Placing Flex-component in the middle of the screen

I have a very general question and have prepared a simple test case.

When using BasicLayout (i.e. absolute positioning) - what is the best approach to place a Flex component in the center of the application?

In the test case below I'm using x="{width/2}" but this gives me at least 2 problems:

1. How do I account for the component dimensions (the ProgressBar in the test case)?

2. Is the binding {width/2} a good idea to use? Wouldn't it send unnecessary DATA_CHANGE events in some cases?

And finally I wonder, how this all applies to full screen applications using StageScaleMode.SHOW_ALL - because currently my application is aligned to the top-left in the full screen mode and there is a white dead zone on the right of it. (Which is at least a change from pure Flash/AS3 behaviour, where the full screen content is displayed in the center of the screen and 2 dead zones on the left and right).

<?xml version="1.0" encoding="utf-8"?>
<s:Application 
    开发者_开发问答xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:s="library://ns.adobe.com/flex/spark"
    xmlns:mx="library://ns.adobe.com/flex/mx"
    width="700" height="525" 
    backgroundColor="#CCFFCC"
    initialize="systemManager.stage.scaleMode=StageScaleMode.SHOW_ALL">

    <fx:Script>
        <![CDATA[
            private function fullScreen(event:MouseEvent):void {
                stage.displayState = 
                    stage.displayState == StageDisplayState.NORMAL ?
                    StageDisplayState.FULL_SCREEN :
                    StageDisplayState.NORMAL;
            }
        ]]>
    </fx:Script>

    <s:states>
        <s:State name="normal" />
        <s:State name="connected" />
    </s:states>

    <s:CheckBox right="10" bottom="10" 
        label="Full screen" 
        click="fullScreen(event)" />

    <mx:ProgressBar indeterminate="true" 
        x="{width/2}" y="{height/2}" 
        label="Connecting..." labelPlacement="center" 
        includeIn="normal" />

</s:Application>            

Way easier than that: just use horizontalCenter and verticalCenter properties of UIComponent. When used inside a BasicLayout the component will always be centered. Like this:

<mx:ProgressBar indeterminate="true" 
                horizontalCenter="0" verticalCenter="0"
                label="Connecting..." labelPlacement="center" />

If you give these properties a value other than '0' the component will be offset from the middle by the amount of pixels you specify as a value. (e.g. horizontalCenter="50" will offset the component 50 pixels to the right of the center of the parent component)

That white space is probably due to your usage of systemManager.stage.scaleMode=StageScaleMode.SHOW_ALL. Just remove that line. Why are you doing that anyway?

edit

Just noticed you're using a fixed 'width' and 'height' on your Application: you'll have to make those '100%' if you want your app to really fill the screen. This is another possible cause for that white space.

To get your application centered (rather than left/right aligned) when using systemManager.stage.scaleMode=StageScaleMode.SHOW_ALL, set systemManager.stage.align to the empty string - i.e.:

// in your Application creationComplete handler
systemManager.stage.scaleMode = StageScaleMode.SHOW_ALL;
systemManager.stage.align = "";

I typically do 2 things,

I place objects inside VGroups/HGroups with alignment, I also use relative widths eg percentages.

Think like in the old days of using HTML tables. It works very well especially if your canvas is resized allot.

All of my canvases have to be resizeable as I like to only write my apps once, for pc, mobile etc, so I just make everything scalable by default.