I’m not really sure what Adobe has been thinking regarding DragManagers in Flex/AIR. The code makes a very basic and very WRONG assumption that in an AIR app using WindowedApplication, you will only ever want to use the NativeDragManager class. DragManager loads up a singleton class to handle drag and drop. Depending on whether your app is AIR or Flex and what your top-level container is you either get DragManagerImpl (the flex drag manager) or NativeDragManagerImpl. Unfortunately, this class has some really annoying limitations. You can’t specify an alpha level for the drag proxy. It’s hardcoded to 0.5 and there’s nothing you can do about it. Here’s what adobe has to say about it…

When a Flex application runs in Adobe® AIR™, you can control whether the application uses the Flex drag manager or the AIR drag manager. These drag managers are implemented by the classes mx.managers.DragManager (Flex drag manager) and flash.desktop.NativeDragManager (AIR drag manager).

Internally, the Flex mx.managers.DragManager class uses an implementation class to determine which drag manager to use. It uses either the Flex mx.managers.DragManagerImpl class, or the AIR mx.managers.NativeDragManagerImpl class.

By default, a Flex application defined by the <mx:Application> tag uses the Flex drag-and-drop manager, even when the Flex application runs in AIR. If you run your Flex application in AIR, and you want to take advantage of the AIR drag-and-drop manager to drag and drop items from outside of AIR, then you must configure the Flex mx.managers.DragManager class to use the AIR drag-and-drop manager.

There are three scenarios that determine which drag-and-drop manager your Flex application uses when running in AIR:

  1. Your main application file uses the <mx:Application> tag. In this scenario, you use the Flex drag-and-drop manager, and cannot drag and drop items from outside of AIR.
  2. Your main application file uses the <mx:WindowedApplication> tag. In this scenario, you use the AIR drag-and-drop manager, and can drag and drop items from outside of AIR.
  3. Your main application file uses the <mx:Application> tag, but loads the AIR drag-and-drop manager as represented by the mx.managers.NativeDragManagerImpl class. In this scenario, you use the AIR drag-and-drop manager, and can drag and drop items from outside of AIR.

Now what am I to do if I want option 4? I want all the window dressing goodness that comes with WindowedApplication like maximize, minimize, and a status bar, but I want the fully customizable Flex version of the drag manager implementation. Time to MONKEYPATCH!

The first thing you’ll need is to copy the framework classes mx.managers.SystemManager and mx.core.Version into your AIR app at the right locations.
monkeypatch1

Next, you edit one line of code in SystemManager.as so that it loads the DragManagerImpl instead of NativeDragManagerImpl. Find the method docFrameHandler. You should see a bit of code that looks like this…

    if (Capabilities.playerType == "Desktop")
    {
        Singleton.registerClass("mx.managers::IDragManager",
            Class(getDefinitionByName("mx.managers::NativeDragManagerImpl")));

        // Make this call to create a new instance of the DragManager singleton.
        // This will allow the application to receive NativeDragEvents that originate
        // from the desktop.
        // if this class is not registered, it's most likely because the NativeDragManager is not
        // linked in correctly. all back to old DragManager.
        if (Singleton.getClass("mx.managers::IDragManager") == null)
            Singleton.registerClass("mx.managers::IDragManager",
                Class(getDefinitionByName("mx.managers::DragManagerImpl")));
    }

Now change to look like this…

    if (Capabilities.playerType == "Desktop")
    {
        Singleton.registerClass("mx.managers::IDragManager",
            Class(getDefinitionByName("mx.managers::DragManagerImpl")));

Enjoy your AIR app with the Flex DragManager goodness.

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Posted by Andrew, filed under AIR, as3, Flex, MonkeyPatch. Date: February 26, 2008, 8:27 pm | 1 Comment »

One Response

  1. Gary Hill Says:

    Working great so far – only note is that you need to relaunch Flex Builder for change to take effect.

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