Daniel R.: Could not resolve * to a component implementation.

Often when refactoring code I'll extract ActionScript code out of an MXML file into an ActionScript based superclass to get a cleaner separation between logic and the view. In doing so I sometimes forget to update MXML variable declrations, leading to the confusing error:

Could not resolve <mx:states> to a component implementation.

In this case my MXML class extends another custom class and looks like:

<?xml version="1.0" encoding="utf-8"?>
<example:CustomCanvas xmlns:example="com.example.*" xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:states>
<mx:State name="default"/>
<mx:State name="custom"/>
</mx:states>
</example:CustomCanvas>

The issue is that the "mx" namespace doesn't match the root component's namespace so the MXML compiler gets confused about it being a property versus a child component. The simple fix is to just change the namespace on the property to match the root component's namespace like this:

<?xml version="1.0" encoding="utf-8"?>
<example:CustomCanvas xmlns:example="com.example.*" xmlns:mx="http://www.adobe.com/2006/mxml">
<example:states>
<mx:State name="default"/>
<mx:State name="custom"/>
</example:states>
</example:CustomCanvas>