Modules and DragManager bug in Flex 3

In my last RIA project based on Flex 3, I find a bug working with modules that I’d like to sharing in this blog.

I create a dashboard where 4 custom components, that extend Panel component, load in a ModuleLoader differents sections.
In those sections there are components based on List component and when I run my project I’ve this kind of error:

TypeError: Error #1034: Type Coercion failed: cannot convert mx.managers::DragManagerImpl@21ef6df9 to mx.managers.IDragManager.

Surfing on web I don’t find anything about this kind of problem, finally on JIRA Adobe Flex bugs, I find it!
You can find workaround on JIRA ticket, it’s so easy to solve it, you only create a new instance of DragManager in your Default Application and everything works well.
This bug is created because you have more instance of DragManager in this sample, but DragManager in Flex framework is used like a Singleton so only one instance per time.
Each module has a List component that use DragManager when you have in the same application 2 or more modules you must put in Default Application file a DragManager instance and that’s it!
I think could be useful for anybody in the same situation.

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I’m the VP of Architecture at DAZN with more than 15 years of experience, a Google Developer Expert on Web Technologies and the London Javascript community Manager I had the chance to work on cutting-edge projects for mobile, desktop, web, TVs, set top boxes and embedded devices. I am currently managing DAZN, a sports video platform based on the cloud with millions of users that are watching live and on-demand contents. I'm the author of Front-End Reactive Architectures published by APress: I think the best way to use any programming language is mastering their models, that’s why I spent a lot of time studying and researching on topics like OOP, Functional and Reactive programming. In my spare time, I wrote for national and international technical magazines and editors, I'm also a technical reviewer for APress, Packt Publishing, Pragmatic Bookshelf and O'Reilly. I was speaker at: O'Reilly media webinars, O'Reilly Software Architecture (San Francisco & London), O'Reilly Fluent (San Jose), O'Reilly Oscon (London), Google Developers Summit (Krakow), Google DevFest (London), Frontend Devs Love (Amsterdam), Voxxed Days (Belgrad & Bristol), JeffConf (Milan), International Javascript Conference (Munich & London), JS Poland (Warsaw), Code Europe (Wroclaw), JSDay (Verona), CybercomDev (Łódź), Jazoon Conference (Bern), JDays (Göteborg), Codemotion (Milan), FullStack Conference (London), React London UG (London), Scrum Gathering (Prague), Agile Cymru (Cardiff), Scotch on the rocks (Edinburgh & London), 360Max (San Francisco), PyCon (Florence), Lean Kanban Conference (London), Adobe Creative Suite CS 5.5 - Launch event (Milan), Mobile World Congress (Barcelona)

8 thoughts on “Modules and DragManager bug in Flex 3”

  1. Good find and thanks! I had run into that very issue awhile back never really finding the culprit and finally the UX changed such that it was no longer an issue. Guess I dodged a bullet back then.

  2. Great solution. Thanks very much.
    I had same problems and this was my solution, I insertet in Main.mxml the following and it works!

  3. In a mx:script put the two lines
    Line1: import mx.managers.DragManager;
    Line2: private var dummy:DragManager;

  4. With modules you can have a module-wide singleton or an application-wide singleton depending on whether you import the singleton class also in the main application or not. I’ve written a post on my blog about this that demonstrates the different behaviors. I think it could be useful to avoid strange behaviors of the singletons in each modular Flex application. You can check it at

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