Many ways to do the same thing…but which is right? It’s an issue developers face every single day.

Here’s one of those issues that I faced this past week: rotating an object in Flex. I was playing with the camera in Flex Mobile, and I quickly learned that it is landscape only even if the device is being held in portrait. Really, Adobe?

“…a Video object attached to the camera will only show upright video in a landscape-aspect orientation. Thus, mobile apps should use a landscape orientation when displaying video and should not auto-rotate.”
     – official Adobe docs for

Sad but true. So, if you wanted to build a video chat app and use the front camera you either always hold the iPhone in landscape or you hold it portrait and rotate the video in code. (Why did I say iPhone?…Alas, Flex Mobile does not provide front camera support for Android…period…none!)

Option 1: rotation Attribute

The first thing to try is to rotate the easy way, like this:

camera.rotation = 90;

Using actionscript, I just set the rotation to 90°. Of course, this rotates about (0,0), the upper left corner in Flex, and not what we need.

Option 2: rotation, Plus transformX and transformY

To rotate an object about its center, you can set transformX and transformY (and transformZ too), like this:

<s:Button id="btn" label="My Button"
    transformX="{btn.width * 0.5}"
    transformY="{btn2.height * 0.5}" />

Using MXML, I set rotation to 90°, plus transformX and transformY to half the width and height of the object (in this case, a button). By moving the rotation point from the top left to the center of the button, I get what I want: an object rotating about its center. [I had forgotten how this works...thanks go to Bindu for reminding me!]

Option 3: Matrix Rotation

Option #2 works because every component in Flex has a full affine transformation matrix behind it (as of Flash Player v9, I think, but only exposed to MXML in Flex 4). In MXML, when you set attributes like rotation, transformX, and transformY, you are just setting the underlying transformation matrix.

You can also set the matrix directly, like this:

var mat:Matrix = new Matrix();
mat.translate(-W/2, -H/2);
mat.translate(+W/2, +H/2);
myObj.transform.matrix = mat;

To get a rotation about the center of our object, we first translate it, then rotate, then translate it back. Note the concat() operation, which just “adds” the current matrix to our new rotation matrix, and thus preserves any previous transforms (maybe our object had scaleX or scaleY applied).

Option 4: Rotate Animation

Lastly, if we want to watch the rotation happen, we can use a Rotate animation, like this:

var rot:Rotate = new Rotate();
rot.angleBy = 90;
rot.duration = 1000;
rot.autoCenterTransform = true; = myObj;;

Here we just instantiate a new spark Rotate effect, set its various properties, and start it with a call to play(). The key property to force rotation about the center is autoCenterTransform, just set it to true.


When I need total control, I find myself doing the full matrix solution. Otherwise for anything simple, the MXML attributes are best.

Here’s an example project showing all options (view source enabled):

Flash is required. Get it here!


Did a little bit of vanilla Flex work recently, and I needed a Tree component to display an object hierarchy. Everyone, by now, hopefully knows that mx:DataGrid and mx:Tree are two of the crappiest, bug ridden, worst performing components from Flex 3. And, everyone by now, has left the buggy world of Flex 3 behind and entered the world Flex 4 and the vastly improved Spark components. With the arrival of Flex 4.5 this summer, Adobe finally gave us a rewritten Spark-based DataGrid. Alas, no updated Tree yet, so I had to write my own. So once again, I turned to the trusty combo of List plus custom ItemRenderer to make pure-Spark custom TreeList component that doesn’t suck.

Alex Harui is the guru of turning a Flex 4 Spark List into a look-alike for the old Flex 3 component using skins and a custom ItemRenderer. Alex has used List + ItemRenderer to make a DataGrid, DateField, ColorPicker, Menu + MenuBar, and even an XML-based TreeList. Unfortunately, Alex’s TreeList assumes incoming XML, and I needed a TreeList that could display a simple object hierarchy (root node with children, and those children have children, etc.). Since I couldn’t find exactly what I wanted, I decided to build it myself.


The key step to getting a hierarchy of objects to display as a list is: flatten the list, duh! Or at least flatten the part of the tree you wish to display. So, I built a simple adapter class that turns an object hierarchy into an ArrayList that can be given directly to a List‘s dataProvider.

Here’s the actual flattener, but with just the comments not the code:

public class MyObjFlattenedList extends ArrayList {
    //the root object
    private var _root:MyObj;
    //list of open nodes
    private var _openList:Array;
    public function MyObjFlattenedList(root:MyObj) {
        _root = root;
        _openList = [];
    public function reset(openList:Array = null):void {
        //init the flattened list, starting at root
        _openList = (openList == null ? [] : openList);
        var a:Array = [];
        addItemToList(_root, a);
        this.source = a;
    private function addItemToList(obj:MyObj, a:Array):void {
        //recursively walk obj and all of its "open" children to build
        //a flattened list that is returned in array a
    public function isItemOpen(obj:MyObj):Boolean {
        //true if obj has children and is "open"
    public function openItem(obj:MyObj):void {
        //add all of obj's children (if any) to the list
    public function closeItem(obj:MyObj):void {
        //remove all of obj's children (if any) from the list

There’s really not much to it. When instantiated with a root object, the object hierarchy is walked recursively to build a flattened list of all the open nodes. Once the initial list is built, openItem() can be called to open a node, and add all its children to the list. Alternately, closeItem() can be called to close a node, and remove all its children from the list.


I used some basic styling and skinning, but the ItemRenderer does the majority of the work. Here’s the abbreviated version of MyObjRenderer.mxml with all the boring stuff edited out:

<?xml version="1.0" encoding="utf-8"?>
            [Bindable] private var _obj:MyObj;
            [Bindable] private var _hasChildren:Boolean = false;
            private var _list:MyObjFlattenedList;
            override public function set data(val:Object):void {
       = val;
                if (val != null) {
                    _obj = val as MyObj;
                    var ownerList:List = owner as List;
                    _list = ownerList.dataProvider as MyObjFlattenedList;
                    btn.selected = _list.isItemOpen(_obj);
                    _hasChildren = (_obj.children != null && _obj.children.length > 0);
            private function toggleHandler():void {
                if (btn.selected) {
                } else {
        <s:State name="normal" />
        <s:State name="hovered" />
        <s:State name="selected" />
    <s:Rect ...>
    <s:HGroup ...>
        <s:ToggleButton id="btn" click="toggleHandler()" visible="{_hasChildren}" ... />
        <s:Group id="dot" visible="{!_hasChildren}" ... />
        <s:Label ... />

Each rendered item has a background Rect and a Label. But most importantly, each row has either a ToggleButton (if the object has children) or some non-interactive visuals (if the object doesn’t have children it just gets a dot). The toggle button is the key interactive element used to open or close the node, everything else is part of the visual gravy added to make the list look good.

Focusing on the functional stuff, if you look in the script block, you’ll see two functions: a data() setter and a toggleHandler() to handle toggle button clicks. As expected, clicking the toggle button calls openItem() or closeItem() on the flattener adapter which adds or removes children from the flattened list, respectively. The setter mostly sets up the local variables, but it notably computes if the object has children or not, and also sets the initial state of the toggle button.


With just a little effort, we can have a nice usable Spark TreeList component that looks decent. More importantly, we have total control and can make the TreeList look like anything our designer can throw at us. As is always the case, the combo of List + ItemRenderer proves to be awesome. I tried to cover all the interesting pieces, but for the details, you’ll need to check out the source code.

Here’s the finished product (view source enabled):

Flash is required. Get it here!

Use it and enjoy.


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