R – Flex custom button component

According to the Flex docs, as long as the property is bindable, you can simply do this (I've included the two extra buttons to demonstrate):

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">

    <mx:Script>
        <![CDATA[

            [Bindable]
            private var currentUser:String = "Bill";

            private function isUserAllowed(user:String):Boolean
            {
                if (user == "Bill")
                {
                    return true;
                }

                return false;
            }

        ]]>
    </mx:Script>

    <mx:VBox>
        <mx:Button label="My Button" enabled="{isUserAllowed(currentUser)}" />
        <mx:HBox>
            <mx:Button label="Try Tom" click="{currentUser = 'Tom'}" />
            <mx:Button label="Try Bill" click="{currentUser = 'Bill'}" />
        </mx:HBox>
    </mx:VBox>

</mx:Application>

Without currentUser marked [Bindable], though, it won't work.

Another way to go, if you wanted to bind more literally to the function (this is also expressed in the docs), would be to have the function respond to an event you dispatch when the current user changes, like so:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" initialize="init()">

    <mx:Script>
        <![CDATA[

            private var _currentUser:String = "Bill";

            public function set currentUser(value:String):void
            {
                if (_currentUser != value)
                {
                    _currentUser = value;
                    dispatchEvent(new Event("userChanged"));
                }
            }           

            [Bindable(event="userChanged")]
            private function isUserEnabled():Boolean
            {
                if (_currentUser == "Bill")
                {
                    return true;
                }

                return false;
            }

        ]]>
    </mx:Script>

    <mx:VBox>
        <mx:Button label="My Button" enabled="{isUserEnabled()}" />
        <mx:HBox>
            <mx:Button label="Try Tom" click="{currentUser = 'Tom'}" />
            <mx:Button label="Try Bill" click="{currentUser = 'Bill'}" />
        </mx:HBox>
    </mx:VBox>

</mx:Application>

So there are a couple of ways. IMO, the second seems somehow more proper, but there's definitely nothing wrong with the first. Good luck!

You can instantiate an arbitrary named type through ActionScript's "reflection API":

var clazz:Class = Class(getDefinitionByName("class.from.your.xml.file.Name"));
var component:Object = new clazz();

http://livedocs.adobe.com/flex/3/langref/flash/utils/package.html#getDefinitionByName()

If you get an error about the type not existing, this is because it isn't linked from elsewhere in your application and the compiler only adds classes that are referenced. You can work around this using the following compiler arg:

includes class [...]

Links one or more classes to the resulting application SWF file, whether or not those classes are required at compile time.

http://livedocs.adobe.com/flex/3/html/compilers_14.html#157203