Flex4 component _ FX: Declarations

 FX: declarations is used to define non-visual sub-objects of an application to indicate a property declaration. These Sub-objects are no longer limited to independent use under the application root directory.

Top-level packages are placed under FX. Such as data type, global function, and global constant.

It is legal to write in flex3 like this:
<XML id = "data_xml"/>, but it does not work in gumbo. The correct method requires <FX: declarations> to declare,
For example:
<FX: declarations>
<FX: Number ID = "D"> 100 </FX: number>
<FX: XML id = "data_xml" Source = "test. xml"/>
<Mx: httpservice id = "yahoonewsservice" url = "http://rss.news.yahoo.com/rss/mostemailed" method = "get"/>
<S: fade id = "F_e" alphafrom = "0.2" alphato = "1"/>
</FX: declarations>

Any invisible virtual object (unlike a label or other visual object, you must declare it before use. Of course, if you place a visual object in <FX: declarations> the statement does not report an error, but it cannot be seen.