AS3 Asynchronous ExternalInterface

As I may have mentioned before, AS3 performs its ExternalInterface operations synchronously. For the most part, this isn’t a big issue, but what happens when the javascript you need to execute grows extensive, or what if it is a slow operation? In these cases, it’s helpful to be able to make an ExternalInterface call that is asynchronous so your actionscript code can continue.

The following example does exactly that. It provides a layer between your JS and your Flash for converting methods to an asynchronous method.

The first part of this method is the AS3 class, AsyncExternalInterface:


This class first tests whether ExternalInterface is available at all, and then whether the necessary JavaScript class is on the page. If the JavaScript is not loaded, it will degrade and use ExternalInterface normally. If everything is in place, then your calls will be passed down to the CallStack JavaScript class. Here’s CallStack.js:


This class acts like a Singleton where you get the instance via the CallStack method made available in window. This means that when you include the JS in the page, you don’t have to do any instantiating yourself, or pass any variable names back to Flash. It’ll figure all that out itself. Of course, if you’re using this with the AS3 class, you don’t need to know about any of that because it’s all done behind the scenes.

What’s happening here is that every function call you make to ExternalInterface is being stored in a stack. This quick storing of the calls means your AS3 can continue on its merry way while JavaScript delays, then makes calls to that stack later. The speed at which the class works through the CallStack is set in the variable INTERVAL_TIME. For our test I’ve set that to 1 second. Before you use this in deployment, you’ll probably want to speed that up dramatically.

The class also has some cleanup methods so that while the stack is empty, it doesn’t waste any time polling an interval.

Finally, it’s time to put the files together into an HTML wrapper.


For our test, Flash is calling “testFunction” four times. The first three have sequential parameters, and the last has no parameters. If you have a FlashLog.txt file set up, you can also note the appearance of the traces vs the alerts. In the file, the traces come after the ExternalInterface calls. Were we using a normal ExternalInterface, you wouldn’t see any of those traces until after the alerts completed. Because of the AsyncExternalInterface solution, though, the traces will appear instantly, before the first alert has time to fire. The full example code can be found over on github. Enjoy!

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