Armindo Cachada's Blog: Struts 2, Adobe Flex and ATG: Adobe Flex: How to use verticalCenter and horizontalCenter in a container with absolute layout

In this tutorial I will explain how you can use the verticalCenter and horizontalCenter constraints in a container with absolute layout.

At this stage you probably already figured out how to use the left, right, top, bottom constraints. But what about horizontalCenter and verticalCenter?

horizontalCenter is the distance from the center of the container in the horizontal where you want your component to appear. If you give it a positive value, the component will show up in the right half of the container. If you give it a negative value the component will appear in the left half.


<mx:Canvas width="500" height="500">
   <mx:Button horizontalCenter="200" label="Right half of the container"/>
   <mx:Button horizontalCenter="-200" label="Left half of the container"/>
</mx:Canvas>

If you try to combine horizontalCenter with left or right constraints, they will always be ignored as the horizontalCenter constraint always takes precedence.


<mx:Canvas width="500" height="500">
   <mx:Button horizontalCenter="200" left="400" label="Button 1"/>
   <mx:Button horizontalCenter="200" right="200" label="Button 2"/>
</mx:Canvas>

In the example above the two buttons will overlap because the value of the left and right coordinate is ignored.

verticalCenter is the distance from the center of the container in the vertical where you want your component to appear. If you give it a positive value, the component will show up in the bottom half of the container. If you give it a negative value the component will appear in the top half.


<mx:Canvas width="500" height="500">
   <mx:Button verticalCenter="200" label="Bottom half of the container"/>
   <mx:Button verticalCenter="-200" label="Top half of the container"/>
</mx:Canvas>

If you try to combine verticalCenter with top or bottom constraints, they will always be ignored as the verticalCenter constraint always takes precedence.


<mx:Canvas width="500" height="500">
   <mx:Button verticalCenter="200" top="400" label="Button 1"/>
   <mx:Button verticalCenter="200" bottom="200" label="Button 2"/>
</mx:Canvas>

In the example above the two buttons will overlap because the value of the top and bottom constraint is ignored.

The code below shows possible combinations of horizontalCenter and verticalCenter that you can use to layout your components:


<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" height="520" width="520">
<mx:Button label="Left Center" horizontalCenter="-200" verticalCenter="0">
</mx:Button>

<mx:Button label="Center" horizontalCenter="0" verticalCenter="0">
</mx:Button>

<mx:Button label="Right Center" horizontalCenter="200" verticalCenter="0">
</mx:Button>


<mx:Button label="Top Left" horizontalCenter="-200" verticalCenter="-200">
</mx:Button>

<mx:Button label="Top Center" horizontalCenter="0" verticalCenter="-200">
</mx:Button>

<mx:Button label="Top Right" horizontalCenter="200" verticalCenter="-200">
</mx:Button>

<mx:Button label="Bottom Left" horizontalCenter="-200" verticalCenter="200">
</mx:Button>

<mx:Button label="Bottom Center" horizontalCenter="0" verticalCenter="200">
</mx:Button>

<mx:Button label="Bottom Right" horizontalCenter="200" verticalCenter="200">
</mx:Button>


<mx:Button label="Ignored right constraint" right="10" horizontalCenter="-200" verticalCenter="50">
</mx:Button>

<mx:Button label="Not ignored right constraint" right="10" verticalCenter="50">
</mx:Button>

<mx:Button label="Ignored top constraint" top="10" horizontalCenter="-200" verticalCenter="-50">
</mx:Button>

<mx:Button label="Not Ignored top constraint" top="10" horizontalCenter="-200">
</mx:Button>

</mx:Application>