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[news.eclipse.platform.swt] Re: Dropping Java objects as text
ByteArrayTransfer is an abstract class. You don't use ByteArrayTransfer directly. It just provides some native implementation to allow you to transfer bytes to another application (after you have serialized your objects into bytes). Each transfer type has a unique id and each id is unique to the platform that you're on. When the bytes are transferred, the header will include the unique transfer type id so that the target can identify it.

If an target application says that it supports "TextTransfer" then the source can safely give it a String. If the target application requests that it can only support TextTransfer then the source must only give it text and nothing else.

In your case, you don't need to create a new transfer type if all you want to do is transfer text (in addition to your existing transfer type). You can continue to use your current transfer type to drag your object to your own application and add the text transfer so that other applications can read it. Just set the transfer types to be your transfer type and the TextTransfer. Eg.

dragSource.setTransfer(new Transfer[] {MyInternalTransfer.getInstance(), TextTransfer.getInstance()});

I assume you already have a "MyInternalTransfer" since it's already working for you. Next, you need to ensure that if the target can support the transfer type that you're providing. "MyInternalTransfer" will only be requested if the target is your own application since no other app knows about it. If the target app (such as NotePad) says that it can only support text then you must only give it the String representation of your object.

public void dragSetData(DragSourceEvent event) {
if (MyInternalTransfer.getInstance().isSupportedType(event.dataType)) {
event.data = myObject; // do your current transfer magic for your object. This is just an example.
}
if (TextTransfer.getInstance().isSupportedType(event.dataType)) {
event.data = myObject.toString();
}
}

Note that TextTransfer will work with any native application (eg. WordPad, NotePad on Windows) that accepts text. It'll work on all platforms.

Please see Snippet185 for a complete example, as it shows you how to drag an object (a file). The target side can choose to accept the object as a String using TextTransfer or it can accept the object as an File using FileTransfer. If you drag the object in this example to NotePad, a string will be transferred. If you drag the object in this example to FileExplorer, you'll get a File transfer. If you drag the object in this example to itself, you can get either.

If both sides (dragSource and dropTarget) can agree on at least one common transfer type then the drag and drop is allowed.

You should implement your drag and drop the same way as Snippet185. No new Transfer (other than what you've already implemented) type is necessary. No native calls are required.

Duong

David Huebel wrote:

Thanks for the reply. The reason I'm confused is that I don't know how to
reliably distinguish between the two cases in dragSetData. I don't want to
create my own TransferData implementation (since that would require
learning native drag-and-drop implementation details on every target
platform) so my ByteArrayTransfer subclass acts just like ByteArrayTransfer
with respect to TransferData instances. 

I have no idea (and the documentation gives no hint) whether
TextTransfer.isSupportedType will reliably return false for the
TransferData offered by a ByteArrayTransfer -- and the code here is
platform-dependent.  Do native drag-and-drop implementations consider byte
arrays and strings to be completely incompatible?  I don't even know the
answer to that on my *own* preferred platform.  So I'd like to set the same
data regardless and let Eclipse pick the Transfer instance using whatever
native voodoo it uses for that....


I guess I should create my own TransferData class so I could rely on
isSupportedType to distinguish between the two cases, but writing
platform-specific code seems pretty onerous for a simple drag-and-drop
task.

-David