If my series of articles are not your appetite, Or you prefer official learning documents, I have organized some online resources and recommended learning routes for your reference :)
First, do you know Flex? If you do not know flex and its development environment, you can look at it here:
Introduction to flex
Flex Language Reference
If you have some knowledge about flex and are ready to start using ArcGIS API for flex, we recommend that you read an ESRI online video to teach you how to configure the development environment, to do this, you must first sharpen your tools.
Creating an application using the ArcGIS API for flex
When you understand both flex mxml and ActionScript languages, and you have configured ArcGIS API for flex and can create a flex application, case-driven learning is the best way to learn, the following is an online sample of ArcGIS API for flex and its source code.
ArcGIS API for flex online sample
In the development process, if you have any questions about ArcGIS API for flex, the best solution is to view the API reference:
ArcGIS flex api reference
Finally, the topic irrelevant to ArcGIS API for Flex is the role of flex in WebGIS. The biggest advantage of using Flex is that you don't need to redesign and architecture the original GIS system and website, and you only need to embed the flex application in time to achieve very good results. Here, I will attach a demonstration of flex 3D map. After reading this demonstration, you will have a more intuitive understanding of how Flex promotes the performance of WebGIS applications:
Http://thunderhead.esri.com/readonlyurl/HeatMap3D.html