externally. The values returned by the getters are determined based on a number of factors. They may be explicit dimensions passed to the setters or they may be ideal dimensions calculated automatically during validation (because no explicit dimensions were specified).

The _explicitWidth and _explicitHeight variables are changed if the width and height setters are called with valid numeric values. In the following example, a Button control is created, and its width property is set to 150 pixels. Internally, the button will store this value in the _explicitWidth variable.

var button:Button = new Button();
button.width = 150;

The explicitWidth and explicitHeight getters expose these values publicly.

The actualWidth and actualHeight variables are the values used for layout. The "actual" dimensions typically default to the values of _explicitWidth and _explicitHeight, but if explicit dimensions are not specified, the component must calculate ideal dimensions. The ideal dimensions could be hard-coded values or they could be determined based on the dimensions of skins or sub-components, and other values like padding and gap. Most components calculate their ideal dimensions differently, but the result should always be passed to the saveMeasurements() method.

Minimum dimensions work similarly, with minWidth and minHeight properties exposed publicly, and _explicitMinWidth, _explicitMinHeight, actualMinWidth, and actualMinHeight variables used internally.