This article was sleeping in my drafts for a while, and I am happy I managed to finish it today. There are multiple ways to create preloader for your ActionScript applications with flex compiler. The easiest and my prefered way is to use Frame metadata.
This metadata is a hint to the compiler that the class containing the metadata would like to be bootstrapped by the cited factory. In other words, when you build an application based on mx.core.Application, that isn’t actually the “root” class of the SWF. The root class is actually a compiler-generated subclass of mx.managers.SystemManager (Modular Applications)