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Unlike our XML export code, which is located within the particular class being
exported, we only have one single large XML import class (RoseXmlHandler, in
src/document/) which reads a complete document in one go.

In cases where we only need to import part of a document -- for example, when
importing a .rgd device file, which consists of just the studio element from
a .rg file -- we do it by importing the file as if it were a complete
document, into a new RosegardenGUIDoc object, and then copying the devices we
need from the imported document to the current one as appropriate. The
plugin case would be quite similar to that, I expect.

You can see an example of this logic in the interaction of BankEditorDialog
(src/gui/studio) and ImportDeviceDialog (src/dialogs -- don't ask why these
two related dialogs are in different parts of the source tree, it's probably
a reorganisational mistake). The former calls doImport on the latter, which
calls importFromRG if the source is a .rgd file, and this function
(ImportDeviceDialog::importFromRG) does what I just described -- it loads a
complete document from the .rgd file, and then copies the device and bank
structures from it into local data members which are subsequently queried
using the various get...() methods by BankEditorDialog.

Note also that RosegardenGUIDoc::openDocument doesn't care whether the file
it's loading is plain or gzipped XML.

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