Multiple Inheritance in Flex

Learned something this weekend about how to do multiple inheritance in Flex after going through the FileIO examples from this DevNet article. It’s something very fundamental to how Flex works, and the fact that I’ve made it this far in my Flex career without knowing it either A, clearly illustrates how powerful composition is in Flex, or B, how rarely I extend more than 1 base class.

Extending a class in Flex via MXML is easy. If you want to extend any class, you merely make it the root tag in your MXML. Below, I’m extending mx.containers.VBox:

<?xml version="1.0" encoding="utf-8"?>
width="400" height="400">

<mx:FormItem label="Username:">

<mx:TextInput id="username_ti" />


<mx:FormItem label="Password:">

<mx:TextInput id="password_ti" />




When mxmlc.exe, the command line compiler Flex uses, converts that to ActionScript, you are basically doing:

import mx.containers.VBox;

class Login extends VBox
        // create children code

Both would look like this:

I’d save this as Login.mxml. However, from this point on, I’d usually use Login.mxml as a component in another component/class. I’d never want to “extend” my Login component to do something else… until recently.

Flex 1.5 uses a lot of mixins, since it’s based off of a lot of the Flash MX 2004 v2 component codebase. As such, most mixins are best used by being applied to an abstract base class, with which you later extend the abstract base class. It’s called abstract because you never actually instantiate the class itself; it’s merely there for inheritance purposes only; it was made only for being extended, not for being used.

Since ActionScript currently does not support multiple inheritance, the easiest way to fake it in ActionScript 1 and 2 is to utilize mixins; classes that utilize the Decorator pattern to add methods and properties to a class at runtime.

The downside to this is, even though you can define the properties and methods in your class so your compiler doesn’t bitch, adding event listeners for events that don’t yet exist is problematic, ecspecially if you don’t have control of the mixin. Case in point, mx.controls.listclasses.DataSelector.

This class adds basic list functionality to your class as well as support for dealing with dataproviders. The problem? The class the mixin is applied to cannot use modelChanged after the first listener. Quite frustrating, as I’ve written about before. You can use a static initializer that is run after the mixin’ to further overrwrite the modelChanged function on the class’ prototype to force it to call yours.

While that works, you’re still left with a quite unweidly class to start writing your own logic. The class itself works mind you, but if you’ve seen the DataSelector template your supposed to copy so your class will compile, it’s a LOT of stuff.

So, best to extend.

…problem was, I didn’t know how to do that in Flex, nor really know how to ask the question. So, I used the prototype hack. This was really the ONLY time I wanted to actually ever extend one of my components, and while I could of fallen back to using pure ActionScript, it’s just too much work to write all of that GUI interaction in ActionScript when MXML is faster, and easier to change without breaking my code.

So, looking through the FileIO examples, and I see they build a component, and then extend it by using the component’s name as the root tag. Well… DUH! Of course, how could I be so short-sighted? That is exactly how you extend, and have been, extending existing built in components, why would it NOT work for MY components?


<?xml version="1.0" encoding="utf-8"?>
private function initLogin():Void {
var so:SharedObject = SharedObject.getLocal("username");
if( != null)

I managed to code Director for a year before learning how to return values from function calls. Apparently, one can go longer than that in Flex without knowing how to do more than 1 level of inheritance.

4 Replies to “Multiple Inheritance in Flex”

  1. Multiple inheritance is the common term for beeing able to inherit from multiple classes at once. Only a few languages support this, like C++ for example. What you do here is normal inheritance. I’m not sure, if there is another name for it.

    But anyway, it’s always a great feeling to learn something new, isn’t it?

  2. Actually, my title had 2 meanings. First, how to inherit from MXML components you make, and secondly, using mixins, you can in face emulate extending multiple classes. The problem is, mixins tend to all assault your class at once, hence it being better to use mixins on a base class, and extend that. So, technically, yes, you are extending 1 base class, but they still allow you to utilize multiple classes in a way.

    Know how to do this in AS3 yet?

  3. Bokel’s point was, and I agree with him is that multiple inheritance is a common name in programming for something, well… this is not =)

    I (and a lot of others) consider multiple inheritance a Bad Thing, and I was suprised to read that actionscript would support it, but the title of this post is misleading.

    Congratz on finding out how usual class inheritance really works though =) Have you checked out intefaces yet?

  4. Yeah… that is a mis-leading title, sorry about that. I was so excited I found something new, I just proof-read my post, not the title. My bad.

    Interfaces are actually easy:

    <MyComponent implements=’ISomeInterface’ />

    Where the ISomeInterface is your interface class name.

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