Just a quick one this, hopefully showcasing how to use classes with MovieClips in Flash CS4.  I'll cover two basic methods, one ties different MovieClips to a base class and the other re-uses a button class.

Making a reusable button

To keep it quick and easy this will be a simple fade in and out effect on a button so start off by making a button design that contains a dynamic text field with an instance name of 'label'.

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Once our MovieClip is created, it needs to be setup to export for ActionScript.

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First, open the properties for the symbol in the Library, then tick the 'Export for ActionScript' box and fill in the correct linkage.  

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Notice here that for the class I've typed in net.anatomicdesign.Button - this is something that I'd encourage any developer to do as it is industry best-practice.  For small things like this it doesn't make too much sense, but on larger projects organising your classes into proper packages is really important so it's good to get into good habits.

From here, create a new ActionScript3 document called Button.as and save it in the folder net/anatomicdesign/ (or whatever the package names are that you choose to use).