用到的MX标签：constraintColumns、ConstraintColumn、constraintRows、ConstraintRow
给大家说说前提情况：在做MXML布局时，要嵌套很多层级，这里面有很多问题：
1、嵌套太多层级会影响效率！
2、嵌套太多层级，代码的可读性、可维护性差！
3、嵌套太多层级是因为Canvas、Vbox、Hbox功能单一，不能满足需要，而不是必须要这样做。
好吧，来看一个MXML代码示例：
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" minWidth="955" minHeight="600">
<mx:constraintColumns>
<mx:ConstraintColumn id="column1" width="250"/>
<mx:ConstraintColumn id="column2" width="100%"/>
</mx:constraintColumns>
<mx:constraintRows>
<mx:ConstraintRow id="row1" height="25"/>
<mx:ConstraintRow id="row2" height="25"/>
<mx:ConstraintRow id="row3" height="100%"/>
<mx:ConstraintRow id="row4" height="35"/>
</mx:constraintRows>
<mx:Button id="topMenuCvs" label="topMenuCvs" width="100%"
top="row1:0" bottom="row1:0"/>
<mx:Button id="topButtonCvs" label="topButtonCvs" width="100%"
top="row2:0" bottom="row2:0"/>
<mx:Button id="leftNavCvs" label="leftNavCvs" height="100%"
top="row3:0" bottom="row3:0" left="column1:0" right="column1:0"/>
<mx:Button id="leftMainCvs" label="leftMainCvs" height="100%"
top="row3:0" bottom="row3:0" left="column2:0" right="column2:0"/>
<mx:Button id="footCvs" label="footCvs" width="100%"
top="row4:0" bottom="row4:0"/>
</mx:Application>
效果图：
<OBJECT height="700" width="800" border="0" classid="clsid:CA8A9780-280D-11CF-A24D-444553540000"
>
<PARAM NAME="_Version" VALUE="65539">
<PARAM NAME="_ExtentX" VALUE="20108">
<PARAM NAME="_ExtentY" VALUE="10866">
<PARAM NAME="_StockProps" VALUE="0">
<PARA ......
