R – Adobe Flex and Air Connecting to a network

• Adobe AIR runs on the same technology as Flash (i.e. your AIR apps will run in the Flash player.) The difference is the additional libraries available in AIR to allow you to more easily create Rich Internet Applications, as opposed to Flash itself which is targeted at animation and games creation.

• The end product of a compiled AIR application is either a .air file or a natively packaged .exe/.dmg file (if targeted at the desktop in AIR 2+)

  The .air file is just a zip file which contains a .swf file and relevant directories from your project (such as unembedded assets).

  The .exe file is the same except for it also contains code to download and install the Adobe AIR Runtime if the user does not have it installed on their system. This can be very useful if you want your application to run only on the desktop as it means users won't get an 'open this file with..' message when they run the app without the runtime installed.

• The Adobe AIR runtime is what allows your operating system to interpret .air files and install/run them on the system. It contains the relevant AIR libraries not included in Flash.

• Adobe AIR when combined with Flex (mxml application) does allow states and transitions. States and transitions are a feature of Flex, so you would need to be using the Flex SDK in your project to use them.

I haven't been able to figure out how exactly the AIR and Flex SDKs are related.

From an encapsulation perspective, they aren't related in any way. However, Adobe does distribute the AIR SDK with their versions of the Flex SDK.

When using an IDE, such as Flash Builder, a certain directory structure is expected, which includes the AIR SDK in a subdirectory of the Flex SDK.

When using Apache Flex, Apache does not have the rights to distribute the AIR SDK, so it is up to the user to combine the AIR SDK and Flex SDK in a way that makes it easy to use the SDK with Flash Builder. However, the Apache Flex folks are working on an "easy install" tool to prepare the Apache Flex SDK for use with Flash Builder. The tool downloads all the relevant non-Apache binaries and provides you with a finished package.

It is perfectly valid to use the Flex UI Framework without AIR; and to use the AIR SDK without Flex. However, in the Flex SDK, specifically in the mobile component set, I believe there are some dependencies to the AIR SDK. And to use AIR, you will need something to compile it. MXMLC from the Flex SDK is one option.

I'm going to attempt to answer some of your specific questions:

My understanding would have been that AIR enables you to develop Flash (i.e. ActionScript, and apparently also JavaScript/HTML) applications that can be run by the AIR runtime without the need of a browser (such as a pure desktop or smartphone application).

This is true; except I do not believe that AIR has HTML/JS support for smartphones in the same way it does on the desktop. The Mobile AIR Runtime does not include an embedded browser like the desktop version does. However, you can make use of the browser on the device using StageWebView.

The Flex SDK appears to be a collection of classes and additional APIs ...to extend the "standard" Flash library.

This is correct. The Flex Framework provides a lot of UI Component classes, in addition to a framework for building your own UI Classes. To be clear, the Flex SDK includes more than just the Flex Framework, including a step through debugger and the command line compiler.

it appears that the Flex SDK includes some older version of the AIR SDK

This is true; the Flex SDK includes a version of the AIR SDK. Flex and AIR are not on the same update schedule; so Flex would only get updated w/ the latest version of AIR when a new Flex release comes out. There are ways to overlay a new Air SDK onto a Flex SDK if you need it. Here are some instructions. I forget the version of AIR included in Adobe Flex 4.6, but I think it is AIR 3.2. AIR 3.4 is the latest from Adobe.

It is worth noting that Flex has been donated to the Apache Foundation; and they have already released Apache Flex 4.8.

Would I need the Flex utilities to compile?

I'm not entirely clear on the answer to this. Most people use MXMLC from the Flex SDK to compile AIR based applications. Even if you use Flash Builder, MXMLC is used under the hood. If you use Flash Professional, I believe it has it's own AIR Compiler but I do not know if it is based on MXMLC. Flash Builder 4.7 will include a brand new compiler--code named Falcon.

Could I just copy any version of the AIR SDK into a Flex SDK to use both with e.g. FlashDevelop?

In theory yes. I don't know anything about FlashDevelop, however I did understand it had support for Flex. I assume, by extension, it also has support for AIR.